GB2449276A - A low-capacitance transmit-receive switch for an EIT electrode - Google Patents
A low-capacitance transmit-receive switch for an EIT electrode Download PDFInfo
- Publication number
- GB2449276A GB2449276A GB0709399A GB0709399A GB2449276A GB 2449276 A GB2449276 A GB 2449276A GB 0709399 A GB0709399 A GB 0709399A GB 0709399 A GB0709399 A GB 0709399A GB 2449276 A GB2449276 A GB 2449276A
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- GB
- United Kingdom
- Prior art keywords
- switches
- signal
- pad
- guard
- transmitter
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- 230000000694 effects Effects 0.000 claims abstract description 10
- 238000002955 isolation Methods 0.000 claims abstract description 10
- 230000003071 parasitic effect Effects 0.000 claims description 3
- 230000002542 deteriorative effect Effects 0.000 claims 1
- 238000002593 electrical impedance tomography Methods 0.000 abstract description 3
- 239000004065 semiconductor Substances 0.000 description 3
- 230000008878 coupling Effects 0.000 description 2
- 238000010168 coupling process Methods 0.000 description 2
- 238000005859 coupling reaction Methods 0.000 description 2
- 230000009977 dual effect Effects 0.000 description 2
- 238000000034 method Methods 0.000 description 2
- 230000001360 synchronised effect Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 1
- 238000002847 impedance measurement Methods 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/002—Switching arrangements with several input- or output terminals
-
- 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
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/51—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used
- H03K17/56—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices
- H03K17/687—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices the devices being field-effect transistors
- H03K17/693—Switching arrangements with several input- or output-terminals, e.g. multiplexers, distributors
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- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
- Biomedical Technology (AREA)
- Medical Informatics (AREA)
- Biophysics (AREA)
- Pathology (AREA)
- Engineering & Computer Science (AREA)
- Radiology & Medical Imaging (AREA)
- Heart & Thoracic Surgery (AREA)
- Physics & Mathematics (AREA)
- Molecular Biology (AREA)
- Surgery (AREA)
- Animal Behavior & Ethology (AREA)
- General Health & Medical Sciences (AREA)
- Public Health (AREA)
- Veterinary Medicine (AREA)
- Amplifiers (AREA)
Abstract
A sensor pad 6 for electrical impedance tomography (EIT) is connected to either a transmitter or a receiver by a switch arrangement which provides high isolation and very low capacitive loading of the sensor pad. The guard pad 6g is driven by a buffered version of the sensor pad signal so that the effect of stray capacitance to the guard pad is minimized. The guard pad drive signal is also coupled to the supply rails of the switches (figure 10), the supply rails of the switch drivers, and the reference signal inputs of the shunt switches SW2, SW5. This reduces the effect of stray capacitance from the sensor pad to the supply rails because the supply rails follow the sensor signal. The additional switches SW7-SW10 increase isolation. The guard pad and supply rail driving signals are generated in the transmitter (figure 6) and receiver, and are selected by a multiplexer switch (53, figure 5).
Description
Improved Multiplexer Switching for Multiple Use of a Sensor Pad
Background
Semiconductor and hardware switches are commonly used to address a signal line so that it can have multifunctional use. For example, an antenna can be switched between being used as an RE transmitter or receiver or by multiple transmitters or receivers. Optimally the switching needs to minimally load any transmitter output or any receiver input and to show good isolation between the functional uses, i.e. so that the signals from one transmitter or receiver does not get to another transmitter or receiver.
One such use is in electrical Impedance Tomography, Eli, a technique applied to determine the distribution of impedance of a space. Eli generally uses contact sensors and six or more plates to cylindrically or spherically enclose the space so that the boundary conditions of the space are fixed. One or more plates act as voltage or current transmitter and the remaining plates are scanned and used as a voltage or current receivers. Typically, the selection of which plates are used as transmitters or receivers or as a pad whose impedance is to be measured is done with semiconductor switches. These route the transmitter signals from the transmitting oscillators to the selected plates and the received signals to the receiver circuits.
Structure of a single CMOS semiconductor switch Figure 1 shows the structure of a single switch with input/output lines 1 and 3, positive and negative power supply lines 2 and 4 and is the control line 5 to make the switch open and close. There will be approximately 7pF total capacitance to the power supply lines from each of lines 1 and 3 and approximately 0.3pF feed-through capacitance across an open switch.
State of art dual T arrangement of switches The switches are often used in a dual T configuration with two open series switches leaving a line unused for carrying a signal and a closed parallel switch connecting it to OV in order to better isolate the transmitter and receiver signals. Figure 2 shows such a switching circuit to change the function of a sensor pad 6 from transmitter to receiver. It uses a T arrangement of switches in the top path series switches SW 1,3 and parallel switch SW 2 and a T arrangement of switches in the bottom path series switches SW 4,6 and parallel switch SW 5. One of the pair of switches in series SW 1,3 or SW 4,6 will be closed if the function transmitter or receiver respectively is chosen. Lines 8 and 10 are connected to OV and are used to connect to for improved isolation of an unused line. If one of the pair of switches is open then one of the switches in parallel SW2 or SW5 will be closed connecting the unused line to OV. This is done in order to stop the feed-through of unwanted signals across the approximate O.3pF feed-through capacitance of an open series switch by.
Figure 2 illustrates the case where the function of a sensor pad 6 is chosen as transmitter. In the top T arrangement from the transmitter 7 SW1 and SW3 are closed so that the sensor pad is connected to the transmitter 7. SW2 is open so that the sensor pad is not connected to OV on line 8. In the bottom T arrangement to the receiver 9 the switches SW4 and SW6 are open so that the sensor pad is not connected to the receiver 9. The switch SW5 is closed connecting this signal line to a known isolating signal 10, in this case OV so that the transmitter signal does not get to the receiver 9 across the 0.3 pF feed-through capacitance across SW4.
The control lines are not shown to the switches. Neither is any level shifting necessary to correctly interface to the switches from an assumed micro controller.
Capacitances to directing guard plates For high impedance measurement, a directing guard plate, driven by an amplifier of gain near one so that it follows the sensor pad voltage, will be used behind and/or around the sensor pad to make the sensor pad more sensitive in one direction. This adds parallel standing capacitance to the sensor pad that may be sensitive to temperature and vary from pad to pad.
Statement of Invention
This invention is an improved circuit arrangement of switches and other circuitry allowing a sensor pad to have multifunctional use while minimising stray capacitance effects of signal lines to power supply rails and guard planes and increasing isolation between possible functions. The stray capacitance effects of signal lines to power supply rails in all parts of the circuit are minimised by AC driving the power rails of the circuitry through amplifiers with level shifted output voltages that AC track the signals carried in that part of the circuit. The circuit has increased isolation as additional parallel to signal line switches close to connect other wise floating unused lines to appropriate isolating signals and additional series to signal line switches open so that an unused signal line is not connected toward the sensor pad part of the circuit.
For simplicity, only two active signal line circuits will be illustrated: one transmitter and one receiver circuit and the term "active signal line circuit" will be coined as a generic term for any circuit such as a transmitter or receiver. The appropriate AC rail driving and isolating signals are supplied in the transmitter circuit and in the receiver circuit, called TxGuardOut and RxGuardOut respectively. The chosen transmitter or receiver signal is not loaded when it passing through switches on its path to or from the sensor pad as the power supply lines of the switches are made to follow in AC voltage terms the transmitter or receiver signal using follower amplifiers or similar circuitry with level shifted output voltages so that no AC difference exists to signals being switched.
For a constant current transmitter TxGuardOut will be the output from a high input impedance amplifier of gain near one following the voltage of the current output. For a voltage transmitter TxGuardOut will be the output from a high input impedance amplifier following the voltage output.
The transmitter signal thus is not loaded by its signal passing through switches on its path to the sensor pad as the power supply lines of the switches all follow in AC voltage the receiver signal TxGuardOut.
For a receiver RxGuardOut will be the output from an high input impedance amplifier of gain near one following the virtual earth input, for an inverting current receiver, or following the voltage input, for a voltage receiver.
The receiver signal thus is not loaded by its signal passing through switches on its path to the sensor pad as the power supply lines of the switches all follow in AC voltage the receiver signal RxGuardOut.
Additionally: 1. Any guard plate is connected to TxGuardOut if the sensor pad is chosen as transmitter or to RxGuardOut if the sensor pad is chosen as a receiver.
2. Another pair of series and parallel switches with appropriate switching of their power supply lines to TxGuardOut or RxGuardOut is added in order to optimise the isolation between transmitter and receiver. These switches connect unused lines not passing signals to either TxGuardOut or RxGuardOut, instead of OV as in existing EIT circuits, so that there is no effect through the approximate O.3pF feed-through capacitance of the two open series switches connected to the line.
In the following figures for simplicity the drive for switch control lines are not shown.
Level shifting may be necessary to correctly intertace these control lines to an assumed micro controller. Ideally, every control line should be following in an AC manner signal lines in the switch that it is controlling. One method of doing this is to use a non-inverting buffer amplifier of gain one with its power lines driven to follow in an AC manner the signal lines in the switch that it is controlling. Another is by supplying DC rail current through resistors that can be capacitively driven by TxGuardOut or RxGuardOut If many transmitter or receiver circuits are used then each one must provide an appropriate AC rail driving signal and level shifted outputs for the two rails of the adjacent series switch and an isolating signal that goes to one of the switch input/output pins of the local parallel connecting switch used for isolation. Other parts of the circuitry will also use AC signals equal to these determined if this function is chosen for the sensor pad.
Implementation as transmitter Figures 3 shows how this is implemented for a circuit that functions as a transmitter.
Additional switching not shown connects the power supply rails to the appropriate AC signal.
TxGuardOut 11 is the appropriate AC signal for all switches SW 1, 2,3,4,5,6,7,8 guard plane 6g and isolating lines 8,10,13 not in the dotted rectangle 15.
RxGuardOut 12 is the appropriate AC signal for the power supply rails of switches SW 9,10 and the isolating line 14 inside the dotted rectangle 15.
Implementation as receiver Figures 4 shows how this is implemented for a circuit that functions as a receiver.
Additional switching not shown connects the power supply rails to the appropriate AC signal.
RxGuardOut is the appropriate AC signal for all switches SW 1,2,3,4,5,6,9, 10 and isolating lines 3,4,7 not in the dotted rectangle 9.
TxGuardOut 11 is the appropriate AC signal for all switches SW 7,8 and the isolating line 6 inside the dotted rectangle 9.
Possible Transmitters The transmitters can be voltage or current transmitters. There can be multiple possible transmitters selected with switches. Figure 5 shows such an arrangement where the transmitter output signal 18 is selected by a multiplexer 19 from a current transmitter 16 or voltage transmitter 17.
The transmitter needs a high input impedance amplifier of gain near one to provide an output signal for the isolating signal lines. Figure 6 shows such an arrangement 7. Amplifier 21 is a current or voltage transmitter or the configuration 20 shown in Figure 5. A high input impedance amplifier 23 is used to generate a signal that follows in voltage the output 18. The output 11 of this amplifier 22 is a voltage, earlier called TxGuardOut 11, following the transmitter output 18. Typically, synchronous demodulators with DC output 24 will be used to measure the voltage drive for a current transmitter or the current injected for a voltage transmitter. This allows the complex impedance of a single pad to be measured.
Power rail driving circuit Figure 7 shows a power rail driving circuit. It consists of an AC follower amplifier 29 with circuits 26, 27 giving DC offset average output levels 25, 30 needed to drive the vdd and vss rails of the switches. One output 25 is typically +10 the other output 30 is typically +5v. Either TxGuardOut 11 or RxGuardOut 12 will be connected to the input 28.
To implement the invention three of these circuits are needed. One will be permanently driven at its input by RxGuardOut and will drive the power lines of the switches SW 9,10 in the dotted rectangle 15 of Figure 3. One will be permanently driven at its input by TxGuardOut and will drive the power lines of the switches SW 9,10 in the dotted rectangle 9 of Figure 4. The third power rail driving circuit will have a 2 way multiplexer with the common line connected to the input 28. The common line thus can be chosen to be either TxGuardOut 11 or RxGuardOut 12. The common line will also be connected to isolating signal lines 3 and 4 so that when the correct drive for the rails is chosen the correct isolating signals is at 3 and 4.
Possible Receivers The receivers can be voltage or current receivers or there can be a choice of multiple possible receivers selected with a multiplexer. The power rails of the multiplexer will need to be driven to follow RxGuardOut.
Voltage receivers Figure 8 shows a high input impedance amplifier 31 used to receive voltage 34.
Typically, a synchronous demodulator with filter 32 follows so that the average complex received voltage is measured and output 33. A low output impedance buffer may be needed to provide RxGuardOut 36.
Current receivers Figure 9 shows a block diagram of an inverting amplifier 37 with feedback 42 used to receive current at 43 and turn it into a voltage output. A demodulator with filter 38 follows it so that the average received current is measured and output 39. A low output impedance buffer 41 may be needed to provide a signal for RxGuardOut at or often OV will be acceptable for RxGuardOut since input 43 is a virtual earth input. Due to the finite gain of the inverting amplifier there will be some deviation from the AC signal on the rails and the virtual earth input. Putting a guard amplifier in front of the inverter but in the feedback loop and using the guard output as RxGuardOut to the circuitry driving the rails of the switches improves the cancellation of parasitic capacitances.
Complete single switch with driving circuitry Figure 10 shows a complete single switch 45 with driving circuitry. The switch 45 has its control line driven by a buffer amplifier 46. The vdd power lines of the buffer 49 and of the switch 46 are driven from the level shifted outputs 45, 47 of the power rail driver 51. Mux 51 selects either TxGuardOut 54 or RxGuardOut 55 to use for the input 52 of the power rail driver 51.
Typical applications of this invention This invention relates to the switching of the driving of the power lines of switches connected to a sensor pad in the best way so that: 1. Any pad can be used as a voltage transmitter and the current transmitted measured and the impedance of the pad calculated.
2. Any pad can be used as a current transmitter and the voltage transmitted measured and the impedance of the pad calculated.
3. Any pad can be used or a current receiver.
4. Any pad can be used or a voltage receiver.
5. Multiple pads can be chosen as transmitters and thus as this will effect the current transmitted or voltage measured, the transmitters are also acting as receivers.
6. Very smafi coupling or single pad impedances can be measured.
7. Very small impedances can exist from a transmitter to a receiver pad, i.e. <1fF = transmit/receive coupling capacitance, and still a useful signal can be received.
8. The feed-through from transmitting signal to the receiving signal line and receiver amplifier is made small by choosing the correct shorting signal.
Options: 1. Different Transmitters a. Oscillator voltage output where the output current is not measured.
b. Oscillator current output where the output voltage is not measured.
c. Oscillator driving a resistor as the current source.
d. Multiple simultaneous current outputs with one guard amplifier on different frequencies.
2. Different Receivers a. Inverter receiver with feedback.
b. Inverter receiver with guard amplifier in feedback loop.
c. Emitter coupled drive to a transistor.
d. For low voltages base coupled drive to a transistor.
e. Use of any receiver with multiple simultaneous demodulations at different frequencies.
Claims (10)
- Claims 1. A device comprising a sensor pad, a guard plate, a set ofswitches, multiplexers for choosing one signal out of many, follower amplifiers, high input impedance guard follower amplifiers, follower amplifiers with two output levels and multiple functional active signal line circuits, such as transmitters and receivers, able to choose the function of the pad with the switches and minimise with the use of bootstrapping the parasitic impedance effects to power supply lines and the guard plane and effects of feed-through capacitance deteriorating the isolation function of open switches.
- 2. A device according to claim 1 where maximum isolation to other signatis accomplished by opening signal path connections to other signal lines and then connecting the floating line to a signal that can function to bootstrap effects of feed-through capacitances across open switches.
- 3. A device according to claim 1 where the active signal line circuits have guard tollower amplifiers that supply an output signal that can be used to minimise by bootstrapping parasitic impedance effects to the power supply lines and to guard planes in other parts of the circuit.
- 4. A device according to claim 1 where the power rails of the switches with one input/output pin connected to the sensor pad are driven by follower amplifiers with two output levels that follow in an AC manner the functional signal chosen so that any stray capacitance to the power rails is minimised by bootstrapping.
- 5. A device according to claim 1 where the power rails of the switches one with one input/output pin connected to an active signal line circuit are driven by follower amplifiers with two output levels that follow in an AC manner the active signal line circuit so that any stray capacitance to the power rails is minimised by bootstrapping.
- 6. A device according to claim 1 where the use of follower amplifiers with two output levels is used to drive the power lines of all the switches,
- 7. A device according to claim 1 where the multiple functional circuits are one transmitter and one receiver.
- 8. A device according to claim 1 where the multiple functional circuits are all different transmitters.
- 9. A device according to claim 1 where the multiple functional circuits are all different receivers.
- 10. A device according to claim 2 where the control lines of the switches are driven by follower amplifiers with their power supply lines driven in an AC manner the same as the power supply lines of the switches so that any stray capacitance to the control lines is minimised by bootstrapping.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0709399A GB2449276A (en) | 2007-05-15 | 2007-05-15 | A low-capacitance transmit-receive switch for an EIT electrode |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0709399A GB2449276A (en) | 2007-05-15 | 2007-05-15 | A low-capacitance transmit-receive switch for an EIT electrode |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB0709399D0 GB0709399D0 (en) | 2007-06-27 |
| GB2449276A true GB2449276A (en) | 2008-11-19 |
Family
ID=38234539
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0709399A Withdrawn GB2449276A (en) | 2007-05-15 | 2007-05-15 | A low-capacitance transmit-receive switch for an EIT electrode |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2449276A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2018151948A1 (en) * | 2017-02-15 | 2018-08-23 | Qualcomm Incorporated | Programmable pad capacitance for supporting bidirectional signaling from unterminated endpoints |
| RU220315U1 (en) * | 2022-07-14 | 2023-09-07 | Олег Игоревич Сазонов | Multichannel bioimpedance signal generator-multiplexer |
Citations (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1247880A (en) * | 1967-10-30 | 1971-09-29 | Ibm | Leakage controlled electric charge switching and storing circuitry |
| US4544854A (en) * | 1983-08-04 | 1985-10-01 | Motorola, Inc. | Analog switch structure having low leakage current |
| US5384570A (en) * | 1992-09-08 | 1995-01-24 | Fujitsu Limited | Voltage storage circuits |
| US5544662A (en) * | 1991-07-09 | 1996-08-13 | Rensselaer Polytechnic Institute | High-speed electric tomography |
| US5588429A (en) * | 1991-07-09 | 1996-12-31 | Rensselaer Polytechnic Institute | Process for producing optimal current patterns for electrical impedance tomography |
| US20050020935A1 (en) * | 2001-11-20 | 2005-01-27 | Thomas Helzel | Electrode for biomedical measurements |
| US6978170B1 (en) * | 1999-10-15 | 2005-12-20 | Kao Corporation | Body fat measuring method and device therefor |
-
2007
- 2007-05-15 GB GB0709399A patent/GB2449276A/en not_active Withdrawn
Patent Citations (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1247880A (en) * | 1967-10-30 | 1971-09-29 | Ibm | Leakage controlled electric charge switching and storing circuitry |
| US4544854A (en) * | 1983-08-04 | 1985-10-01 | Motorola, Inc. | Analog switch structure having low leakage current |
| US5544662A (en) * | 1991-07-09 | 1996-08-13 | Rensselaer Polytechnic Institute | High-speed electric tomography |
| US5588429A (en) * | 1991-07-09 | 1996-12-31 | Rensselaer Polytechnic Institute | Process for producing optimal current patterns for electrical impedance tomography |
| US5384570A (en) * | 1992-09-08 | 1995-01-24 | Fujitsu Limited | Voltage storage circuits |
| US6978170B1 (en) * | 1999-10-15 | 2005-12-20 | Kao Corporation | Body fat measuring method and device therefor |
| US20050020935A1 (en) * | 2001-11-20 | 2005-01-27 | Thomas Helzel | Electrode for biomedical measurements |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2018151948A1 (en) * | 2017-02-15 | 2018-08-23 | Qualcomm Incorporated | Programmable pad capacitance for supporting bidirectional signaling from unterminated endpoints |
| RU220315U1 (en) * | 2022-07-14 | 2023-09-07 | Олег Игоревич Сазонов | Multichannel bioimpedance signal generator-multiplexer |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0709399D0 (en) | 2007-06-27 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |