CHOPPER-STABILIZED ANALOG-TO-DIGITAL CONVERTER

• US 20090079607A1
• Filed: 09/26/2007
• Published: 03/26/2009
• Est. Priority Date: 09/26/2007
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Denison, Timothy J., Anderson, Joel A., Heinks, Michael W.
• Granted Patent: US 7,714,757 B2
• Time in Patent Office: Days
• US Class Current: 341/143
• CPC Class Codes: A61B 5/316 Modalities, i.e. specific diagnostic methods
• CPC Class Codes: A61B 5/33 specially adapted for cooperation with other devices
• 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: A61B 5/7242 using integration
• CPC Class Codes: H03M 3/34 by chopping
• CPC Class Codes: H03M 3/39 Structural details of delta-sigma modulators, e.g. incremental delta-sigma modulators of digital delta-sigma modulators H03M7/3004

Abstract

This disclosure describes a chopper-stabilized sigma-delta analog-to-digital converter (ADC). The ADC is configured to provide accurate output at low frequency with relatively low power. The chopper-stabilized ADC substantially reduces or eliminates noise and offset from an output signal produced by the mixer amplifier. Dynamic limitations, i.e., glitching that result from chopper stabilization at low power are substantially eliminated or reduced through a combination of chopping at low impedance nodes within the mixer amplifier and feedback. The signal path of the ADC 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. In this manner, the chopper-stabilized ADC can be used in a low power system, such as an implantable medical device (IMD), to provide a stable, low-noise output signal.

Claims

1. A chopper-stabilized analog-to-digital converter (ADC) comprising:

a first modulator that modulates an amplitude of an analog input signal at a clock frequency to produce a modulated signal;

a mixer amplifier that amplifies the modulated signal to produce an amplified signal and demodulates the amplified signal at the clock frequency to produce an output signal;

circuitry that converts the output signal into a digital value that approximates the analog input signal;

circuitry that converts the digital value into a reconstructed analog output signal;

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

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

2. The ADC of claim 1, wherein a non-inverting input of the mixer amplifier inputs the modulated signal from the first modulator and an inverting input of the mixer amplifier inputs the modulated output signal from the feedback path.

3. The ADC of claim 1, wherein the mixer amplifier is a differential input mixer amplifier that receives, and 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 reconstructed analog output signal out of phase with one another.

4. The ADC of claim 3, 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.

5. The ADC of claim 1, wherein the mixer amplifier includes:

a third modulator that demodulates the amplified signal to a baseband frequency; and

an integrator that integrates the demodulated signal produced by the third modulator.

6. The ADC 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, and wherein the input signal has a frequency of less than or equal to approximately 1.0 Hz.

7. The ADC of claim 1, wherein the mixer amplifier outputs a differential signal, and the circuitry that converts the output signal produced by the mixer amplifier into the digital value includes:

a differential input comparator that produces a control signal with a first amplitude when an amplitude of the differential signal input to a positive input of the mixer amplifier is greater than an amplitude of the differential signal input to a negative input of the mixer amplifier;

a compensator that suppresses the output signal produced by the mixer amplifier at frequencies lower than a high pass corner frequency; and

an up/down counter that receives the control signal and increases an amplitude of the digital value when the control signal is received with the first amplitude, and decreases the amplitude of the digital value when the control signal is received with the second amplitude.

8. The ADC of claim 1, wherein the circuitry that converts the output signal produced by the mixer amplifier into the digital value includes:

a comparator that produces a control signal with a first amplitude when an amplitude of the output signal is greater than a reference voltage and produces the control signal with a second amplitude when the amplitude of the output signal is less than the reference voltage;

a compensator that suppresses the output signal produced by the mixer amplifier at frequencies lower than a high pass corner frequency; and

an up/down counter that receives the control signal and increases an amplitude of the digital value when the control signal is received with the first amplitude, and decreases the amplitude of the digital value when the control signal is received with the second amplitude.

9. An implantable medical device comprising:

a physiological sensor that generates an input signal indicative of a physiological condition; and

a chopper-stabilized analog-to-digital converter (ADC) comprising;

a first modulator that modulates an amplitude of the input signal at a clock frequency to produce a modulated signal;

a mixer amplifier that amplifies the modulated signal to produce an amplified signal and demodulates the amplified signal at the clock frequency to produce an output signal;

circuitry that converts the output signal into a digital value that approximates the analog input signal;

circuitry that converts the digital value into reconstructed analog output signal;

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

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

10. The device of claim 9, wherein a non-inverting input of the mixer amplifier inputs the modulated signal from the first modulator and an inverting input of the mixer amplifier inputs the modulated output signal from the feedback path.

11. The device of claim 9, wherein the mixer amplifier is a differential input mixer amplifier that receives, and 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 reconstructed analog output signal out of phase with one another.

12. The device of claim 11, 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.

13. The device of claim 9, wherein the mixer amplifier includes:

a third modulator that demodulates the amplified signal to a baseband frequency; and

an integrator that integrates the demodulated signal produced by the third modulator.

14. The device of claim 9, further comprising a power source to power the device, 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, and wherein the input signal has a frequency of less than or equal to approximately 1.0 Hz.

15. The device of claim 9, wherein the mixer amplifier outputs a differential signal, and the circuitry that converts the output signal produced by the mixer amplifier into the digital value includes:

a differential input comparator that produces a control signal with a first amplitude when an amplitude of the differential signal input to a positive input of the mixer amplifier is greater than an amplitude of the differential signal input to a negative input of the mixer amplifier;

a compensator that suppresses the output signal produced by the mixer amplifier at frequencies lower than a high pass corner frequency; and

an up/down counter that receives the control signal and increases an amplitude of the digital value when the control signal is received with the first amplitude, and decreases the amplitude of the digital value when the control signal is received with the second amplitude.

16. The device of claim 9, wherein the circuitry that converts the output signal produced by the mixer amplifier into the digital value includes:

a comparator that produces a control signal with a first amplitude when an amplitude of the output signal is greater than a reference voltage and produces the control signal with a second amplitude when the amplitude of the output signal is less than the reference voltage;

a compensator that suppresses the output signal produced by the mixer amplifier at frequencies lower than a high pass corner frequency; and

an up/down counter that receives the control signal and increases an amplitude of the digital value when the control signal is received with the first amplitude, and decreases the amplitude of the digital value when the control signal is received with the second amplitude.

17. The device of claim 9, wherein the sensor includes one of an accelerometer, a pressure sensor, and a voltage sensor.

18. The device of claim 9, wherein the sensor includes one of electrocardiogram (ECG), electromyogram (EMG), or electroencephalogram (EEG) sensor.

19. The device of claim 9, further comprising a processor that controls delivery of a therapy to a patient based on the digital signal from ADC.

20. The device of claim 9, wherein the implantable medical device includes one of a cardiac pacemaker, a cardiac defibrillator, an electrical neurostimulator, and an implantable drug delivery device.

21. A chopper-stabilized analog-to-digital (ADC) converter comprising:

means for modulating an amplitude of an analog input signal at a clock frequency to produce a modulated signal;

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

means for converting the output signal into a digital value that approximates the analog input signal;

means for converting the digital value into a reconstructed analog output signal;

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

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

22. A method comprising:

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

amplifying the modulated signal to produce an amplified signal;

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

converting the output signal into a digital value that approximates the analog input signal;

converting the digital value into a reconstructed analog output signal;

modulating an amplitude of the reconstructed analog output signal at the clock frequency; and

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

23. The method of claim 22, applying the modulated signal to a non-inverting input of a mixer amplifier and applying the modulated feedback signal to an inverting input of the mixer amplifier.

24. The method of claim 22, demodulating the amplified signal comprises demodulating the amplified signal at the clock frequency to produce a differential output signal.