GB2449904A - A high-frequency current source for Electrical impedance Tomography (EIT), with compensation for amplifier gain error - Google Patents

A high-frequency current source for Electrical impedance Tomography (EIT), with compensation for amplifier gain error Download PDF

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GB2449904A
GB2449904A GB0710950A GB0710950A GB2449904A GB 2449904 A GB2449904 A GB 2449904A GB 0710950 A GB0710950 A GB 0710950A GB 0710950 A GB0710950 A GB 0710950A GB 2449904 A GB2449904 A GB 2449904A
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amplifier
node
input node
current source
voltage
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Wei Wang
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De Montfort University
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De Montfort University
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    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
    • G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
    • G05F1/10—Regulating voltage or current
    • G05F1/46—Regulating voltage or current wherein the variable actually regulated by the final control device is dc
    • G05F1/461—Regulating voltage or current wherein the variable actually regulated by the final control device is dc using an operational amplifier as final control device
    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
    • G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
    • G05F1/10—Regulating voltage or current
    • G05F1/46—Regulating voltage or current wherein the variable actually regulated by the final control device is dc
    • G05F1/56—Regulating voltage or current wherein the variable actually regulated by the final control device is dc using semiconductor devices in series with the load as final control devices
    • G05F1/561—Voltage to current converters
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/053—Measuring electrical impedance or conductance of a portion of the body
    • A61B5/0536—Impedance imaging, e.g. by tomography

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  • Health & Medical Sciences (AREA)
  • Physics & Mathematics (AREA)
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  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Automation & Control Theory (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
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  • Radiology & Medical Imaging (AREA)
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  • Heart & Thoracic Surgery (AREA)
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Abstract

A high-frequency voltage-controlled current source (VCCS) 20 for EIT provides a high output impedance over a broad frequency range by dynamic adjustment of at least one resistor R1-R5 in dependence on frequency. The controller 18 may be a computer and the variable resistors may be digital potentiometers. The dynamic adjustment compensates for the finite gain of the operational amplifier 12 at high frequencies and thus provides a voltage-to-current converter suitable for EIT across a broad frequency range, thereby enabling measurement of frequency-dependent dispersions that may be indicative of underlying pathology. The resistors R1-R5 may be trimmed by laser during manufacture of the current source, thereby to improve the output impedance.

Description

TITLE
A high output impedance voltage controlled current source
FIELD OF THE INVENTION
Embodiments of the present invention relate to a voltage controlled current source
BACKGROUND TO THE INVENTION
1 0 A voltage controlled current source (VCCS) is a key component in a number of electronic apparatus For example Electrical Impedance Tomography (EIT) is a known medical imaging technique in which impedance variations of part of the body are determined from surface electrical 1 5 measurements. Typically multiple conducting electrodes are attached to the skin of the subject. A small alternating electric current is typically provided by a VCCS to one or more electrodes and the resulting electrical potentials across the other electrodes are measured It is important that the small alternating current provided to each electrode is the same so that impedance measurements can be compared The current should be stable and accurately controlled It is important that the current source's output impedance is large enough compared to the measured object impedance so that variations in the measured impedance do not significantly affect the current For example, if the measured impedance is in the range of 1-2K = typically, the output impedance of the current source should exceed 1-2Mg to ensure 1% accuracy. in addition, if the contact impedance is considered, the equivalent output impedance of the current source should be much higher A popular current source is the Howland model as illustrated in Fig 1 This current source 10 comprises a voltage input node 11 for receiving an input voltage, a current output node 13 for providing a constant current to a load; an operational amplifier 12 comprising a first amplifier input node 15, a second amplifier input node 17 and an amplifier output node 19, a first resistor Ri, second resistor R2, third resistor R3 and fourth resistor R4 connected as illustrated.
For the ideal operational amplifier, the gain is infinite and the input impedance is infinite, so R2 R2 _1* R1 *R. [R.R4 R3] When R2 = R1 R1 then I = -J--.V R1 The output current is dominated by the input voltage and R3, and this circuit is ideally a current = R4 source under the condition R, I? . If the resistors were perfectly matched, the output 1 0 impedance of the circuit would be infinite.
It has been suggested that a limitation with the Howland model is that stray capacitance associated with the output node will reduce the output impedance. One proposed solution is to provide a Generalized Impedance Converter (GIC) in parallel with the output node to 1 5 compensate for such stray capacitance. Although this produces high output impedance values, it does so only over a limited frequency range.
This is problematic for some application because in a multi-frequency EIT system, EIT is performed using electric currents over a broad range of different frequencies. This allows the impedance of the body to be mapped over the broad range of frequencies typically 0-50MHz to cover frequency dependant dispersions that may be indicative of underlying pathology.
It would be desirable to provide a current source that achieves the highest possible output impedance across a broad range of frequencies using a simple structure.
BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION
According to various embodiments of the invention there is provided a voltage controlled current source comprising: a voltage input node for receiving an input voltage, a current output node for providing a constant current to a load; an operational amplifier having a gain and comprising a first amplifier input node, a second amplifier input node and an amplifier output node; a first resistor connected between the voltage input node and the second amplifier input node; a second resistor connected between the amplifier output node and the second amplifier input node, a third resistor connected between a reference voltage and the first amplifier input node, a fourth resistor connected between the current output node and the first amplifier input node; a fifth resistor connected between the amplifier output node and the current output node, wherein one of more of the first, second, third, fourth or fifth resistors is a variable resistor, and compensation means, used for compensating for frequency dependent variation of the amplifier gain, arranged to control the value of one or more of the resistors Ri, R2, R3, R4 and R5 1 0 The first, second, third, fourth and fifth resistors may have respective resistances Ri, R2, R3, R4 and R5 and the compensation means may be arranged to maintain an output impedance over a frequency range by maintaining R2/Rl equal to [(R4 +R5)/R3j + P where P is a parameter dependent upon the frequency dependent variation of the amplifier gain 1 5 The compensation means may be arranged to maintain an output impedance exceeding 2M0 over the frequency range The first, second, third, fourth and fifth resistors may have respective resistances Ri, R2, R3, R4 and R5 and the compensation means may be arranged to maintain a transconductance of the voltage controlled current source over a frequency range by maintaining (Q *R2)/(R1*R5) constant, where Q is a parameter dependent upon the frequency dependent variation of the amplifier gain The frequency range may be 0 to 30MHz. The compensation means may be arranged to vary the ratio of the R2 to Ri and/or arranged to vary R5.
Embodiments of the present invention maintain an acceptably and consistently high output impedance over a broad bandwidth of frequencies As an example, embodiments of the Invention maintain an output impedance up to 5OM over frequency range 0-100MHz and up to 2MQ at frequency bandwidth 0-30MHz with up to 2k = loading resistance in practice.
Embodiments of the invention uses a simple cicuit with a low component count In addition, the performance of the circuit is dependent on the performance of the operational amplifier and the performance of the current source can be straightforwardly improved by improving the
specification of the operational amplifier.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of various embodiments of the present invention reference will now be made by way of example only to the accompanying drawings in which.
Fig 1 schematically illustrates a Howland model current source, Fig 2 schematically illustrates a current source according to one embodiment of the invention, and Fig 3 schematically illustrates a current source according to another embodiment of the invention.
DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION
Fig. 2 schematically illustrates one embodiment of a voltage controlled current source (VCCS) This current source 20 comprises a voltage input node 11 for receiving an input voltage, a current output node 13 for providing a constant current to a load; an operational amplifier 12 having a gain and comprising a first amplifier input node 15, a second amplifier input node 17 and an amplifier output node 19, a first resistor Ri, second resistor R2, third resistor R3 and fourth resistor R4, and a fifth resistor R5.
The first resistor Ri is connected between the voltage input node 11 and the second amplifier input node 17. The second resistor R2 is connected between the amplifier output node 19 and the second amplifier input node 17. The third resistor R3 is connected between a reference voltage (ground) 14 and the first amplifier input node 15. The fourth resistor R4 is connected between the current output node 13 and the first amplifier input node 15 The fifth resistor R5 is connected between the amplifier output node 19 and the current output node 13.
One or more of the resistors Ri, R2, R3, R4 and R5 may be variable resistors.
One or more of the variable resistors may be variable' in the sense that they are arranged to be trimmed in-situ using for example a YAG laser to ablate resistor material. Such variable resistors are typically varied once to the desired value.
One or more of the variable resistors may be variable' in the sense that they are dynamically changeable This may be achieved via a port 16 using a controller such as a computer 18. For example, the port 16 is arranged to control the value of the resistors R5 in Fig 2. One example of a dynamically changeable variable resistor is a digital potentiometer.
Transcoriductance If the operational amplifier 12 is considered as an ideal operational amplifier, the output current of the circuit can be described as 1?, + R1(R2R4+RS 1 R5 I? R5 (R, + R4) R R3) When R2R4+R5 -2 then R R1 R The VCCS output current is dominated by the input voltage V/and value of resistors Ri, R2, R5.
It forms an ideal current source under these conditions The transconductance, the ratio can be worked out using Eq 3, in which Setting V1 to the dynamic range R of the operational amplifier and Ito the amplitude A of constant current required, then equals R/A The values of RI and R2 and/or R5 are set to achieve this equality. This may occur at the manufacturing or initialization stage or alternatively may occur dynamically during use by using, for example, one or more digital potentiometers.
Vi can also be connected to the non-inverting input terminal of operational amplifier Under the same conditions as above, the output current is 1=V*_L.!?-2-.4 R5 R Output impedance The output impedance of the VCCS 20 is found by setting J" 0 in Eq 1, to get I -5 [R5 R3 + R4 R R, + R4 R5 J which gives the output impedance as: R511 -v R JI?2 R5+R4 -R3 When the condition = R5 = k is satisfied, the circuit will perform as an ideal current
R
source and the output impedance is infinite The larger the ratio of the larger the output impedance is. However, the ratio of ---has to be R51R4 smaller than the close-loop gain k of the operational amplifier, which is designed with respect to the stability of the operational amplifier and the working bandwidth The values of Ri, R2, R3, R4 and R5 may be set to maximize under the constraint that it equals R2/R1 - 1 5 R51R3. This may occur at the manufacturing or initialization stage or alternatively may occur dynamically during use by using, for example, one or more digital potentiometers = ?4 k and the tolerance of commercial products R1 to R5 is a, then in the worst R1 R3 situation R5(l + -a)3 z=-R3 7 4/ca Low tolerance resistors ensure higher output impedance The current source using 0 1% tolerant resistors has a ten times larger output impedance than one using 1% tolerant resistors. A manufacturer may select the performance level by controlling the tolerance of the resistors, for example, using laser trimming Non-Ideal Operational Amplifier Fig 3 schematically illustrates an alternative embodiment in which operational amplifier 12 is non-ideal. The operational amplifier has a finite differential gain AOd and input impedance RId The current absorbed by the operational amplifier is I, V and VE' are the input voltages at the inverting and non-inverting terminals respectively, and V0 is the output voltage of the operational amplifier Therefore, the basic equations are given as -J" = , 8 E RId -If the loading current is I and the loading voltage is V, at the circuit nodes V V1.and V, the current sum is zero JI-12-JE=O -9 14 -0 -11 Here, I to /5 denote the current passing through the resistors R1 to R5.
Combining the equations, the loading current can be described as.
R2 (i ____ 1 1=-R1+R2tR5R3+R4A.R,) v i ( R. R2 R3 R I R1
____ -_____ _____
AOd 1\R1 + R2 R1 + R4) AOd. R, R + R3 (i R3 I R3 + R5 R3 + R4 A(,d. -R3 + R4 + R5 + R1 *R2 -R3 *R4 I + R1 R5.(R3 R4) R1 + R2 R3 + R4) AOd. R, R1 + R2 According to this equation the current is decided by input voltage 1/, if the second part of equation is forced to equal to zero Output Impedance If input voltage L' 0, the output impedance is Z0 = VII, that is J,' +J-+1:. I R5.(R+R) ________ I+R2 4d I+R2 1+J) 4iI --13 -I J_I-RI / +l+Ji J(1A+l+/) i.(i +) i) 4h Because normally the value of open-loop gain and the input impedance of an operational amplifier are relatively large, it can be seen that 1 is relatively unimportant and can be R,1 ignored. Hence, the effect of _!_ is the main error source and the output impedance can be given as
R I
z -R5.(R3+R,) R+R,A -14 R, R3_R2-R1.(R4+R5) R3+R4+R5 I R3.(R+R2) R3 which can be expressed as \ z = R5.(R3 + R4) A,,,, -15 R1 (1+k)2._L When the current source condition R4+ R5 is applied, the output impedance of the circuit is dominated by the open-loop gain AOd and the larger the open-loop gain, the larger the output impedance The output impedance Z0 can reach infinity, when R3R2 -R.(R4 +R5) R+R4 +R5 10 -16 R3.(R1+R2) R3 or 1?2 R4+R5 +Ii+R4+R5.(1+.L -17 l?3 R3) R,) AOd which can be re-expressed as R R4 R5 ( R,+R5'( R2"1 I --= +P where P=il+ i*i1±i.----18 R1 R3 R3) R1) Typically, the open-loop gain AOd is frequency dependent and decreases with increasing frequency. For some types of operational amplifier the product of the open-loop gain AOd and the 1 5 frequency is a constant Thus as frequency increases, the open loop gain decreases and the output impedance decreases.
If the variation of the open-loop gain A,)d with frequency is known, then the equality of Eq 18 can be maintained by dynamically varying one or more of the resistors Ri, R2, R3, R4, R5 via port 16 using controller 18. Dynamic adjustment may, for example, be achieved using digital potentiometers for one or more of the resistors Ri R2, R3, R4, R5 and then controlling the digital potentiometers via port 16 using controller 18.
Transconductance The output current is , V,R, R5 R
I
R5 J I±-.(I+k) AQd v.kl 1 1= J.Q where R5 140d The output current is also affected by the finite open-loop gain of the amplifier The value of is the designed current of the current source Therefore, the current is also R5 affected by the open-loop gain A01 and the closed loop gain k of the operational-amplifier 1 0 Typically, the open-loop gain A(, d is frequency dependent and decreases with increasing frequency For some type of operational amplifier the product of the open-loop gain A(,d and the frequency is a constant. Thus as frequency increases, the open loop gain decreases and Q decreases.
1 5 If the variation of the open-loop gain A,d with frequency is known, then the linearity between Vi and I can be maintained by dynamically adjusting either k or R5 to compensate for the change in the open-loop gain AUd. Dynamic adjustment may, for example, be achieved using digital potentiometers for one or more of the resistors Ri, R2, R5 and then controlling the digital potentiometers via port 16 using controller 18.
Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.
The VCCS may have external pins for connection to an external analogue or digital resistor network including potentiometers for fine adjustment of the value of one of the resistors Ri, R2, R3, R4 and R5 The VCCS may have an externat pin for a control line of an internal digital potentiometer that has been integrated on same chip to adjust the value of one of the resistors RI, R2, R3, R4 and R5.
The VCCS may have an external pin for a control line of an external digital potentiometer to adjust the value of one of the resistors Ri, R2, R3, R4 and R5.
Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant 1 0 claims protection in respect of any patentable feature or combination of features hereinbefore referred to andlor shown in the drawings whether or not particular emphasis has been placed thereon.
I/we claim:

Claims (1)

1. A voltage controlled current source comprising: a voltage input node for receiving an input voltage, a current output node for providing a constant current to a load; an operational amplifier having a gain and comprising a first amplifier input node, a second amplifier input node and an amplifier output node, a first resistor connected between the voltage input node and the second amplifier input node; a second resistor connected between the amplifier output node and the second amplifier input node; a third resistor connected between a reference voltage and the first amplifier input node; a fourth resistor connected between the current output node and the first amplifier input node; a fifth resistor connected between the amplifier output node and the current output node, wherein one or more of the first, second, third, fourth or fifth resistors is a variable resistor; and 1 5 compensation means, configured to compensate for frequency dependent variation of the amplifier gain by controlling the value of one or more of the first, second, third, fourth or fifth resistors.
2. A voltage controlled current source as claimed in claim 1, wherein the first, second, third, fourth and fifth resistors have respective resistances Ri, R2, R3, R4 and R5 and wherein the compensation means is arranged to maintain an output impedance over a frequency range by maintaining R2/R1 equal to [(R4 +R5)/R3] + P where P is a parameter dependent upon the frequency dependent variation of the amplifier gain.
3. A voltage controlled current source as claimed in claim 1 or 2, wherein the compensation means is arranged to maintain an output impedance exceeding 1 5OMc over frequency range 0-100MHz and up to 2Mc = at frequency bandwidth 0-30MHz with up to 2k loading.
4 A voltage controlled current source as claimed in claim 1, 2 or 3, wherein the first, second, third, fourth and fifth resistors have respective resistances Ri, R2, R3, R4 and R5 and wherein the compensation means is arranged to maintain a transconductance of the voltage controlled current source over a frequency range by maintaining (Q *R2)/(R1*R5) constant, where Q is a parameter dependent upon the frequency dependent variation of the amplifier gain A voltage controlled current source as claimed in claim 4, wherein the frequency range is not less than 5M output impedance with frequency bandwidth 0-30MHz with 2k loading.
6. A voltage controlled current source as claimed in any one of claims 2 to 5, wherein the compensation means is arranged to vary the ratio of the R2 to Ri.
7. A voltage controlled current source as claimed in claims 2 or 6, wherein the compensation means is arranged to vary R5.
8 A voltage controlled current source comprising a voltage input node for receiving an input voltage; a current output node for providing a constant current to a load at an output frequency; an operational amplifier having a gain and comprising a first amplifier input node, a second 1 5 amplifier input node and an amplifier output node, a first resistor connected between the voltage input node and the second amplifier input node; a second resistor connected between the amplifier output node and the second amplifier input node; a third resistor connected between a reference voltage and the first amplifier input node; a fourth resistor connected between the current output node and the first amplifier input node; a fifth resistor connected between the amplifier output node and the current output node, wherein one or more of the first, second, third, fourth or fifth resistors is a variable resistor; and compensation means, configured to vary the value of one or more of the first, second, third, fourth or fifth resistors in a manner dependent upon a variation of the output frequency.
9 A voltage controlled current source as claimed in claim 8, wherein the first, second, third, fourth and fifth resistors have respective resistances Ri, R2, R3, R4 and R5 and wherein the compensation means is arranged to maintain R2IR1 equal to [(R4 +R5)fR3] + P where P is a parameter dependent upon the output frequency.
10. A voltage controlled current source as claimed in claim 8 or 9, wherein the compensation means is arranged to maintain an output impedance exceeding 5M at frequency bandwidth 0- 30MHz with ik loading.
11. A voltage controlled current source as claimed in claim 8, 9 or 10, wherein the first, second, third, fourth and fifth resistors have respective resistances Ri, R2, R3, R4 and R5 and wherein the compensation means is arranged to maintain (0 *R2)/(R1*R5) constant, where 0 is a parameter dependent upon the output frequency 12. A voltage controlled current source as claimed in claim 11, wherein the frequency range is at least 0-30MHz with 2kQ and 0-50MHz with lkQ loading 13 A voltage controlled current source as claimed in any one of claims 9 to 12, wherein the 1 0 compensation means is arranged to vary the ratio of the R2 to Ri.
14. A voltage controlled current source as claimed in claims 9 to 13, wherein the compensation means is arranged to vary R5.
1 5 15. A method for controlling a voltage controlled current source that is operable to provide a constant current at any one of a plurality of output frequencies wherein the voltage controlled current source comprises.
a voltage input node for receiving an input voltage, a current output node for providing a constant current to a load; an operational amplifier having a gain and comprising a first amplifier input node, a second amplifier input node and an amplifier output node; a first resistor connected between the voltage input node and the second amplifier input node; a second resistor connected between the amplifier output node and the second amplifier input node; a third resistor connected between a reference voltage and the first amplifier input node; a fourth resistor connected between the current output node and the first amplifier input node; a fifth resistor connected between the amplifier output node and the current output node; wherein one or more of the first, second, third, fourth or fifth resistors is a variable resistor; the method comprising dynamically adjusting a value of one or more of the first, second, third, fourth or fifth resistors as the output frequency varies 16 A method for controlling a voltage controlled current source that is operable to provide a constant current at any one of a plurality of output frequencies wherein the voltage controlled current source comprises.
a voltage input node for receiving an input voltage, a current output node for providing a constant current to a load, an operational amplifier having a gain and comprising a first amplifier input node, a second amplifier input node and an amplifier output node, a first resistor connected between the voltage input node and the second amplifier input node; a second resistor connected between the amplifier output node and the second amplifier input node; a third resistor connected between a reference voltage and the first amplifier input node; a fourth resistor connected between the current output node and the first amplifier input node; a fifth resistor connected between the amplifier output node and the current output node; 1 0 wherein one or more of the first, second, third, fourth or fifth resistors is a variable resistor; the method comprising detecting a variation in the output frequency; and dynamically adjusting a value of one or more of the first, second, third, fourth or fifth resistors in response to the detected variation 17. A method of manufacturing a voltage controlled current source comprising a voltage input node for receiving an input voltage; a current output node for providing a constant current to a load; an operational amplifier having a gain and comprising a first amplifier input node, a second amplifier input node and an amplifier output node, and resistors, the method controlling the output impedance of the voltage controlled current source by controlling the tolerance of one or more of the resistors
GB0710950A 2007-06-07 2007-06-07 A high-frequency current source for Electrical impedance Tomography (EIT), with compensation for amplifier gain error Withdrawn GB2449904A (en)

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US10117624B2 (en) 2009-11-20 2018-11-06 Wei Wang Electrical impedance detection and ultrasound scanning of body tissue
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