Capacitive interface circuit for low power sensor system

• US 20080079444A1
• Filed: 04/24/2007
• Published: 04/03/2008
• Est. Priority Date: 09/28/2006
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Denison, Timothy J.
• Granted Patent: US 8,000,789 B2
• Time in Patent Office: Days
• US Class Current: 324/679
• CPC Class Codes: G01D 5/24 by varying capacitance

Abstract

This disclosure describes a capacitive interface circuit for a low power system. The capacitive interface circuit is configured to achieve very low noise sensing of capacitance-based transducers, such as a micro-electro-mechanical system (MEMS)-based sensor, with high resolution and low power. The capacitive interface circuit uses a differential amplifier and correlated triple sampling (CTS) to substantially eliminate, or at least reduce, kT/C noise, as well as amplifier offset and flicker (1/f) noise, from the output of the amplifier. The capacitive interface circuit may further include an output stage that reduces glitching, i.e., clock transients, in the output signal by allowing transients in the amplifier output to settle. In this manner, the circuit can be used in a low power system to produce a stable, low-noise output.

Claims

1. A capacitive interface circuit comprising:

a differential amplifier that amplifies a sensor signal to produce an output signal;

a sampling capacitor that stores the output signal; and

a correlated sampling circuit that reduces one or more noise components in the output signal.

2. The circuit of claim 1, wherein the differential amplifier comprises a first input coupled to a first capacitive sensor element, a second input coupled to a second capacitive sensor element, and an output coupled to an input of the sampling capacitor.

3. The circuit of claim 2, wherein the correlated tripling sampling circuit couples an output of the sampling capacitor to ground during a first clock phase, couples the first and second inputs to ground during a first portion of the first clock phase, decouples the first and second inputs from ground during a second portion of the first clock phase, decouples the output of the sampling capacitor from ground during a second clock phase, couples the first and second capacitive sensor elements to a reference voltage during the second clock phase, and decouples the first and second capacitive sensor elements from the reference voltage during the first clock phase.

4. The circuit of claim 3, wherein the sampling capacitor receives the one or more noise components during both the second portion of the first clock phase and the second clock phase, and blocks at least a portion of the one of more noise components during the second clock phase to reduce the one or more noise components in the output signal.

5. The circuit of claim 4, wherein the one or more noise components include kT/C noise.

6. The circuit of claim 4, wherein the one or more noise components include at least one of kT/C noise, amplifier offset, and flicker noise.

7. The circuit of claim 1, wherein:

the differential amplifier comprises a first input coupled to a first capacitive sensor element, a second input coupled to a second capacitive sensor element, and an output;

the sampling capacitor has an input coupled to the output of the differential amplifier to receive the output signal, and

the correlated sampling circuit comprises;

a first switch that couples an output of the sampling capacitor to ground during the first clock phase and decouples the output of the sampling capacitor from ground during the second clock phase,

a second switch that couples the first input of the amplifier to ground during the first portion of the first clock phase and decouples the first input of the amplifier from ground during a second portion of the first clock phase,

a third switch that couples the second input of the amplifier to ground during the first portion of the first clock phase and decouples the second input of the amplifier from ground during a second portion of the first clock phase, and

a fourth switch that couples a reference voltage to the first and second capacitive sensor elements during the second clock phase and decouples the reference voltage from the first and second capacitive sensor elements during the first clock phase.

8. The circuit of claim 1, wherein the correlated sampling circuit controls inputs to the differential amplifier such that the differential amplifier amplifies the one or more noise components in at least a portion of a first clock phase, and amplifies the sensor signal with the one or more noise components during a second clock phase.

9. The circuit of claim 8, wherein the sampling capacitor stores the one or more amplified noise components from the differential amplifier in at least a portion of the first clock phase, receives the amplified sensor signal with the one or more amplified noise components from the differential amplifier during the second clock phase, and blocks the one or more amplified noise signals to reduce the one or more noise components in the output signal.

10. The circuit of claim 1, wherein the differential amplifier receives the sensor signal from a pair of capacitive sensor elements, wherein each of the sensor elements has a first capacitor plate coupled to an input of the amplifier and a second capacitor plate coupled to a reference voltage.

11. The circuit of claim 1, wherein the differential amplifier receives the sensor signal from a pair of capacitive sensor elements, wherein each of the sensor elements has a first capacitor plate coupled to an input of the amplifier and a second capacitor plate coupled to ground.

12. The circuit of claim 1, further comprising a ping-pong demodulator that samples and holds an output of the sampling capacitor in first and second output stages on an alternating basis.

13. The circuit of claim 12, wherein each of the output stages holds the output of the sampling capacitor for an amount of time sufficient to permit settling of at least some undesirable transients.

14. The circuit of claim 12, further comprising a feedback path between an output of the ping-pong demodulator and an input to the ping-pong demodulator.

15. The circuit of claim 12, further comprising first and second differential feedback paths, wherein the first differential feedback path is coupled between an output of the ping-pong demodulator and a first input of the differential amplifier, and wherein the second differential feedback path is coupled between an output of the ping-pong demodulator and a second input of the differential amplifier.

16. The circuit of claim 1, wherein the correlated sampling circuit operates at a clock rate of less than approximately 500 Hz.

17. The circuit of claim 1, wherein the correlated sampling circuit operates at a clock rate of less than approximately 300 Hz.

18. The circuit of claim 1, wherein the circuit consumes less than approximately 2 microamps of current.

19. The circuit of claim 1, wherein the circuit consumes less than approximately 1 microamp of current.

20. The circuit of claim 1, wherein the circuit is housed within an implantable medical device.

21. A sensor device comprising:

a sensor;

a differential amplifier that amplifies a sensor signal received from the sensor to produce an output signal;

a sampling capacitor that stores the output signal; and

a correlated sampling circuit that reduces one or more noise components in the output signal.

22. The device of claim 21, wherein the differential amplifier comprises a first input coupled to a first capacitive sensor element, a second input coupled to a second capacitive sensor element, and an output coupled to an input of the sampling capacitor.

23. The device of claim 22, wherein the correlated tripling sampling circuit couples an output of the sampling capacitor to ground during a first clock phase, couples the first and second inputs to ground during a first portion of the first clock phase, decouples the first and second inputs from ground during a second portion of the first clock phase, decouples the output of the sampling capacitor from ground during a second clock phase, couples the first and second capacitive sensor elements to a reference voltage during the second clock phase, and decouples the first and second capacitive sensor elements from the reference voltage during the first clock phase.

24. The device of claim 23, wherein the sampling capacitor receives the one or more noise components during both the second portion of the first clock phase and the second clock phase, and blocks at least a portion of the one of more noise components during the second clock phase to reduce the one or more noise components in the output signal.

25. The device of claim 24, wherein the one or more noise components include at least one of kT/C noise, amplifier offset, and flicker noise.

26. The device of claim 21, wherein:

the differential amplifier comprises a first input coupled to a first capacitive sensor element, a second input coupled to a second capacitive sensor element, and an output;

the sampling capacitor has an input coupled to the output of the differential amplifier to receive the output signal, and

the correlated sampling circuit comprises;

a first switch that couples an output of the sampling capacitor to ground during the first clock phase and decouples the output of the sampling capacitor from ground during the second clock phase,

a second switch that couples the first input of the amplifier to ground during the first portion of the first clock phase and decouples the first input of the amplifier from ground during a second portion of the first clock phase,

a third switch that couples the second input of the amplifier to ground during the first portion of the first clock phase and decouples the second input of the amplifier from ground during a second portion of the first clock phase, and

a fourth switch that couples a reference voltage to the first and second capacitive sensor elements during the second clock phase and decouples the reference voltage from the first and second capacitive sensor elements during the first clock phase.

27. The device of claim 21, wherein the correlated sampling circuit controls inputs to the differential amplifier such that the differential amplifier amplifies the one or more noise components in at least a portion of a first clock phase, and amplifies the sensor signal with the one or more noise components during a second clock phase.

28. The device of claim 27, wherein the sampling capacitor stores the one or more amplified noise components from the differential amplifier in at least a portion of the first clock phase, receives the amplified sensor signal with the one or more amplified noise components from the differential amplifier during the second clock phase, and blocks the one or more amplified noise signals to reduce the one or more noise components in the output signal.

29. The device of claim 21, further comprising a ping-pong demodulator that samples and holds an output of the sampling capacitor in first and second output stages on an alternating basis, wherein each of the output stages holds the output of the sampling capacitor for an amount of time sufficient to permit settling of at least some undesirable transients.

30. The device of claim 21, wherein the correlated sampling circuit operates at a clock rate of less than approximately 500 Hz.

31. The device of claim 21, wherein the circuit consumes less than approximately 2 microamps of current.

32. The device of claim 21, wherein the device is housed within an implantable medical device.

33. The device of claim 21, wherein the sensor includes a capacitance-based sensor.

34. The device of claim 21, wherein the sensor includes at least one of an accelerometer, a pressure sensor, a gyroscope, and a humidity sensor.

35. A method comprising:

amplifying a differential sensor signal to produce an output signal;

storing the output signal on a sampling capacitor; and

applying a correlated sampling process to reduce one or more noise components in the output signal.

36. The method of claim 35, wherein the differential amplifier comprises a first input coupled to a first capacitive sensor element, a second input coupled to a second capacitive sensor element, and an output coupled to an input of the sampling capacitor.

37. The method of claim 36, wherein the correlated sampling process comprises coupling an output of the sampling capacitor during a first clock phase, coupling the first and second inputs to ground during a first portion of the first clock phase, decoupling the first and second inputs from ground during a second portion of the first clock phase, decoupling the output of the sampling capacitor from ground during a second clock phase, coupling the first and second capacitive sensor elements to a reference voltage during the second clock phase, and decoupling the first and second capacitive sensor elements from the reference voltage during the first clock phase.

38. The method of claim 37, further comprising receiving the one or more noise components at the sampling capacitor during both the second portion of the first clock phase and the second clock phase, and blocking at least a portion of the one of more noise components via the sampling capacitor during the second clock phase to reduce the one or more noise components in the output signal.

39. The method of claim 38, wherein the one or more noise components include at least one of kT/C noise, amplifier offset, and flicker noise.

40. The method of claim 35, wherein:

the differential amplifier comprises a first input coupled to a first capacitive sensor element, a second input coupled to a second capacitive sensor element, and an output;

the sampling capacitor has an input coupled to the output of the differential amplifier to receive the output signal, and

the correlated sampling process comprises;

coupling an output of the sampling capacitor to ground during the first clock phase and decoupling the output of the sampling capacitor from ground during the second clock phase,

coupling the first input of the amplifier to ground during the first portion of the first clock phase and decoupling the first input of the amplifier from ground during a second portion of the first clock phase,

coupling the second input of the amplifier to ground during the first portion of the first clock phase and decoupling the second input of the amplifier from ground during a second portion of the first clock phase, and

coupling a reference voltage to the first and second capacitive sensor elements during the second clock phase and decoupling the reference voltage from the first and second capacitive sensor elements during the first clock phase.

41. The method of claim 35, wherein the correlated sampling process comprises controlling inputs to the differential amplifier such that the differential amplifier amplifies the one or more noise components in at least a portion of a first clock phase, and amplifies the sensor signal with the one or more noise components during a second clock phase.

42. The method of claim 41, further comprising storing the one or more amplified noise components from the differential amplifier in the sampling capacitor in at least a portion of the first clock phase, receiving the amplified sensor signal with the one or more amplified noise components from the differential amplifier in the sampling capacitor during the second clock phase, and blocking the one or more amplified noise signals via the sampling capacitor to reduce the one or more noise components in the output signal.

43. The method of claim 35, further comprising receiving the sensor signal from a pair of capacitive sensor elements, wherein each of the sensor elements has a first capacitor plate coupled to an input of the amplifier and a second capacitor plate coupled to a reference voltage.

44. The method of claim 35, further comprising receiving the sensor signal from a pair of capacitive sensor elements, wherein each of the sensor elements has a first capacitor plate coupled to an input of the amplifier and a second capacitor plate coupled to ground.

45. The method of claim 35, further comprising sampling and holding an output of the sampling capacitor in first and second output stages of a ping-pong demodulator on an alternating basis, wherein each of the output stages holds the output of the sampling capacitor for an amount of time sufficient to permit settling of at least some undesirable transients.

46. The method of claim 45, further comprising applying a feedback signal from an output of the ping-pong demodulator to an input to the ping-pong demodulator.

47. The method of claim 45, further comprising applying feedback signals via a first differential feedback path coupled between an output of the ping-pong demodulator and a first input of the differential amplifier, and a second differential feedback path coupled between an output of the ping-pong demodulator and a second input of the differential amplifier.

48. The method of claim 35, further comprising operating the correlated sampling process at a clock rate of less than approximately 500 Hz.

49. The method of claim 35, further comprising operating the correlated sampling process at a clock rate of less than approximately 300 Hz.

50. The method of claim 35, wherein the differential amplifier and the sampling capacitor are housed within an implantable medical device.

51. A circuit comprising:

means for amplifying a differential sensor signal to produce an output signal;

means for storing the output signal on a sampling capacitor, and

means for applying a correlated sampling process to reduces one or more noise components in the output signal.