EP4658163A1

EP4658163A1 - Method and apparatus for impedance method accuracy improvement

Info

Publication number: EP4658163A1
Application number: EP23918981.4A
Authority: EP
Inventors: Hui Shen; Jim Ni
Original assignee: Analog Devices International ULC
Current assignee: Analog Devices International ULC
Priority date: 2023-01-31
Filing date: 2023-01-31
Publication date: 2025-12-10
Also published as: WO2024159393A1

Abstract

A method and apparatus for measuring impedances. An apparatus comprises a signal generator (116) to generate a signal at an output of the signal generator (116), at least five signal paths, wherein each signal path comprises a current limiting device and a branch impedance, wherein each signal path in the at least five signal paths forms a network having an impedance of interest, a calibration path comprising a resistive device, a configurable network to selectively couple the output of the signal generator to each of the signal paths in the at least five signal paths and the calibration path, and circuitry (150), coupled to the configurable network, to determine the impedance of interest, the branch impedance of each signal path in the at least five signal paths, and a voltage at the branch impedance in each signal path in the at least five signal paths, based at least in part on current measurements of each of the signal paths in the at least five signal paths, a voltage measurement of the calibration path, and a current measurement of the calibration path.

Description

METHOD AND APPARATUS FOR IMPEDANCE METHOD ACCURACY IMPROVEMENT

BACKGROUND

Technical Field

The described aspects the field of integrated circuits, in particular to impedance measurements.
Introduction
Impedance measurements of the body, referred herein as bio-impedance, has many applications in healthcare and consumer applications. Impedance measurements can be made by electrodes provided in body-worn systems, or wearable devices, such as wrist watches, chest bands, head bands, patches, and so on. Circuitry coupled to the electrodes can derive the unknown impedance of the body on which the electrodes are placed. Impedance measurements can be particularly useful for vital-signs monitoring, sensing of tissues and fluid level in the body for purposes of detecting signs of pulmonary edema, or assess body composition. Moreover, electrical impedance tomography is an emerging non-invasive technique of medical imaging. Due to various challenges, making an accurate bio-impedance measurement is not trivial.
SUMMARY
The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.
An example aspect includes an apparatus for measuring impedances, comprising a signal generator to generate a signal at an output of the signal generator, at least five signal paths, wherein each signal path of the at least five signal paths comprises a current limiting device and a branch impedance, wherein each signal path in the at least five signal paths forms a network having an impedance of interest, a calibration path comprising a resistive device, a configurable network to selectively couple the output of the signal generator to each of the signal paths in the at least five signal paths and the calibration path, and circuitry, coupled to the configurable network, to determine the impedance of interest, the branch impedance of each signal path in the at least five signal paths, and a voltage at the branch impedance in each signal path in the at least five signal paths, based at least in part on current measurements of each of the signal paths in the at least five signal paths, a voltage measurement of the calibration path, and a current measurement of the calibration path.
Another example aspect includes a method for measuring impedances, the impedances including an impedance of interest and at least five branch impedances, comprising generating a signal at an output of a signal generator, selectively coupling the output of the signal generator to at least five signal paths and a calibration path, wherein each signal path comprises a current limiting device and a branch impedance, wherein each signal path in the at least five signal paths forms a network having an impedance of interest and the calibration path comprises a resistive device, measuring a current through each signal path in the at least five signal paths and in the calibration path, measuring a voltage at the resistive element in each signal path in the at least five signal paths, measuring a voltage across the calibration path, and determining the impedance of interest, based at least in part on the voltage measurements and the current measurements.
To the accomplishment of the foregoing and related ends, the one or more aspects comprise the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative features of the one or more aspects. These features are indicative, however, of but a few of the various ways in which the principles of various aspects may be employed, and this description is intended to include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed aspects will hereinafter be described in conjunction with the appended drawings, provided to illustrate and not to limit the disclosed aspects, wherein like designations denote like elements, wherein dashed lines may indicate optional elements, and in which:
FIG. 1 illustrates a system having electrodes and circuitry for performing one exemplary way of making a four-wire impedance measurement of bio-impedance, according to some aspects of the disclosure;
FIG. 2 illustrates input capacitances present in circuitry that performs a four-wire impedance measurement of bio-impedance, according to some aspects of the disclosure;
FIG. 3 illustrates current leakage present in circuitry that performs a four-wire impedance measurement of bio-impedance, according to some aspects of the disclosure;
FIG. 4 illustrates a calibration measurement, according to some aspects of the disclosure;
FIGS. 5-9 illustrate five current measurements, according to aspects of the disclosure;
FIG. 10 illustrates current leakage present in the measurement seen in FIG. 5, according to some aspects of the disclosure;
FIGS. 11-15 illustrate five current measurements which avoid current leakage, according to aspects of the disclosure;
FIGS. 16-30 illustrate current limiting resistors in various signal paths, and how voltages may be measured, in accordance with aspects of the present disclosure.
FIGS. 31-33 illustrate on-chip current limiting resistors in accordance with aspects of the present disclosure.
FIG. 34 is a flow diagram illustrating a method for measuring impedances, according to some aspects of the disclosure.

DETAILED DESCRIPTION

Various aspects are now described with reference to the drawings. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects. It may be evident, however, that such aspect (s) may be practiced without these specific details.
Overview
Accurately measuring bio-impedance is important for sensing properties of the body. Unfortunately, contact impedances can significantly degrade the accuracy of bio-impedance measurements. To address this issue, circuitry for implementing a four-wire impedance measurement can be configured to make multiple current measurements. The multiple current measurements set up a system of equations to allow the unknown bio-impedance and contact impedances to be derived. The result is an accurate bio-impedance measurement that is not negatively impacted by large contact impedances. Moreover, bad contacts with undesirably large impedances can be identified.
Four-Wire Impedance Measurement
One technique for impedance measurement is a four-terminal sensing scheme, or four-wire impedance measurement scheme. Sometimes it is referred to as Kelvin sensing. The technique involves using four electrodes placed on the body to sense or derive an unknown bio-impedance.
FIG. 1 illustrates a system 100 having electrodes and circuitry for performing one exemplary way making a four-wire impedance measurement of bio-impedance, according to some aspects of the disclosure. In the FIGURE, the unknown bio-impedance is shown as ZBODY. The system 100 includes electrodes 104, 106, 108, and 110 (or contacts to the body) . The electrodes 104, 106, 108, and 110 have respective contact impedances ZE1, ZE2, ZE3, and ZE4. Contact impedances ZE1, ZE2, ZE3, and ZE4 can represent skin-electrode impedance of the electrodes 104, 106, 108, and 110, respectively. Circuitry 150, packaged as an integrated circuit or chip, has pins (or connections) to which the electrodes are connected. Pin CE0 is electrically coupled to electrode 104. Pin AIN2 is electrically coupled to electrode 106. Pin AIN3 is electrically coupled to electrode 108. Pin AIN1 is electrically coupled to electrode 110.
The system 100 has four branches: a branch that includes electrode 104 and pin CE0, a branch that includes electrode 106 and pin AIN2, a branch that includes electrode 108 and pin AIN3, and a branch that includes electrode 110 and pin AIN1. Two branches are for sensing a first end of the unknown bio-impedance ZBODY, and two other branches are for sensing a second end of the unknown bio-impedance ZBODY. The branch that includes electrode 104 is coupled to the first end of unknown bio-impedance ZBODY. The branch that includes electrode 106 is coupled to the first end of unknown bio-impedance ZBODY. The branch that includes electrode 108 is coupled to the second end of unknown bio-impedance ZBODY. The branch that includes electrode 110 is coupled to the second end of unknown bio-impedance ZBODY. The four branches are connected to respective pins of circuitry 150. Parts of the branches outside of circuitry 150 can represent cables with patches at the end of the cables. Parts of the branches outside of circuitry 150 can also represent conductors or wires having electrodes at the end of the conductors or wires. The conductors and electrodes can be fitted in a wearable device. Optionally, capacitances shown CISO1, CISO2, CISO3, CISO4 can be included between respective pairs of electrodes and pins to provide isolation and protection between the body of the human user and the circuitry within circuitry 150 (e.g., to block DC signals) .
Circuitry 150 can include a multiplexer (mux) 112. Mux 112 can be controlled in a manner to connect signal paths of the different pins to different parts of circuitry 150. Mux 112, as used herein, represents a configurable network controllable to connect different parts of circuitry 150 to different pins. For instance, mux 112 can connect different parts of circuitry 150 to different branches connected to the pins (the branches having respective electrodes) . Different configurations of mux 112 can form different signal paths or different impedance networks (impedance networks being synonymous with signal paths) .
Circuitry 150 can include a signal generator 116 (e.g., sinusoidal signal generator) . Signal generator can generate a signal having a peak voltage of VPEAK. The signal generator generates the signal at an output of the signal generator.
Circuitry 150 can include voltage measurement circuitry 118 to measure a voltage across a positive input and a negative input of the voltage measurement circuitry 118. In some aspects, voltage measurement circuitry 118 can include an instrumentation amplifier (inAmp) 120 with a positive terminal and a negative terminal to sense a voltage difference between the positive terminal and negative terminal, and outputs a voltage output representative of that voltage difference. Voltage measurement circuitry 118 can include a Discrete Fourier Transform (DFT) block 122 and summation block 124 to generate a voltage measurement based on the voltage output from inAmp 120. Components for generating a voltage measurement (e.g., a difference in voltage between two inputs) can differ depending on the implementation.
Circuitry 150 can further include current measurement circuitry 126 to measure a current at an input of the current measurement circuitry 126. In some aspects, current measurement circuitry 126 can include a transimpedance amplifier (TIA) 128 to convert a current at an input terminal of the TIA 128 to a voltage output representative of the current. Current measurement circuitry 126 can include a DFT block 130 and summation block 132 to generate a current measurement based on the voltage output from TIA 128. Components for generating a current measurement (e.g., an amount of current flowing through an input) can differ depending on the implementation.
To make an impedance measurement, a voltage is generated across the unknown bio-impedance shown as ZBODY. The voltage across the unknown bio-impedance ZBODY can be viewed as VA-VB. The voltage across the unknown bio-impedance ZBODY can be generated or imposed by signal generator 116. Meanwhile, the voltage across the unknown bio-impedance ZBODY is measured by the voltage measurement circuitry 118, and the current through the unknown bio-impedance ZBODY is also measured, by current measurement circuitry 126. The measured voltage and the measured current can be used to derive the impedance value of the unknown bio-impedance ZBODY. Specifically, the impedance value of the unknown bio-impedance ZBODY is related to the voltage measurement divided by the current measurement.
In conventional two-wire impedance measurements, measurement issues can arise from impedances of cables (including contact impedances) being added to the unknown bio-impedance ZBODY, thus corrupting the impedance measurement. For simplicity, the impedances present are lumped together as a contact impedance in each branch. In theory, a four-wire impedance measurement can avoid such issues. When the unknown bio-impedance ZBODY is much higher than the impedances of the cables, the measurements can be sufficiently accurate.
However, in practice, a four-wire impedance measurement can have certain other limitations or non-idealities that can significantly impact the accuracy of the bio-impedance measurement. These limitations can be significant, e.g., when making impedance measurements at low frequencies, high frequencies, certain frequencies, or various frequencies. In some situations, one or more of the contact impedances ZE1, ZE2, ZE3, and ZE4 can be greater than the unknown bio-impedance ZBODY. For instance, mechanical and/or environmental reasons (e.g., humidity, movement, hair on skin, etc. ) can cause poor3410 contacts, and can severely increase one or more of the contact impedances. In some severe cases, the (magnitude of) contact impedances can be greater than 2 kΩ. In some situations, the optional capacitors CISO1, CISO2, CISO3, CISO4 can also significantly increase or affect the impedances of the cables. In some situations, the contact impedances ZE1, ZE2, ZE3, and ZE4 can have an imbalance with each other (e.g., imbalance can be greater than 1 kΩ) . These limitations have been found to degrade the accuracy of the four-wire impedance measurement.
One of the problems causing these limitations that degrade the accuracy of the bio-impedance measurement is that there can be large input capacitances at pin AIN2 and pin AIN3 (e.g., around 40 pF) . FIG. 2 illustrates input capacitances present in circuitry that performs a four-wire impedance measurement of bio-impedance, according to some aspects of the disclosure. Grounded input capacitance 202 can be present at pin AIN2, and grounded input capacitance 204 can also be present at pin AIN3. Grounded input capacitance 202, contact impedance ZE2, and capacitance CISO2 can form a filter. This filter can be problematic because the contact impedance ZE2 is unknown, and thus the effect of the filter is unknown as well. Grounded input capacitance 204, contact impedance ZE3, and capacitance CISO3 can also form another filter. This other filter can be problematic because the contact impedance ZE3 is unknown, and thus the effect of this other filter is unknown as well. Ideally, voltage VA should be the same as the voltage VC, and voltage VB should be the same as the voltage VD. Due to the grounded input capacitances 202 and 204, at certain frequencies, voltage VA is not the same as the voltage VC, and voltage VB is not the same as the voltage VD. The voltage across VA and VB may not be the same as the voltage across VC and VD. The negative effect of the grounded input capacitances 202 and 204 can be observable at low frequencies and when contact impedances are high, e.g., in the range of hundreds or thousands of Ohms. Furthermore, the grounded input capacitances 202 and 204 can attribute to imbalances in the contact impedances. Imbalances in the contact impedances of the branches can produce different cut-off frequencies, thereby causing different attenuations in each branch.
Another problem that may degrade the accuracy of the bio-impedance measurement is current leakage. FIG. 3 illustrates current leakage present in circuitry that performs a four-wire impedance measurement of bio-impedance, according to some aspects of the disclosure. The current leakage arises because the impedance ZS-of the branch having electrode 108 can be similar to the impedance ZF-of the branch having electrode 110 driving the TIA 128. This results in some of the current IBODY flowing through the unknown bio-impedance ZBODY to flow through the branch having electrode 108, and not all of the current IBODY would flow through the branch having electrode 110. In other words, the current IZS-through the branch having electrode 108 is ideally zero, and the current IZF-through the branch having electrode 110 is ideally equal to current IBODY. In reality, the current IZS-is not zero. As a result, the current IZF-through the branch having electrode 110 is not equal to current IBODY, and part of current IBODY is not measured by the current measurement circuitry 126. The current measurement is corrupted, and thus the impedance measurement is also corrupted. This issue can be exacerbated by high contact impedances in the branches.
An Exemplary Scheme for Deriving Contact Impedances Through Multiple Measurements and Signal Processing
By configuring mux 112 and making multiple current measurements, it is possible to derive the (unknown) impedances of the system, including the unknown bio-impedance ZBODY, and the contact impedances ZE1, ZE2, ZE3, and ZE4, based on a system of equations. The system of equations are formed through a calibration measurement, and several other current measurements of different signal paths formed by configuring mux 112. Mux 112 can selectively couple the output of the signal generator 116 and the input of the current measurement circuitry 126 to different pins (e.g., RCAL1, RCAL2, CE0, AIN2, AIN3, and AIN1) . Accordingly, mux 112 can connect the output of the signal generator 116 to the input of the current measurement circuitry 126 through different signal paths, or different impedance networks involving at least some of the unknown impedances. The different signal paths, individually, can include two or more of the unknown impedances of the system: the unknown bio-impedance ZBODY, and the contact impedances ZE1, ZE2, ZE3, and ZE4. Unique signal paths or unique impedance networks of at least some of the unknown impedances, and the current measurements of the unique signal paths or unique impedance networks, setup a system of equations for the unknown impedances. The unique signal paths or unique impedance networks, together, include each one of the unknown impedances at least once. Each unique signal path or unique impedance network would include at least some of the unknown impedances of the system. Effectively, the signal generator 116 can excite unique signal paths or unique impedance networks formed by mux 112, and the current measurement circuitry 126 can make measurements of current going through the unique signal paths or unique impedance networks.
To determine five unknown impedances (the bio-impedance and the four contact impedances) , at least five equations are needed. With a sufficient number of equations, it is possible to derive the five unknown impedances through signal processing (i.e., calculations) . Through suitable processing, the current measurements allow the bio-impedance and the contact impedances to be determined. The current measurements can be performed by the current measurement circuitry 126. The signal processing can be performed in the digital domain, e.g., by digital circuitry 190. Digital circuitry 190 can include specialized digital hardware to perform the signal processing. Digital circuitry 190 can include a microprocessor or microcontroller configured to carry out instructions that implement the signal processing. The digital circuitry 190 can be provided on-chip with circuitry 150 or off-chip (as shown) . Digital circuitry 190 can be implemented to control mux 112 to form unique signal paths or unique impedance networks from the signal generator 116 to the current measurement circuitry 126. Computer-readable storage 192 can store the measurements. Computer-readable storage 192 can store the instructions that implement the signal processing. The computer-readable storage 192 can be provided on-chip with circuitry 150 or off-chip (as shown) .
FIG. 4 illustrates a calibration measurement, according to some aspects of the disclosure. The calibration measurement is performed to determine a peak voltage from the signal generator 116, if it is not already measured or if it is not already known. The system of equations (shown as equations 2-6 below) being formed by the current measurements of unique signal paths going through at least some of the unknown impedances use the peak voltage measured in the calibration measurement as a numerical constant. The unknown impedances would be derived based further on the peak voltage measured in the calibration measurement. Determining the peak voltage from the signal generator 116 can be performed in various ways. An output from the signal generator 116 can be applied to a resistor with a known resistance value, and the current measurement circuitry 126 can measure a current through the resistor. The calibration measurement is represented by: VCAL=ICAL·RCAL (reproduced as equation 1 below) . RCAL is a resistor with a known stable resistance value. ICAL is measured by the current measurement circuitry 126. Accordingly, VCAL, which is the voltage from signal generator 116 can be derived.
The resistor with a known resistance value can be provided on-chip with circuitry 150 or off-chip (as shown) . The calibration measurement is optional if the peak voltage from the signal generator is known. The calibration measurement may only need to be performed once, and does not need to be performed every time impedance measurements are being made.
In the example shown, for the calibration measurement, an (off-chip) resistor RCAL having a known, stable resistance value is coupled across pins RCAL1 and RCAL2. The mux 112 is configured to couple the signal path from pin RCAL1 to the signal generator 116 and to couple the signal path from pin RCAL2 to the current measurement circuitry 126. The mux 112 forms a signal path from the output of signal generator 116 to input of current measurement circuitry 126, and the signal path includes resistor RCAL. The mux 112 connects the output of signal generator 116 to input of current measurement circuitry 126 through the resistor RCAL. The measured current performed by current measurement circuitry is ICAL. With the known resistance value of the resistor RCAL, it is possible to derive the voltage VCAL=ICAL·RCAL across the resistor RCAL. The measured current ICAL and the known resistance value of resistor RCAL form equation 1, seen below. The voltage VCAL represents the (calibrated) peak voltage from signal generator 116. The measurement of the voltage VCAL across RCAL is determined by measuring a current through RCAL, i.e., through the signal path that includes RCAL, by current measurement circuitry 126.
FIGS. 5-9 illustrate five current measurements, according to aspects of the disclosure. The five current measurements setup a system of five equations, and the five unknown impedances (the bio-impedance and the four contact impedances) can be derived from solving the system of five equations. Note that, in the individual branches, impedances in a cable connected to a pin and a contact impedance are lumped together and represented as a contact impedance (e.g., ZE1, ZE2, ZE3, and ZE4) , for simplicity. The contact impedances thus represent individual branch impedances.
In FIG. 5, the mux 112 is configured to couple the signal path from pin CE0 to the output of the signal generator 116 and to couple the signal path from pin AIN1 to the input of the current measurement circuitry 126. The measured current obtained by current measurement circuitry 126 is I1. The measured current I1, measured current ICAL, and the known resistance value of RCAL form equation 2, seen below. Mux 112 has formed a signal path from the signal generator 116 to the current measurement circuitry 126. The signal path includes unknown contact impedance ZE1, unknown bio-impedance ZBODY, and unknown contact impedance ZE4 (in series) . The signal path includes a branch with electrode 104 and pin CE0, and a branch with electrode 110 and pin AIN1. Equation 2 encapsulates the relationship between the three unknown impedances ZE1, ZBODY, and ZE4 in the signal path and the measured current I1, measured current ICAL, and the known resistance value of RCAL. Note that the product of the measured current ICAL and the known resistance value of RCAL is equivalent to the voltage VCAL obtained from the calibration measurement.
In FIG. 6, the mux 112 is configured to couple the signal path from pin CE0 to the output of the signal generator 116 and to couple the signal path from pin AIN2 to the input of the current measurement circuitry 126. The measured current obtained by current measurement circuitry 126 is I2. The measured current I2, measured current ICAL, and the known resistance value of RCAL form equation 3, seen below. Mux 112 has formed a signal path from the signal generator 116 to the current measurement circuitry 126. The signal path includes unknown contact impedance ZE1 and unknown contact impedance ZE2 (in series) . The signal path includes a branch with electrode 104 and pin CE0, and a branch with electrode 106 and pin AIN2. Equation 3 encapsulates the relationship between the two unknown impedances ZE1 and ZE2 in the signal path and the measured current I2, measured current ICAL, and the known resistance value of RCAL.
In FIG. 7, the mux 112 is configured to couple the signal path from pin CE0 to the output of the signal generator 116 and to couple the signal path from pin AIN3 to the input of the current measurement circuitry 126. The measured current obtained by current measurement circuitry 126 is I3. The measured current I3, measured current ICAL, and the known resistance value of RCAL form equation 4, seen below. Mux 112 has formed a signal path from the signal generator 116 to the current measurement circuitry 126. The signal path includes unknown contact impedance ZE1, unknown bio-impedance ZBODY, and unknown contact impedance ZE3 (in series) . The signal path includes a branch with electrode 104 and pin CE0, and a branch with electrode 108 and pin AIN3. Equation 4 encapsulates the relationship between the three unknown impedances ZE1, ZBODY, and ZE3 in the signal path and the measured current I3, measured current Ica, and the known resistance value of RCAL.
In FIG. 8, the mux 112 is configured to couple the signal path from pin AIN2 to the output of the signal generator 116 and to couple the signal path from pin AIN1 to the input of the current measurement circuitry 126. The measured current obtained by current measurement circuitry 126 is I4. The measured current I4, measured current Ica, and the known resistance value of RCAL form equation 5, seen below. Mux 112 has formed a signal path from the signal generator 116 to the current measurement circuitry 126. The signal path includes unknown contact impedance ZE2, unknown bio-impedance ZBODY, and unknown contact impedance ZE4 (in series) . The signal path includes a branch with electrode 106 and pin AIN2, and a branch with electrode 110 and pin AIN1. Equation 5 encapsulates the relationship between the three unknown impedances ZE2, ZBODY, and ZE4 in the signal path and the measured current I4, measured current Ica, and the known resistance value of RCAL.
In FIG. 9, the mux 112 is configured to couple the signal path from pin AIN3 to the output of the signal generator 116 and to couple the signal path from pin AIN1 to the input of the current measurement circuitry 126. The measured current obtained by current measurement circuitry 126 is I5. The measured current I5, measured current ICAL, and the known resistance value of RCAL form equation 6, seen below. Mux 112 has formed a signal path from the signal generator 116 to the current measurement circuitry 126. The signal path includes unknown contact impedance ZE3 and unknown contact impedance ZE4 (in series) . The signal path includes a branch with electrode 108 and pin AIN3, and a branch with electrode 110 and pin AIN1. Equation 6 encapsulates the relationship between the two unknown impedances, ZE3 and ZE4, in the signal path and the measured current I5, measured current ICAL, and the known resistance value of RCAL.
With five equations (equations 2-6) and five unknown impedances ZBODY, ZE1, ZE2, ZE3, and ZE4, the values for the five unknown impedances ZBODY, ZE1, ZE2, ZE3, and ZE4 can be derived and determined. As illustrated by FIGS. 5-9, each unique signal path includes two branch impedances. Moreover, as seen in FIGS. 5, 7, and 8, some of the unique signal paths can each include the bio-impedance and two branch impedances. Each unique signal path includes at least some of the unknown impedances, and together, the unique signal paths include each unknown impedance at least once.
The five equations (equations 2-6) can be rewritten to equations 7-11 that gives the unknown impedances ZBODY, ZE1, ZE2, ZE3, and ZE4 in terms of one or more ones of the current measurements (one or more of I1, I2, I3, I4, and I5) , the measured current ICAL, and the known resistance value of RCAL. Digital circuitry 190, such as a microcontroller or microprocessor, can be implemented to compute the unknown impedances based on the measurements seen in FIGS. 4-9 and equations 7-11. Computer-readable storage 192 can store the measurements, and instructions for processing the measurements to derive the impedances.
The measurements seen in FIGS. 4-9 can be performed in any order. In some cases, more than five measurements can be made to generate more than five equations.
The scheme illustrated by FIGS. 4-9 can have several advantages. Note that a voltage measurement across the unknown bio-impedance ZBODY is no longer needed (which is normally required in the four-wire impedance measurement illustrated by FIG. 1) . As a result, an expensive inAmp 120 is no longer required in circuitry 150. Furthermore, the error due to the grounded capacitances at pins AIN2 and AIN3 (acting as a low pass filter) , which causes the voltages of VA not being the same as VC and the voltages of VB not being the same as VD, is no longer relevant since a voltage measurement is not being made. Moreover, the scheme can effectively derive five impedances ZBODY, ZE1, ZE2, ZE3, and ZE4.
Another Exemplary Scheme for Deriving Contact Impedances Through Multiple Measurements and Signal Processing
In the previous scheme illustrated by the measurements seen in FIGS. 4-9, there is one limitation: current leakage. FIG. 10 illustrates current leakage present in the measurement seen in FIG. 5, according to some aspects of the disclosure. When making a current measurement, such as current I1 (as illustrated by FIG. 5) , the branches which are not connected to the signal generator 116 or the current measurement circuitry 126 ideally has infinite impedance. With infinite impedance, the branches which are not connected to the signal generator 116 or the current measurement circuitry 126 would have zero current. In other words, IZE2 (current through branch having electrode 106 and pin Ain2) and IZE3 (current through branch having electrode 108 and pin AIN3) are ideally zero. As a result, IZE1 would be equal to IBODY (current through the unknown bio-impedance) , and would also be equal to IIIA (current through branch) . This would mean that no current is leaking through the branches having electrode 106 and electrode 108, and the current measurement circuitry 126 is measuring the current through the unknown bio-impedance ZBODY accurately (ITIA=IBODY) . In reality, the branches which are not connected to the signal generator 116 or the current measurement circuitry 126 do not have infinite impedances, and can have grounded capacitances 1002 and 1004 (e.g., in the pF or μF range) . The grounded capacitances 1002 and 1004 represent circuitry (e.g., circuitry in mux 112) capable of sinking current in the branches. As a result, a part of the current IZE1 may flow through the branches which are not connected to the signal generator 116 or the current measurement circuitry 126. This means that IZE2 and IZE3 is not zero, and IZE1 may not be equal to IBODY, and may not be equal to IIIA. As a result, current is leaking through the branches having electrode 106 and electrode 108, and the current measurement circuitry 126 is measuring the current through the unknown bio-impedance ZBODY inaccurately (ITIA≠IBODY) .
To address this limitation, the current measurements setting up a system of equations having the unknown impedances can be modified. Specifically, the configuration of the mux 112 is adapted for each measurement, and a different system of equations is used for deriving the unknown impedances. Instead of leaving some of the signal paths floating, all signal paths are connected either to the signal generator 116 or the current measurement circuitry 126. The unique signal paths or unique impedance networks, instead of each including just a subset of the unknown impedances or just two of four branches, the unique signal paths or unique impedance networks would include all of the bio-impedance and the branch impedances, and all four branches. As a result, leaked current can be captured by the system of equations.
For four current measurements, one of the signal paths is connected to the signal generator 116, and the other three of the signal paths are connected to the current measurement circuitry 126. One of the four branches is connected to the output of signal generator 116, and three other ones of the four branches are connected to the input of current measurement circuitry 126. For another current measurement, two signal paths are connected to the signal generator 116, and the other two of the signal paths are connected to the current measurement circuitry 126. Two of the four branches are connected to the output of signal generator 116, and two other ones of the four branches are connected to the input of current measurement circuitry 126. Accordingly, no floating branches will cause current leakage or sink current. The five current measurements form a different system of equations, since the overall signal path formed by the mux 112 from the signal generator 116 to the current measurement circuitry 126 now involves parallel impedances (i.e., parallel unknown impedances) . However, the system of equations having five equations can still enable the five unknown impedances to be determined.
By configuring mux 112 and making multiple current measurements, it is possible to derive the unknown impedances of the system, including the unknown bio-impedance ZBODY, and the contact impedances ZE1, ZE2, ZE3, and ZE4, based on a system of equations. The system of equations are formed through a calibration measurement, and several current measurements of different, unique signal paths formed by configuring mux 112. Mux 112 can selectively couple the output of the signal generator 116 and the input of the current measurement circuitry 126 to different pins (e.g., RCAL1, RCAL2, CE0, AIN2, AIN3, and AIN1) . Accordingly, mux 112 can connect the output of the signal generator 116 to the input of the current measurement circuitry 126 through different signal paths, or different impedance networks involving all of the unknown impedances. The different, unique signal paths, form unique impedance networks, where each unique impedance network combines all of the unknown impedances of the system: the unknown bio-impedance ZBODY, and the contact impedances ZE1, ZE2, ZE3, and ZE4, with a unique topology. Unique signal paths or unique impedance networks each involving all of the unknown impedances, and the current measurements of the unique signal paths or unique impedance networks, setup a system of equations for the unknown impedances. Effectively, the signal generator 116 can excite unique signal paths or unique impedance networks formed by mux 112, and the current measurement circuitry 126 can make measurements of current going through the unique signal paths or unique impedance networks.
To determine five unknown impedances (the bio-impedance and the four contact impedances) , at least five equations are needed. With a sufficient number of equations, it is possible to derive the five unknown impedances through signal processing (i.e., calculations) . Through suitable processing, the current measurements allow the bio-impedance and the contact impedances to be determined. The current measurements can be performed by the current measurement circuitry 126. The signal processing can be performed in the digital domain, e.g., by digital circuitry 190. Digital circuitry 190 can include specialized digital hardware to perform the signal processing. Digital circuitry 190 can include a microprocessor or microcontroller configured to carry out instructions that implement the signal processing. The digital circuitry 190 can be provided on-chip with circuitry 150 or off-chip (as shown) . Digital circuitry 190 can be implemented to control mux 112 to form unique signal paths or unique impedance networks from the signal generator 116 to the current measurement circuitry 126. Computer-readable storage 192 can store the measurements. Computer-readable storage 192 can store the instructions that implement the signal processing. The computer-readable storage 192 can be provided on-chip with circuitry 150 or off-chip (as shown) .
In this modified scheme, the calibration measurement can be performed based on the configuration seen in FIG. 4 and equation 1, which yields VCAL. FIGS. 11-15 illustrate five current measurements, according to aspects of the disclosure. The five current measurements setup a system of five equations, and the five unknown impedances (the bio-impedance and the four contact impedances) can be derived from solving the system of five equations. Note that, in the individual branches, impedances in a cable connected to a pin is lumped together and represented as a contact impedance (e.g., ZE1, ZE2, ZE3, and ZE4) , for simplicity. The contact impedances thus represent individual branch impedances.
In FIG. 11, the mux 112 is configured to couple the signal path from pin CE0 to the output of the signal generator 116, to couple the signal path from pin AIN2 to the input of current measurement circuitry 126, to couple the signal path from pin AIN3 to the input of current measurement circuitry 126, to couple the signal path from pin AIN1 to the input of current measurement circuitry 126. The measured current done by current measurement circuitry 126 is I1. The configuration of mux 112 in FIG. 11 forms an overall signal path that includes ZE1 in series with (ZE2 in parallel with (ZBODY in series with (ZE3 and ZE4 in parallel) ) . The branch with electrode 104 and pin CE0 is connected to the output of the signal generator 116. The branch with electrode 106 and pin AIN2 is connected to the input of current measurement circuitry 126. The branch with electrode 108 and pin AIN3 is connected to the input of current measurement circuitry 126. The branch with electrode 110 and pin AIN1 is connected to the input of current measurement circuitry 126. The measured current I1, measured voltage VCAL, form equation 12, seen below. Equation 12 encapsulates the relationship between the measured current I1, measured voltage VCAL, and the unknown impedances in the overall signal path from the signal generator 116 to current measurement circuitry 126 (formed by the mux 112 in the configuration shown in FIG. 11) .
In FIG. 12, the mux 112 is configured to couple the signal path from pin AIN2 to the output of signal generator 116, to couple the signal path from pin CE0 to the input of current measurement circuitry 126, to couple the signal path from pin AIN3 to the input of current measurement circuitry 126, to couple the signal path from pin AIN1 to the input of current measurement circuitry 126. The measured current done by current measurement circuitry 126 is I2. The configuration of mux 112 in FIG. 12 forms an overall signal path that includes ZE2 in series with (ZE1 in parallel with (ZBODY in series with (ZE3 and ZE4 in parallel) ) ) . The branch with electrode 104 and pin CE0 is connected to the input of the current measurement circuitry 126. The branch with electrode 106 and pin AIN2 is connected to the output of signal generator 116. The branch with electrode 108 and pin AIN3 is connected to the input of current measurement circuitry 126. The branch with electrode 110 and pin AIN1 is connected to the input of current measurement circuitry 126. The measured current I2, measured voltage VCAL, form equation 13, seen below. Equation 13 encapsulates the relationship between the measured current I2, measured voltage VCAL, and the unknown impedances in the overall signal path from the signal generator 116 to current measurement circuitry 126 (formed by the mux 112 in the configuration shown in FIG. 12) .
In FIG. 13, the mux 112 is configured to couple the signal path from pin AIN3 to the output of signal generator 116, to couple the signal path from pin CE0 to the input of current measurement circuitry 126, to couple the signal path from pin AIN2 to the input of current measurement circuitry 126, to couple the signal path from pin AIN1 to the input of current measurement circuitry 126. The measured current done by current measurement circuitry 126 is I3. The configuration of mux 112 in FIG. 13 forms an overall signal path that includes ZE3 in series with (ZE4 in parallel with (ZBODY in series with (ZE1 and ZE2 in parallel) ) ) . The branch with electrode 104 and pin CE0 is connected to the input of the current measurement circuitry 126. The branch with electrode 106 and pin AIN2 is connected to input of the current measurement circuitry 126. The branch with electrode 108 and pin AIN3 is connected to the output of signal generator 116. The branch with electrode 110 and pin AIN1 is connected to the input of current measurement circuitry 126. The measured current I3, measured voltage VCAL, form equation 14, seen below. Equation 14 encapsulates the relationship between the measured current I3, measured voltage VCAL, and the unknown impedances in the overall signal path from the signal generator 116 to current measurement circuitry 126 (formed by the mux 112 in the configuration shown in FIG. 13) .
In FIG. 14, the mux 112 is configured to couple the signal path from pin AIN1 to the signal generator 116, to couple the signal path from pin CE0 to the current measurement circuitry 126, to couple the signal path from pin AIN2 to the current measurement circuitry 126, to couple the signal path from pin AIN3 to the current measurement circuitry 126. The measured current done by current measurement circuitry 126 is I4. The configuration of mux 112 in FIG. 14 forms an overall signal path that includes ZE4 in series with (ZE3 in parallel with (ZBODY in series with (ZE1 and ZE2 in parallel) ) ) . The branch with electrode 104 and pin CE0 is connected to the input of the current measurement circuitry 126. The branch with electrode 106 and pin AIN2 is connected to input of the current measurement circuitry 126. The branch with electrode 108 and pin AIN3 is connected to the input of current measurement circuitry 126. The branch with electrode 110 and pin AIN1 is connected to the output of signal generator 116. The measured current I4, measured voltage VCAL, form equation 15, seen below. Equation 15 encapsulates the relationship between the measured current I4, measured voltage VCAL, and the unknown impedances in the overall signal path from the signal generator 116 to current measurement circuitry 126 (formed by the mux 112 in the configuration shown in FIG. 14) .
In FIG. 15, the mux 112 is configured to couple the signal path from pin CE0 to the signal generator 116, to couple the signal path from pin AIN2 to the signal generator 116 (as well) , to couple the signal path from pin AIN3 to the current measurement circuitry 126, to couple the signal path from pin AIN1 to the current measurement circuitry 126. The measured current done by current measurement circuitry 126 is I5. The configuration of mux 112 in FIG. 15 forms an overall signal path that includes (ZE1 and ZE2 in parallel) in series with ZBODY and in series with (ZE3 and ZE4 in parallel) . The branch with electrode 104 and pin CE0 is connected to the output of the output of signal generator 116. The branch with electrode 106 and pin AIN2 is connected to the output of the output of signal generator 116. The branch with electrode 108 and pin AIN3 is connected to the input of current measurement circuitry 126. The branch with electrode 110 and pin AIN1 is connected to the input of current measurement circuitry 126. The measured current Is, measured voltage VCAL, form equation 16, seen below. Equation 16 encapsulates the relationship between the measured current Is, measured voltage VCAL, and the unknown impedances in the overall signal path from the signal generator 116 to current measurement circuitry 126 (formed by the mux 112 in the configuration shown in FIG. 15) .
An alternative to the signal path illustrated by FIG. 15 is to connect the branch with electrode 104 and pin CE0 and the branch with electrode 106 and pin AIN2 is connected to the input of current measurement circuitry 126, and to connect the branch with electrode 108 and pin AIN3 and the branch with electrode 110 and pin AIN1 to the output of signal generator 116.
Equations 17-21 show equations 12-16 in an expanded form based on the notation for parallel impedances.
With five equations (equations 12-16) and five unknown impedances ZBODY, ZE1, ZE2, ZE3, and ZE4, the values for the five unknown impedances ZBODY, ZE1, ZE2, ZE3, and ZE4 can be derived and determined. As illustrated by FIGS. 11-15, each unique signal path includes all of the unknown impedances. Moreover, as seen in FIGS. 5, 7, and 8, some of the unique signal paths can each include the bio-impedance and two branch impedances. Each unique signal path includes at least some of the unknown impedances, and together, the unique signal paths include each unknown impedance at least once.
Algebraic manipulations can be applied to equations 17-21 to rewrite equations 12-21 so that the unknown impedances ZBODY, ZE1, ZE2, ZE3, and ZE4 are defined in terms of the current measurements (e.g., I1, I2, I3, I4, and I5) , the measured current ICAL, and the known resistance value of RCAL. The following pseudocode can be implemented in digital circuitry 190, such as a microcontroller or microprocessor, to determine and compute the unknown impedances based on the measurements seen in FIGS. 4, and 11-15.
The measurements seen in FIGS. 4, and 11-15 can be performed in any order. In some cases, more than five measurements can be made to generate more than five equations.
The scheme illustrated by FIGS. 4, and 11-15 can have several advantages (similar to the scheme seen in FIGS. 4-9) . Note that a voltage measurement across the unknown bio-impedance ZBODY is no longer needed (which is normally required in the four-wire impedance measurement illustrated by FIG. 1) . As a result, an expensive inAmp 120 is no longer required in circuitry 150. Furthermore, the error due to the grounded capacitances at pins AIN2 and AIN3 (acting as a low pass filter) , which causes to the voltages of VA not being the same as VC and the voltages of VB not being the same as VD, is no longer relevant since a voltage measurement is not being made. Moreover, the scheme can effectively and accurately derive five impedances ZBODY, ZE1, ZE2, ZE3, and ZE4. In addition to these advantages, this scheme can now ensure accuracy even in the presence of high impedances, and big imbalances between contact impedances.
Current Limiting Resistors in Signal Paths
FIGS. 16-30 illustrate current limiting resistors in various signal paths, and how voltages may be measured in accordance with aspects of the present disclosure. The digital circuitry shown in FIGS. 1-15 is not shown for ease of understanding, however, such circuitry may be included in FIGS. 16-30 without departing from the scope of the present invention.
FIG. 16 illustrates that each of the signal paths may include a limiting resistance, RLIMIT, to limit the amount of current that can pass through the various signal paths. Such limiting resistance on each signal path may be required for patient safety per safety standards, such as IEC-60601-1. Since each of the current measurements use each pin in the signal paths, a limiting resistance RLIMIT may be placed on each signal path as shown in FIG. 16.
FIGS. 17 and 18 illustrate parasitic capacitances in accordance with an aspect of the present disclosure.
As shown in FIG. 17, there may be some parasitic capacitance on the trace between the device and where the signal path makes contact with the human body to measure ZBODY. For example, Cp0 shows the parasitic capacitance between pin CE0 and ground, which will attenuate the amplitude of the output sinusoidal signal from the CE0 pin. In such a situation, VC0, the voltage between CISO1 and ZE1, may not be equal to VCAL. Such parasitic capacitance may introduce measurement error into Equation 12. Such errors increase when there is a current limiting resistance RLIMIT, because RLIMIT and CE0 acts like an RC filter circuit.
Similarly, in FIG. 18, Cp2 shows the parasitic capacitance between pin AIN2 and ground. Pin AIN2 is connected to the TIA input and is used as an input for current measurement. The TIA input may be treated as a virtual ground, so the voltage amplitude on the AIN2 pin should be equal to zero. However, the VC2 voltage will not be equal to AIN2 and may have voltage fluctuation due to the voltage drop on the limiting resistance RLIMIT. This voltage inequality between VC2 and AIN2 causes some current to go through Cp2 to external ground. Thus, the TIA input current will not be equal to the current coming from ZE2 and may cause a current measurement error. Such parasitic capacitance may introduce measurement error into the equations used to determine ZBODY, such as Equation 12. Such errors increase when there is a current limiting resistance RLIMIT, because RLIMIT and CE0 acts like an RC filter circuit. Similar parasitic capacitances may also be present between ZE3 and CISO3 and ZE4 and CISO4.
FIGS. 19 and 20 illustrate measurements accounting for the parasitic capacitances in accordance with an aspect of the present disclosure.
FIGS. 19 and 20 illustrate how the RC filter (s) and various voltage differences/changes, e.g., VC0, VC2, etc. in the signal paths may affect the measurement of ZBODY. Because there are unknown voltage differences between VCAL and VC0, VC2, VC3, etc., the determination of ZE1, ZE2, ZE3, ZE4 may contain inaccuracies. These inaccuracies affect the determination of ZBODY using the equations listed above.
In an aspect of the present disclosure, FIG. 19 illustrates a current measurement similar to that of FIG. 11 above.
In an aspect of the present disclosure, FIG. 20 illustrates a voltage measurement to compensate for the parasitic capacitances in the current measurement of FIG. 19.
FIG. 20 illustrates that a path from inAmp is coupled through the mux to a new pin P1, on the circuit. This new pin is electrically coupled between CISO2 and ZE2, such that VC0 can be measured directly.
Further, a path from inAmp is coupled through the mux to a new pin P2, on the circuit. Because VC0 and VC2 can now be measured via P1 and P2, a new equation can be used to reduce and/or eliminate errors in the measurements, as I1 can now be determined by Equation 12A below:
By replacing VCAL with V1, where V1 = the voltage difference between VC0 and VC2 in this measurement, the effects of the parasitic capacitances Cp0 and Cp2 are reduced in the overall determination of ZBODY.
FIGS. 21 and 22 illustrate measurements accounting for the parasitic capacitances in accordance with an aspect of the present disclosure.
In an aspect of the present disclosure, FIG. 21 illustrates a current measurement similar to that of FIG. 8 above, again showing voltages VC0 and VC2.
In an aspect of the present disclosure, FIG. 22 illustrates a voltage measurement to compensate for the parasitic capacitances in the current measurement of FIG. 21.
FIG. 22 illustrates that a path from inAmp is coupled through the mux to a new pin P1, on the circuit, associated with CE0. This new pin is electrically coupled between CISO2 and ZE2, such that VC0 can be measured directly.
Further, a path from inAmp is coupled through the mux to a new pin P2, on the circuit, associated with AIN2. Because VC0 and VC2 can now be measured via P1 and P2, a new equation can be used to reduce and/or eliminate errors in the measurements, as I2 can now be determined by Equation 13A below:
By replacing VCAL with V2, where V2 = the voltage difference between VC0 and VC2 in this measurement, the effects of the parasitic capacitances Cp0 and Cp2 are reduced in the overall determination of ZBODY.
FIGS. 23 and 24 illustrate measurements accounting for the parasitic capacitances in accordance with an aspect of the present disclosure.
In an aspect of the present disclosure, FIG. 23 illustrates a current measurement similar to that of FIG. 9 above, now showing voltages VC1 and VC3.
In an aspect of the present disclosure, FIG. 24 illustrates a voltage measurement to compensate for the parasitic capacitances in the current measurement of FIG. 23.
FIG. 24 illustrates that a path from inAmp is coupled through the mux to a new pin P3, on the circuit, associated with AIN3. This new pin P3 is electrically coupled between CISO3 and ZE3, such that VC3 can be measured directly.
Further, a path from inAmp is coupled through the mux to a new pin P4, associated with AIN1, on the circuit. This new pin P4 is electrically coupled between CISO4 and ZE4, such that VC1 can be measured directly. Because VC3 and VC1 can now be measured via P3 and P4, a new equation can be used to reduce and/or eliminate errors in the measurements, as I3 can now be determined by Equation 14A below:
By replacing VCAL with V3, where V3 = the voltage difference between VC3 and VC1 in this measurement, the effects of the parasitic capacitances in these signal paths, e.g., CP3 and CP4, are reduced in the overall determination of ZBODY.
FIGS. 25 and 26 illustrate measurements accounting for the parasitic capacitances in accordance with an aspect of the present disclosure.
In an aspect of the present disclosure, FIG. 25 illustrates a current measurement similar to that of FIG. 14 above, now showing voltages VC1 and VC3.
In an aspect of the present disclosure, FIG. 26 illustrates a voltage measurement to compensate for the parasitic capacitances in the current measurement of FIG. 25.
FIG. 26 illustrates that a path from inAmp is coupled through the mux to pin P3, on the circuit, associated with AIN3. This pin P3 is electrically coupled between CISO3 and ZE3, such that VC3 can be measured directly.
Further, a path from inAmp is coupled through the mux to pin P4, associated with AIN1, on the circuit. This pin P4 is electrically coupled between CISO4 and ZE4, such that VC1 can be measured directly. Because VC3 and VC1 can now be measured via P3 and P4, a new equation can be used to reduce and/or eliminate errors in the measurements, as I4 can now be determined by Equation 15A below:
By replacing VCAL with V4, where V4 = the voltage difference between VC3 and VC1 in this measurement, the effects of the parasitic capacitances in these signal paths, e.g., CP3 and CP4, are reduced in the overall determination of ZBODY.
FIGS. 27 and 28 illustrate measurements accounting for the parasitic capacitances in accordance with an aspect of the present disclosure.
In an aspect of the present disclosure, FIG. 27 illustrates a current measurement similar to that of FIG. 15 above, now showing voltages VC0 and VC3.
In an aspect of the present disclosure, FIG. 28 illustrates a voltage measurement to compensate for the parasitic capacitances in the current measurement of FIG. 27.
FIG. 28 illustrates that a path from inAmp is coupled through the mux to pin P3, on the circuit, associated with AIN3. This pin P3 is electrically coupled between CISO3 and ZE3, such that VC3 can be measured directly.
Further, a path from inAmp is coupled through the mux to pin P1, associated with CE0, on the circuit. This pin P0 is electrically coupled between CISO1 and ZE1, such that VC0 can be measured directly. Because VC3 and VC0 can now be measured via P3 and P4, a new equation can be used to reduce and/or eliminate errors in the measurements, as I5 can now be determined by Equation 15A below:
By replacing VCAL with V5, where V5 = the voltage difference between VC3 and VC0 in this measurement, the effects of the parasitic capacitances in these signal paths, e.g., CP3 and CP0, are reduced in the overall determination of ZBODY.
FIGS. 29 and 30 illustrate calibration measurements in accordance with an aspect of the present disclosure.
In an aspect of the present disclosure, FIG. 29 illustrates a current measurement similar to that of FIG. 4 above.
In an aspect of the present disclosure, FIG. 30 illustrates a voltage measurement to compensate for the parasitic capacitances in the current measurement of FIG. 29.
VCAL can now be determined by Equation 1A below:
V_CAL0=I_CAL0*R_CAL
This voltage measurement of VCAL0 may also reduce measurement errors in the overall calculation of ZBODY when used with respect to and/or in conjunction with equations 12A through 16A described above.
FIGS. 31-33 illustrate on-chip current limiting resistors in accordance with aspects of the present disclosure.
FIG. 31 illustrates that the RLIMIT resistors may be located on circuitry 150, as an integrated part of a circuit, integrated circuit, or other device, rather than being externally coupled to the pins of circuitry 150 as shown in FIGS. 16-30.
FIG. 32 illustrates a current measurement similar to that of FIG. 19, where the limiting resistors are part of circuitry 150. FIG. 33 illustrates a voltage measurement similar to that of FIG. 20, with the limiting resistors being part of circuitry 150.
Method for Measuring Impedances
FIG. 34 is a flow diagram illustrating a method for measuring impedances, according to some aspects of the disclosure.
The impedances include a bio-impedance (the impedance of interest ZBODY) and four branch impedances (the signal path impedances ZE1-ZE4) . In 3402, signal generator 116 generates a signal at an output of a signal generator. In 3404, circuitry such as mux 112 selectively couples the output of the signal generator to at least five signal paths and a calibration path, wherein each signal path comprises a current limiting device and a branch impedance, wherein each signal path in the at least five signal paths forms a network having an impedance of interest and the calibration path comprises a resistive device. In 3406, measurement circuitry, such as current measurement circuitry 126, measures a current through each signal path in the at least five signal paths and in the calibration path. In 3408, measurement circuitry, such as voltage measurement circuitry 118, measures a voltage at the resistive element in each signal path in the at least five signal paths. In 3410, measurement circuitry, such as voltage measurement circuitry, measures a voltage across the calibration path. In 3412, circuitry, such as digital circuitry 190, determines the impedance of interest, based at least in part on the voltage measurements and the current measurements.
Additional information and figures in accordance with aspects of the present disclosure may be found in the appendix attached to the present disclosure.
Additional Technical Advantages
Measuring bio-impedance can be particularly useful for measuring body impedance for detecting fluid level of the lungs or measuring thoracic impedance. Measuring bio-impedance can also be useful in electrical impedance tomography to determine a composition of the body (e.g., imaging of tissues and bones) in a non-invasive manner by making bio-impedance measurements at different frequencies. Measuring bio-impedance can be useful in measuring respiration activity, where respiration activity can be obtained by observing variation in thorax impedance. Measuring bio-impedance and the contact impedances means that respiration activity can be obtained even in the presence of motion, since variations in contact impedances can be taken into account. Users such as athletes and patients can greatly benefit from such applications.
Knowing the contact impedances ZE1, ZE2, ZE3, and ZE4 in addition to the unknown bio-impedance ZBODY, and accounting for the parasitic capacitances and safety guidelines in making such measurements, can enable the circuitry to infer whether the contacts (i.e., contacts being formed by the electrodes contacting the body) are good or not, e.g., as part of a diagnostic process. For instance, high contact impedances can indicate that patches/electrodes are not properly attached to the body. Accordingly, information about the quality of the contacts can be inferred from derived contact impedances.
For example, the digital circuitry 190 can determine quality of contacts corresponding to the four electrodes based on the impedances of the four branches. If a given impedance of a branch is too high, the digital circuitry 190 can infer that the contact for the branch is bad and output a signal that indicates the presence of a bad contact and optionally an identifier that identifies which contact is bad. The digital circuitry 190 can compare the impedances of the four branches against predetermined threshold (s) to determine whether a given impedance is too high.
User feedback can be provided based on the inferred information about the quality of the contacts. In another instance, smart drug delivery applications may require proper contacts to the body to ensure correct and effective drug delivery. If the contact is improper, drug can pool on the skin due to poor absorption and contact to the skin. Other applications, such as electrocardiography or defibrillation, may also require proper contacts to the body. Being able to infer the quality of the contacts based on the derived contact impedances can provide feedback to the user regarding the quality of the contacts in such contexts as well.
Some efforts to extract contact quality or contact impedance have limitations, and the schemes for measuring impedances described herein can improve upon those efforts. In some systems, efforts to extract contact quality or contact impedance ignore bio-impedance, or assume that the bio-impedance is zero, close to zero, or very small compared to the contact impedances. This assumption can be reasonable when the electrodes are measuring electrical activity of the heart, since in such situations, the electrodes are placed close to each other (e.g., on the thorax) and the skin has been prepared to make the body impedance very small. The impedances measurement schemes described herein do not make such an assumption. Not making this assumption can be beneficial in contexts where the body impedance can be large. For instance, body impedance cannot be ignored when electrodes are placed on other parts of the body, far apart from each other, where the bio-impedance can be in the range of the contact impedances. In another instance, the bio-impedance can be much greater than the contact impedances if the electrodes have very low impedances. In yet another instance, the lack of skin preparation can also make the contact impedances much larger than the bio-impedance being measured. For all these reasons, the impedances measurement schemes described herein can be used in a variety of situations. For instance, the impedances measurement scheme can be used to, non-invasively, obtain the body's composition, determine thoracic impedance, determine respiration activity in the presence of motion, etc.
Variations and Implementations
The unique signal paths illustrated by the disclosure are not meant to be limiting. Other topologies, schemes for exciting and measuring the signal paths can be implemented, and are envisioned by the disclosure.
Moreover, certain aspects discussed above can be provisioned in digital signal processing technologies for medical imaging, patient monitoring, medical instrumentation, and home healthcare. The aspects herein can also be beneficial to other applications requiring an accurate impedance measurement using at least four electrodes.
In the discussions of the aspects above, various electrical components can readily be replaced, substituted, or otherwise modified in order to accommodate particular circuitry needs. Moreover, it should be noted that the use of complementary electronic devices, hardware, software, etc. offer an equally viable option for implementing the teachings of the present disclosure.
Parts of various circuitry for deriving unknown impedances can include electronic circuitry to perform the functions described herein. In some cases, one or more parts of the circuitry can be provided by a processor specially configured for carrying out the functions described herein. For instance, the processor may include one or more application specific components, or may include programmable logic gates which are configured to carry out the functions describe herein. The circuitry can operate in analog domain, digital domain, or in a mixed signal domain. In some instances, the processor may be configured to carrying out the functions described herein by executing one or more instructions stored on a non-transitory computer medium. In some aspects, an apparatus can include means for performing or implementing one or more of the functionalities describe herein.
It is also imperative to note that all of the specifications, dimensions, and relationships outlined herein (e.g., the number of processors, logic operations, etc. ) have only been offered for purposes of example and teaching only. Such information may be varied considerably without departing from the spirit of the present disclosure. The specifications apply only to one non- limiting example and, accordingly, they should be construed as such. In the foregoing description, example aspects have been described with reference to particular processor and/or component arrangements. Various modifications and changes may be made to such aspects without departing from the scope of the disclosure. The description and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.
Note that with the numerous examples provided herein, interaction may be described in terms of two, three, four, or more electrical components. However, this has been done for purposes of clarity and example only. It should be appreciated that the system can be consolidated in any suitable manner. Along similar design alternatives, any of the illustrated components, modules, and elements of the FIGURES may be combined in various possible configurations, all of which are clearly within the broad scope of this Specification. In certain cases, it may be easier to describe one or more of the functionalities of a given set of flows by only referencing a limited number of electrical elements. It should be appreciated that the electrical circuits of the FIGURES and its teachings are readily scalable and can accommodate a large number of components, as well as more complicated/sophisticated arrangements and configurations. Accordingly, the examples provided should not limit the scope or inhibit the broad teachings of the electrical circuits as potentially applied to a myriad of other architectures.
Note that in this Specification, references to various features (e.g., elements, structures, modules, components, steps, operations, characteristics, etc. ) included in “one aspect” , “example aspect” , “an aspect” , “another aspect” , “some aspects” , “various aspects” , “other aspects” , “alternative aspect” , and the like are intended to mean that any such features are included in one or more aspects of the present disclosure, but may or may not necessarily be combined in the same aspects.
It is also important to note that the functions related to deriving unknown impedances, illustrate only some of the possible functions that may be executed by, or within, systems illustrated in the FIGURES. Some of these operations may be deleted or removed where appropriate, or these operations may be modified or changed considerably without departing from the scope of the present disclosure. In addition, the timing of these operations may be altered considerably. The preceding operational flows have been offered for purposes of example and discussion. Substantial flexibility is provided by aspects described herein in that any suitable arrangements, chronologies, configurations, and timing mechanisms may be provided without departing from the teachings of the present disclosure.
While the foregoing disclosure discusses illustrative aspects and/or aspects, it should be noted that various changes and modifications could be made herein without departing from the scope of the described aspects and/or embodiments as defined by the appended claims. Furthermore, although elements of the described aspects and/or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and/or embodiment may be utilized with all or a portion of any other aspect and/or embodiment, unless stated otherwise.

Claims

A circuit for measuring impedances, comprising:

a signal generator to generate a signal at an output of the signal generator;

at least five signal paths, wherein each signal path of the at least five signal paths comprises a current limiting device and a branch impedance, wherein each signal path in the at least five signal paths forms a network having an impedance of interest;

a calibration path comprising a resistive device;

a configurable network to selectively couple the output of the signal generator to each of the signal paths in the at least five signal paths and the calibration path; and

circuitry, coupled to the configurable network, to determine the impedance of interest, the branch impedance of each signal path in the at least five signal paths, and a voltage at the branch impedance in each signal path in the at least five signal paths, based at least in part on current measurements of each of the signal paths in the at least five signal paths, a voltage measurement of the calibration path, and a current measurement of the calibration path.
The circuit of claim 1, wherein one of the signal paths in the at least five signal paths comprises two of the branch impedances coupled in parallel.
The circuit of claim 1, wherein one of the signal paths in the at least five signal paths comprises two of the branch impedances coupled in parallel, and the two branch impedances coupled in parallel are in series with the impedance of interest.
The circuit of claim 1, wherein the current limiting device in each signal path in the at least five signal paths is integrated with the configurable network.
The circuit of claim 1, wherein each signal path of the at least five signal paths further comprises a capacitive device.
The circuit of claim 5, wherein the voltage at the branch impedance is measured between the capacitive device and the branch impedance.
A method for measuring impedances, the impedances including an impedance of interest and at least five branch impedances, comprising:

generating a signal at an output of a signal generator;

selectively coupling the output of the signal generator to at least five signal paths and a calibration path, wherein each signal path comprises a current limiting device and a branch impedance, wherein each signal path in the at least five signal paths forms a network having an impedance of interest and the calibration path comprises a resistive device;

measuring a current through each signal path in the at least five signal paths and in the calibration path;

measuring a voltage at the resistive element in each signal path in the at least five signal paths;

measuring a voltage across the calibration path; and

determining the impedance of interest, based at least in part on the voltage measurements and the current measurements.
The method of claim 7, wherein one of the signal paths in the at least five signal paths comprises two of the branch impedances coupled in parallel.
The method of claim 7, wherein one of the signal paths in the at least five signal paths comprises two of the branch impedances coupled in parallel, and the two branch impedances coupled in parallel are in series with the impedance of interest.
The method of claim 7, wherein the current limiting device in each signal path in the at least five signal paths is integrated with the configurable network.
The method of claim 7, wherein each signal path of the at least five signal paths further comprises a capacitive device.
The method of claim 11, wherein the voltage at the branch impedance is measured between the capacitive device and the branch impedance.