Chopper-stabilized instrumentation amplifier for impedance measurement

• US 7,391,257 B1
• Filed: 01/31/2007
• Issued: 06/24/2008
• Est. Priority Date: 01/31/2007
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Denison, Timothy J., Santa, Wesley A.
• Primary Examiner(s): Choe; Henry
• Application Number:
  US11/700,405
• Time in Patent Office: 510 Days
• Field of Search: 330/9, 330/10, 330/69, 327/124, 327/707
• US Class Current: 330/9
• CPC Class Codes: A61B 2560/0209 adapted for power saving
• CPC Class Codes: A61B 5/0002 Remote monitoring of patients using telemetry, e.g. transmission of vital signals via a communication network A61B5/07 takes precedence; transmission systems for measured values G08C; transmission H04B; transmission of digital information H04L; wireless communication networks H04W
• CPC Class Codes: A61B 5/053 Measuring electrical impedance or conductance of a portion of the body
• CPC Class Codes: A61B 5/103 Detecting, measuring or recording devices for testing the shape, pattern, colour, size or movement of the body or parts thereof, for diagnostic purposes A61B5/08 takes precedence; measuring aids for tailors A41H1/00; measuring instruments specially adapted for dentistry A61C19/04
• CPC Class Codes: A61B 5/30 Input circuits therefor
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7217 of noise originating from a therapeutic or surgical apparatus, e.g. from a pacemaker
• CPC Class Codes: A61B 5/7228 Signal modulation applied to the input signal sent to patient or subject; demodulation to recover the physiological signal modulation H03C
• CPC Class Codes: A61N 1/36521 the parameter being derived from measurement of an electrical impedance
• CPC Class Codes: A61N 1/3702 Physiological parameters A61N1/365 takes precedence; evoked response A61N1/371
• CPC Class Codes: A61N 1/3704 Circuits specially adapted therefor, e.g. for sensitivity control
• CPC Class Codes: A61N 2001/083 Monitoring integrity of contacts, e.g. by impedance measurement
• CPC Class Codes: H03F 1/26 Modifications of amplifiers to reduce influence of noise generated by amplifying elements
• CPC Class Codes: H03F 2200/153 Feedback used to stabilise the amplifier
• CPC Class Codes: H03F 2200/252 Multiple switches coupled in the input circuit of an amplifier are controlled by a circuit, e.g. feedback circuitry being controlling the switch
• CPC Class Codes: H03F 2200/261 Amplifier which being suitable for instrumentation applications
• CPC Class Codes: H03F 2200/271 the DC-isolation amplifier, e.g. chopper amplifier, modulation/demodulation amplifier, uses capacitive isolation means, e.g. capacitors
• CPC Class Codes: H03F 2200/372 Noise reduction and elimination in amplifier
• CPC Class Codes: H03F 2200/375 Circuitry to compensate the offset being present in an amplifier
• CPC Class Codes: H03F 2203/45028 the differential amplifier amplifying transistors are folded cascode coupled transistors
• CPC Class Codes: H03F 2203/45138 : Two or more differential amplifiers in IC-block form are combined, e.g. measuring amplifiers
• CPC Class Codes: H03F 2203/45212 : the differential amplifier being designed to have a reduced offset
• CPC Class Codes: H03F 2203/45264 : the dif amp comprising frequency or phase stabilisation means
• CPC Class Codes: H03F 2203/45396 : the AAC comprising one or more switches
• CPC Class Codes: H03F 2203/45512 : the FBC comprising one or more capacitors, not being switched capacitors, and being coupled between the LC and the IC
• CPC Class Codes: H03F 2203/45534 : the FBC comprising multiple switches and being coupled between the LC and the IC
• CPC Class Codes: H03F 2203/45551 : the IC comprising one or more switched capacitors
• CPC Class Codes: H03F 3/387 : with semiconductor devices only
• CPC Class Codes: H03F 3/45183 : Long tailed pairs H03F3/4521, H03F3/45237 take precedence
• CPC Class Codes: H03F 3/45192 : Folded cascode stages
• CPC Class Codes: H03F 3/45475 : using IC blocks as the active amplifying circuit
• CPC Class Codes: H03F 3/45744 : by offset reduction
• CPC Class Codes: H03F 3/45968 : by offset reduction

Abstract

This disclosure describes a chopper stabilized instrumentation amplifier. The amplifier is configured to achieve stable measurements at low frequency with very low power consumption. The instrumentation amplifier uses a differential architecture and a mixer amplifier to substantially eliminate noise and offset from an output signal produced by the amplifier. Dynamic limitations, i.e., glitching, that result from chopper stabilization at low power are substantially eliminated through a combination of chopping at low impedance nodes within the mixer amplifier and feedback. The signal path of the amplifier operates as a continuous time system, providing minimal aliasing of noise or external signals entering the signal pathway at the chop frequency or its harmonics. The amplifier can be used in a low power system, such as an implantable medical device, to provide a stable, low-noise output signal.

Claims

1. A chopper-stabilized instrumentation amplifier comprising:

an alternating current (ac) source that generates an ac stimulation current at a clock frequency for application to a load;

a mixer amplifier coupled to receive an input signal from the load in response to the stimulation current, wherein the mixer amplifier amplifies the input signal to produce an amplified signal and demodulates the amplified signal at the clock frequency to produce an output signal;

a modulator that modulates an amplitude of the output signal at the clock frequency; and

a feedback path that applies the modulated output signal as a feedback signal to the input signal.

2. The amplifier of claim 1, wherein the mixer amplifier includes a differential input mixer amplifier, and the input signal is a differential input signal, and wherein the feedback path includes a first feedback path branch coupled to a first input of the mixer amplifier and a second feedback path branch coupled to a second input of the mixer amplifier, and wherein the second modulator includes a modulator in the first feedback path branch and a modulator in the second feedback path branch that modulate the amplitude of the output signal out of phase with one another.

3. The amplifier of claim 2, wherein each of the first and second feedback path branches includes a feedback capacitance, each of the first and second inputs of the mixer amplifier is coupled to receive the differential input signal via an input capacitance, and a gain of the mixer amplifier is at least partially dependent on a ratio of the feedback capacitance to the input capacitance.

4. The amplifier of claim 1, wherein the feedback path is a first feedback path, the amplifier further comprising:

an integrator that integrates the output signal;

a second modulator that modulates the integrated output signal at the clock frequency to produce a second feedback signal; and

a second feedback path that applies the second feedback signal to the input signal.

5. The amplifier of claim 4, wherein the mixer amplifier includes a differential input mixer amplifier, and the input signal is a differential input signal, and wherein the second feedback path includes a first feedback path branch coupled to a first input of the mixer amplifier and a second feedback path branch coupled to a second input of the mixer amplifier, and wherein the second modulator includes a modulator in the first feedback path branch and a modulator in the second feedback path branch that modulate the amplitude of the integrated output signal out of phase with one another.

6. The amplifier of claim 5, wherein each of the first and second feedback path branches of the second feedback path includes a feedback capacitance, and wherein the second feedback path is dominant at frequencies lower than a high pass cutoff frequency, and the first feedback path is dominant at frequencies above the high pass cutoff frequency.

7. The amplifier of claim 1, wherein the mixer amplifier includes a differential input mixer amplifier having a first input and a second input, and the input signal is a differential input signal, and wherein each of the first and second inputs of the mixer amplifier is coupled to receive the differential input signal via an input capacitor, the amplifier further comprising a second feedback path including a first feedback path branch that couples an output of the mixer amplifier to the alternating current source via a first switched capacitor, and a second feedback path branch that couples the output of the mixer amplifier to the alternating current source via a second switched capacitor.

8. The amplifier of claim 7, wherein each of the first and second switched capacitors is configured to apply a scaled compensatory charge to a respective one of the input capacitors.

9. The amplifier of claim 1, further comprising a power source to power the amplifier, wherein the power source delivers less than approximately 2.0 microamps of electrical current to the amplifier during operation, and delivers a voltage of less than approximately 2.0 volts to the circuit.

10. The amplifier of claim 1, wherein the mixer amplifier is coupled to the load via first capacitors, and wherein the alternating current source is coupled to the load via second capacitors and resistors.

11. The amplifier of claim 1, wherein the load is a biological tissue load, and the alternating current source and the mixer amplifier are galvanically isolated from the load.

12. The amplifier of claim 1, further comprising a high pass filter coupled between the load and the mixer amplifier.

13. The amplifier of claim 1, wherein the instrumentation amplifier resides within an implantable medical device.

14. The amplifier of claim 1, wherein the implantable medical device includes one of a cardiac pacemaker, a cardiac defibrillator, an electrical neurostimulator, and an implantable drug delivery device.

15. The amplifier of claim 1, wherein the input signal indicates an impedance of the load.

16. The amplifier of claim 1, wherein the ac source comprises a first modulator that modulates first and second voltages at a clock frequency to produce the stimulation current for application to the load.

17. The amplifier of claim 1, wherein the mixer amplifier integrates the demodulated signal to produce the output signal.

18. The amplifier of claim 1, further comprising blanking circuitry that selectively decouples the mixer amplifier from the input signal and selectively disables the modulator and the alternating current source.

38. The amplifier of claim 1, wherein the mixer amplifier includes a differential mixer amplifier, the input signal comprises a differential input signal, and the feedback signal comprises a differential feedback signal.

19. A chopper-stabilized instrumentation amplifier comprising:

means for generating an alternating current (ac) stimulation current at a clock frequency for application to a load, wherein the application of the stimulation current to the load produces an input signal;

means for amplifying the input signal to produce an amplified signal;

means for demodulating the amplified signal at the clock frequency to produce an output signal;

means for modulating an amplitude of the output signal at the clock frequency; and

means for applying the modulated output signal as a feedback signal to the input signal.

40. The amplifier of claim 19, wherein the amplifying means includes a differential mixer amplifier, the input signal comprises a differential input signal, and the feedback signal comprises a differential feedback signal.

20. A method comprising:

generating an alternating current (ac) stimulation current at a clock frequency;

applying the stimulation current to a load to produce an input signal;

amplifying the input signal in a mixer amplifier to produce an amplified signal;

demodulating the amplified signal in the mixer amplifier at the clock frequency to produce an output signal;

modulating an amplitude of the output signal at the clock frequency to produce a feedback signal; and

applying the modulated output signal as a feedback signal to the input signal via a first feedback path.

21. The method of claim 20, wherein the mixer amplifier includes a differential input mixer amplifier, and the input signal is a differential input signal, the method further comprising applying the feedback signal via a first feedback path branch, of the first feedback path, coupled to a first input of the mixer amplifier and a second feedback path branch, of the first feedback path, coupled to a second input of the mixer amplifier, wherein modulating an amplitude of the output signal comprises modulating the amplitude of the output signal in the first feedback path branch and modulating the amplitude of the output signal in the second feedback path branch out of phase with modulation in the first feedback path branch.

22. The method of claim 21, wherein each of the first and second feedback path branches includes a feedback capacitance, each of the first and second inputs of the mixer amplifier is coupled to receive the differential input signal via an input capacitance, and a gain of the mixer amplifier is at least partially dependent on a ratio of the feedback capacitance to the input capacitance.

23. The method of claim 20, wherein the feedback path is a first feedback path, the method further comprising:

integrating the output signal;

modulating the integrated output signal at the clock frequency to produce a second feedback signal; and

applying the second feedback signal to the input signal via a second feedback path.

24. The method of claim 23, wherein the mixer amplifier includes a differential input mixer amplifier, and the input signal is a differential input signal, and wherein the second feedback path includes a first feedback path branch, of the second feedback path, coupled to a first input of the mixer amplifier and a second feedback path branch, of the second feedback path, coupled to a second input of the mixer amplifier, and modulating the integrated output signal comprises modulating the amplitude of the integrated output signal in the first feedback path branch and modulating the amplitude of the integrated output signal in the second feedback path branch out of phase with modulation in the first feedback path branch.

25. The method of claim 24, wherein each of the first and second feedback path branches of the second feedback path includes a feedback capacitance, and wherein the second feedback path is dominant at frequencies lower than a high pass cutoff frequency, and the first feedback path is dominant at frequencies above the high pass cutoff frequency.

26. The method of claim 20, wherein the mixer amplifier includes a differential input mixer amplifier having a first input and a second input, and the input signal is a differential input signal, and wherein each of the first and second inputs of the mixer amplifier is coupled to receive the differential input signal via an input capacitor, the method further comprising applying feedback from an output of the mixer amplifier to the alternating current source via a first switched capacitor, and applying feedback from the output of the mixer amplifier to the alternating current source via a second switched capacitor.

27. The method of claim 26, further comprising applying a scaled compensatory charge to each of the input capacitors via the respective first and second switched capacitors.

28. The method of claim 20, further comprising powering the mixer amplifier with less than approximately 2.0 microamps of electrical current and less than approximately 2.0 volts of electrical voltage during operation.

29. The method of claim 20, further comprising coupling the mixer amplifier to the load via first capacitors, and coupling the alternating current source to the load via second capacitors and resistors.

30. The method of claim 20, wherein the load is a biological tissue load, and the first modulator and the mixer amplifier are galvanically isolated from the load.

31. The method of claim 20, further comprising applying a high pass filter to the input signal prior to amplification of the input signal.

32. The method of claim 20, wherein the mixer amplifier resides within an implantable medical device.

33. The method of claim 20, wherein the implantable medical device includes one of a cardiac pacemaker, a cardiac defibrillator, an electrical neurostimulator, and an implantable drug delivery device.

34. The method of claim 20, further comprising estimating an impedance of the load based on the input signal.

35. The method of claim 20, further comprising integrating the demodulated signal in the mixer amplifier to produce the output signal.

36. The method of claim 20, further comprising selectively decoupling the mixer amplifier from the input signal and selectively disabling the modulator and the alternating current source.

39. The method of claim 20, wherein the mixer amplifier includes a differential mixer amplifier, the input signal comprises a differential input signal, and the feedback signal comprises a differential feedback signal.

37. A biomedical impedance sensing device comprising:

an alternating current (ac) source that generates an ac stimulation current at a clock frequency for application to a biological load;

a mixer amplifier coupled to receive an input signal from the load in response to the stimulation current, wherein the mixer amplifier amplifies the input signal to produce an amplified signal and demodulates the amplified signal at the clock frequency to produce an output signal;

a modulator that modulates an amplitude of the output signal at the clock frequency; and

a feedback path that applies the modulated output signal as a feedback signal to the input signal.

41. The device of claim 37, wherein the mixer amplifier includes a differential mixer amplifier, the input signal comprises a differential input signal, and the feedback signal comprises a differential feedback signal.