Chopper-stabilized instrumentation amplifier

• US 7,847,628 B2
• Filed: 05/15/2008
• Issued: 12/07/2010
• 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.
• Primary Examiner(s): CHOE, HENRY
• Application Number:
  US12/121,267
• Publication Number:
  US 20080211574A1
• Time in Patent Office: 936 Days
• Field of Search: 330/9, 330/10, 330/69, 327/124, 327/307
• US Class Current: 330/9
• CPC Class Codes: A61B 5/053 Measuring electrical impedance or conductance of a portion of the body
• CPC Class Codes: A61B 5/24 Detecting, measuring or recording bioelectric or biomagnetic signals of the body or parts thereof
• 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 5/7225 Details of analog processing, e.g. isolation amplifier, gain or sensitivity adjustment, filtering, baseline or drift compensation A61B5/30, A61B5/316 take precedence; transformers for specific applications H01F38/00; amplifiers H03F
• 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: A61B 5/7242 using integration
• CPC Class Codes: H03F 3/38 DC amplifiers with modulator at input and demodulator at output; Modulators or demodulators specially adapted for use in such amplifiers switched capacitor amplifiers H03F3/005

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. An electrical circuit comprising:

a first modulator configured to modulate an amplitude of an input signal at a selected frequency to produce a modulated signal;

an amplifier configured to amplify the modulated signal to produce an amplified signal;

a first demodulator configured to demodulate the amplified signal at the selected frequency to produce an output signal;

an input capacitor coupled between an output of the first modulator and an input of the amplifier;

a second modulator configured to modulate an amplitude of the output signal at the selected frequency to produce a modulated output signal; and

a feedback path coupled between an output of the second modulator and a node between the input capacitor and the input of the amplifier, wherein the feedback path includes a feedback capacitor.

2. The circuit of claim 1, wherein the amplifier is a differential amplifier, the input signal is a differential input signal, the feedback capacitor includes first and second feedback capacitors, the input capacitor includes first and second input capacitors, the input of the amplifier includes first and second inputs, the node includes a first node between the first input capacitor and the first input of the amplifier and a second node between the second input capacitor and the second input of the amplifier, the feedback path includes a first feedback path branch coupled to the first node via the first feedback capacitor and a second feedback path branch coupled to the second node via the second feedback capacitor, and 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 circuit of claim 1, wherein the amplifier and the first demodulator are combined to form a mixer amplifier, and wherein a gain of the amplifier is at least partially dependent on a ratio of a capacitance value of the feedback capacitor to a capacitance value of the input capacitor.

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

an integrator that integrates the output signal;

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

a second feedback path that applies the second feedback signal to the modulated signal at the node between the input capacitor and the input of the amplifier via a second feedback capacitor.

5. The circuit of claim 4, wherein the amplifier is a differential amplifier, the input of the amplifier includes first and second inputs, the input signal is a differential input signal, the second feedback path includes a first feedback path branch coupled to the first input of the amplifier and a second feedback path branch coupled to the second input of the amplifier, and wherein the third 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 circuit of claim 5, wherein first and second feedback path branches of the second feedback path include first and second feedback path branch capacitors, respectively, 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 circuit of claim 1, wherein the amplifier is a differential amplifier, the input signal is a differential input signal, the feedback capacitor includes first and second feedback capacitors, the input capacitor includes first and second input capacitors, the input of the amplifier includes first and second inputs, and the node includes a first node between the first input capacitor and the first input of the amplifier and a second node between the second input capacitor and the second input of the amplifier, the amplifier further comprising a second feedback path including a first feedback path branch that couples an output of the amplifier to a first input of the first modulator via a first switched capacitor, and a second feedback path branch that couples the output of the amplifier to a second input of the first modulator via a second switched capacitor, 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.

8. The circuit of claim 1, wherein the amplifier comprises an integrator that integrates the demodulated signal to produce the output signal.

9. The circuit of claim 1, further comprising a physiological sensor that generates the input signal, wherein the input signal is indicative of a physiological condition, and wherein the physiological sensor includes one of an accelerometer, a pressure sensor, and a voltage sensor.

10. The circuit of claim 1, further comprising a physiological sensor that generates the input signal, wherein the input signal is indicative of a physiological condition, and wherein the physiological sensor includes one of an electrocardiogram (ECG), electromyogram (EMG), or electroencephalogram (EEG) sensor.

11. A method comprising:

modulating an amplitude of an input signal at a selected frequency to produce a modulated input signal;

amplifying the modulated input signal with an amplifier to produce an amplified signal, wherein an input capacitor couples the modulated input signal to an input of the amplifier;

demodulating the amplified signal at the selected frequency to produce an output signal;

modulating an amplitude of the output signal at the selected frequency to produce a modulated output signal; and

applying the modulated output signal to the modulated input signal via a feedback path at a node between the input capacitor and the input of the amplifier via a feedback capacitor.

12. The method of claim 11, wherein the amplifier is a differential amplifier, the input signal is a differential input signal, the feedback capacitor includes first and second feedback capacitors, the input capacitor includes first and second input capacitors, the input of the amplifier includes first and second inputs, and the node includes a first node between the first input capacitor and the first input of the amplifier and a second node between the second input capacitor and the second input of the amplifier, the method further comprising applying the feedback signal via a first feedback path branch and a second feedback path branch of the first feedback path, wherein the first feedback path branch is coupled to the first node via the first feedback capacitor, and the second feedback path branch is coupled to the second node via the second feedback capacitor, and wherein modulating the 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.

13. The method of claim 11, wherein the feedback path is a first feedback path and the feedback capacitor is a first feedback capacitor, the method further comprising:

integrating the output signal;

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

applying the second feedback signal to the modulated signal at the node between the input capacitor and the input of the amplifier via a second feedback capacitor and a second feedback path.

14. The method of claim 13, wherein the amplifier is a differential amplifier, the input signal is a differential input signal, the input of the amplifier includes first and second inputs, the second feedback path includes a first feedback path branch, coupled to the first input of the amplifier and a second feedback path branch coupled to the second input of the 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.

15. The method of claim 14, wherein the first and second feedback path branches of the second feedback path include first and second feedback path branch capacitors, respectively, 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.

16. The method of claim 11, wherein the amplifier is a differential amplifier, the input signal is a differential input signal, the feedback capacitor includes first and second feedback capacitors, the input capacitor includes first and second input capacitors, the input of the amplifier includes first and second inputs of the amplifier, and the node includes a first node between the first input capacitor and the first input of the amplifier and a second node between the second input capacitor and the second input of the amplifier, the method further comprising applying feedback from an output of the amplifier to a first input of the first modulator via a first switched capacitor, applying feedback from the output of the amplifier to a second input of the first modulator via a second switched capacitor, and applying a scaled compensatory charge to each of the input capacitors via the respective first and second switched capacitors.

17. The method of claim 11, further comprising integrating the demodulated signal to produce the output signal.

18. The method of claim 11, further comprising sensing the input signal with a physiological sensor, wherein the sensed input signal is a physiological signal comprising one of an accelerometer signal, a pressure sensor signal, a voltage sensor signal, an electrocardiogram (ECG) signal, electromyogram (EMG) signal, or electroencephalogram (EEG) signal.

19. The method of claim 18, further comprising adjusting a therapy for delivery to a patient based on the output signal.

20. An implantable medical device comprising:

a physiological sensor that senses a physiological signal as an input signal;

a first modulator configured to modulate an amplitude of the input signal at a selected frequency to produce a modulated signal;

an amplifier configured to amplify the modulated signal to produce an amplified signal;

a first demodulator configured to demodulate the amplified signal at the selected frequency to produce an output signal;

an input capacitor coupled between an output of the first modulator and an input of the amplifier;

a second modulator configured to modulate an amplitude of the output signal at the selected frequency to produce a modulated output signal; and

a feedback path coupled between an output of the second modulator and a node between the input capacitor and the input of the amplifier, wherein the feedback path includes a feedback capacitor.

21. The device of claim 20, wherein the sensor is configured to sense one of an electrocardiogram (ECG) signal, electromyogram (EMG) signal, an electroencephalogram (EEG) signal, a cardiac signal, a pressure signal, or a respiratory signal.

22. The device of claim 20, further comprising a therapy delivery module configured to deliver therapy to a patient.

23. The device of claim 20, wherein the amplifier is a differential amplifier, the input signal is a differential input signal, the feedback capacitor includes first and second feedback capacitors, the input capacitor includes first and second input capacitors, the input of the amplifier includes first and second inputs, the node includes a first node between the first input capacitor and the first input of the amplifier and a second node between the second input capacitor and the second input of the amplifier, the feedback path includes a first feedback path branch coupled to the first node via the first feedback capacitor and a second feedback path branch coupled to the second node via the second feedback capacitor, and 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.

24. The device of claim 20, wherein the amplifier and the first demodulator are combined to form a mixer amplifier, and wherein a gain of the amplifier is at least partially dependent on a ratio of a capacitance value of the feedback capacitor to a capacitance value of the input capacitor.

25. The device of claim 20, wherein the feedback path is a first feedback path and the feedback capacitor is a first feedback capacitor, the amplifier further comprising:

an integrator that integrates the output signal;

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

a second feedback path that applies the second feedback signal to the modulated signal at the node between the input capacitor and the input of the amplifier via a second feedback capacitor.

26. The device of claim 25, wherein the amplifier is a differential amplifier, the input of the amplifier includes first and second inputs, the input signal is a differential input signal, the second feedback path includes a first feedback path branch coupled to the first input of the amplifier and a second feedback path branch coupled to the second input of the amplifier, and wherein the third 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.

27. The device of claim 26, wherein first and second feedback path branches of the second feedback path include first and second feedback path branch capacitors, respectively, 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.

28. The circuit of claim 20, wherein the amplifier is a differential amplifier, the input signal is a differential input signal, the feedback capacitor includes first and second feedback capacitors, the input capacitor includes first and second input capacitors, the input of the amplifier includes first and second inputs, and the node includes a first node between the first input capacitor and the first input of the amplifier and a second node between the second input capacitor and the second input of the amplifier, the amplifier further comprising a second feedback path including a first feedback path branch that couples an output of the amplifier to a first input of the first modulator via a first switched capacitor, and a second feedback path branch that couples the output of the amplifier to a second input of the first modulator via a second switched capacitor, 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.

29. An implantable medical device comprising:

means for sensing a physiological signal as an input signal;

means for modulating an amplitude of the input signal at a selected frequency to produce a modulated input signal;

means for amplifying the modulated input signal with an amplifier to produce an amplified signal, wherein an input capacitor couples the modulated input signal to an input of the amplifying means;

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

means for modulating an amplitude of the output signal at the selected frequency to produce a modulated output signal; and

means for applying the modulated output signal to the modulated input signal via a first feedback path at a node between the input capacitor and the input of the amplifying means via a feedback capacitor.

30. The device of claim 29, wherein the means for amplifying includes a differential amplifier, the input signal is a differential input signal, the feedback capacitor includes first and second feedback capacitors, the input capacitor includes first and second input capacitors, the input of the amplifier includes first and second inputs, and the node includes a first node between the first input capacitor and the first input of the amplifier and a second node between the second input capacitor and the second input of the amplifier, the device further comprising means for applying the feedback signal via a first feedback path branch and a second feedback path branch of the first feedback path, wherein the first feedback path branch is coupled to the first node via the first feedback capacitor, and the second feedback path branch is coupled to the second node via the second feedback capacitor, and wherein the means for modulating the amplitude of the output signal comprises means for modulating the amplitude of the output signal in the first feedback path branch and means for modulating the amplitude of the output signal in the second feedback path branch out of phase with modulation in the first feedback path branch.

31. The device of claim 29, wherein the feedback path is a first feedback path and the feedback capacitor is a first feedback capacitor, the device further comprising:

means for integrating the output signal;

means for modulating the integrated output signal at the selected frequency to produce a second feedback signal; and

means for applying the second feedback signal to the modulated signal at the node between the input capacitor and the input of the amplifying means via a second feedback capacitor via a second feedback path.

32. The device of claim 31, wherein the amplifying means includes is a differential amplifier, the input signal is a differential input signal, the input includes first and second inputs, the second feedback path includes a first feedback path branch, of the second feedback path, coupled to the first input of the amplifying means and a second feedback path branch, of the second feedback path, coupled to the second input of the amplifying means, and the means for modulating the integrated output signal comprises means for modulating the amplitude of the integrated output signal in the first feedback path branch and means for 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.

33. The device of claim 29, wherein the amplifying means includes a differential amplifier, the input signal is a differential input signal, the feedback capacitor includes first and second feedback capacitors, the input capacitor includes first and second input capacitors, the input of the amplifying means includes first and second inputs, and the node includes a first node between the first input capacitor and the first input of the amplifier and a second node between the second input capacitor and the second input of the amplifier, the device further comprising means for applying feedback from an output of the amplifier to a first input of the means for modulating an amplitude of the input signal via a first switched capacitor, means for applying feedback from the output of the amplifier to a second input of the means for modulating an amplitude of the input signal via a second switched capacitor, and means for applying a scaled compensatory charge to each of the input capacitors via the respective first and second switched capacitors.

34. The device of claim 29, further comprising means for adjusting a therapy for delivery to a patient based on the output signal.