Capacitance-to-digital converter utilizing digital feedback and auxiliary DAC

• US 9,385,747 B1
• Filed: 12/18/2014
• Issued: 07/05/2016
• Est. Priority Date: 06/24/2013
• Status: Active Grant

• Current Assignee: Musarubra US LLC (Musarubra US SellCo LLC)
• Original Assignee: Silicon Laboratories Incorporated
• Inventors: Perrott, Michael H., Nervegna, Louis
• Primary Examiner(s): JEANGLAUDE, JEAN BRUNER
• Application Number:
  US14/575,167
• Publication Number:
  US 20160182081A1
• Time in Patent Office: 565 Days
• Field of Search: 341/172, 341/155, 341/144, 341/143
• US Class Current: 1/1
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01N 27/26 by investigating electrochemical variables; by using electrolysis or electrophoresis
• CPC Class Codes: G01R 17/105 for measuring impedance or resistance
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: H03M 3/362 in feedback mode, e.g. by reducing the order of the modulator
• CPC Class Codes: H03M 3/464 Details of the digital/analogue conversion in the feedback path
• CPC Class Codes: H03M 3/476 Non-linear conversion systems
• CPC Class Codes: H03M 3/496 Details of sampling arrangements or methods

Abstract

A capacitance-to-digital converter circuit utilizes a capacitor bridge circuit to sense a difference in capacitance between sense capacitors and fixed capacitors in the bridge circuit. The sense capacitors vary according to a sensed parameter. Auxiliary capacitor digital to analog converters (DACs) are coupled to the capacitor bridge circuit to cancel the sensed difference. An analog to digital converter (ADC) receives a signal generated by the capacitor bridge circuit and the auxiliary capacitor DACs and converts the received signal to a digital signal. A digital accumulator accumulates the ADC output, whose output represents the difference in capacitance between the sense capacitors and the fixed capacitors. The accumulator output is used to control the auxiliary capacitor DACs to offset the difference in capacitance between the sense capacitors and the fixed capacitors. The accumulator output also provides the basis for the capacitance-to-digital circuit output.

Claims

1. A capacitance-to-digital converter circuit comprising:

a capacitor bridge circuit to sense a difference in capacitance between one or more sense capacitors and other capacitors in the bridge circuit;

auxiliary capacitor digital to analog converters (DACs) coupled to the capacitor bridge circuit to reduce a magnitude of an error signal corresponding to the sensed difference;

an analog to digital converter (ADC) coupled to the capacitor bridge circuit and the auxiliary capacitor DACs to convert the error signal whose magnitude is reduced by the auxiliary capacitor DACs to a digital signal;

a digital accumulator coupled to the ADC to accumulate the digital signal from the ADC; and

wherein the auxiliary capacitor DACs are controlled to offset the difference in capacitance between the one or more sense capacitors and the other capacitors in the bridge circuit based on the accumulated digital signal.

2. The capacitance-to-digital converter circuit as recited in claim 1 wherein the one or more sense capacitors are responsive to change in capacitance in accordance with change in a sensed parameter.

3. The capacitance-to-digital converter circuit as recited in claim 1 further comprising a feedback capacitance circuit coupled to the bridge circuit and controlled by the digital signal from the ADC to cancel a residual error caused by incomplete cancellation by the auxiliary capacitor DACs of the difference in capacitance between the sense capacitors and the other capacitors in the bridge circuit.

4. The capacitance-to-digital converter circuit as recited in claim 3 wherein the feedback capacitance circuit comprises a first feedback capacitance selectively coupled to a first node of the capacitor bridge circuit according to a first value of the digital signal and coupled to a second node of the capacitor bridge circuit according to a second value of the digital signal and wherein the feedback capacitance circuit further comprises a second feedback capacitance selectively coupled to the second node of the capacitor bridge circuit according to the first value of the digital signal and coupled to the first node of the capacitor bridge circuit according to the second value of the digital signal.

5. The capacitance-to-digital converter circuit as recited in claim 1 wherein a first of the auxiliary capacitor DACs comprises a first plurality of unit capacitors selectively coupled to a first node or a second node of the capacitor bridge circuit based on the accumulated digital signal and wherein a second of the auxiliary capacitor DACs comprises a second plurality of unit capacitors selectively coupled to the second node or the first node of the capacitor bridge circuit according to the accumulated digital signal.

6. The capacitance-to-digital converter circuit as recited in claim 4 further comprising a digital control circuit to provide control signals to the auxiliary DACs, the digital control circuit-including the digital accumulator, the digital control circuit further including a digital delta sigma modulator coupled to receive the accumulated digital signal from the digital accumulator.

7. The capacitance-to-digital converter circuit as recited in claim 1 wherein the accumulated digital signal corresponds to the difference in capacitance between the one or more sense capacitors and the other capacitors in the bridge circuit.

8. The capacitance-to-digital converter circuit as recited in claim 1 further comprising a digital control circuit including the digital accumulator, and wherein the digital control circuit further comprises fast settling logic coupled to receive the digital signal from the ADC and responsive to a predetermined number of consecutive bits of the digital signal having an equal value, to supply a step value to the digital accumulator to improve settling time of the capacitance-to-digital converter.

9. The capacitance-to-digital converter circuit as recited in claim 1 further comprising a digital control circuit including the digital accumulator, and wherein the digital control circuit further comprises a digital accumulate and dump circuit coupled to the digital accumulator to compute a scaled average of a predetermined number of samples of the digital accumulator and provide the scaled average as an output signal, the output signal corresponding to the difference in capacitance between the one or more sense capacitors and the other capacitors in the bridge circuit.

10. The capacitance-to-digital converter circuit as recited in claim 3 further comprising an analog integrator circuit including an amplifier and switched capacitors coupled to the bridge circuit to rectify and accumulate the error signal generated by the capacitor bridge circuit and the feedback capacitance circuit and the auxiliary capacitor DACs and wherein the analog integrator circuit forms part of the ADC.

11. The capacitance-to-digital converter circuit as recited in claim 10 wherein the ADC corresponds to a delta sigma modulator.

12. The capacitance-to-digital converter circuit as recited in claim 10 wherein the capacitor bridge circuit further comprises a first switch to couple a first node of the capacitor bridge circuit to one of a first and second reference voltages according to a first clock signal and a second switch to couple a second node of the capacitor bridge circuit to one of the second and the first reference voltages according to a second clock signal, the second clock signal being an inverse of the first clock signal, and wherein a third node of the capacitor bridge circuit is coupled to a first input of the amplifier and is coupled to one of the switched capacitors and a fourth node of the capacitor bridge circuit is coupled to a second input of the amplifier and is coupled to another one of the switched capacitors.

13. A method comprising:

sensing a difference in capacitance between one or more sense capacitors and other capacitors in a capacitor bridge circuit;

offsetting the sensed difference using auxiliary capacitor digital-to-analog converters (DACs) coupled to the bridge circuit to generate a remaining sensed difference;

converting the remaining sensed difference to a digital signal using an analog-to-digital converter (ADC);

accumulating the digital signal using a digital accumulator, the accumulated digital signal corresponding to the sensed difference between the one or more sense capacitors and the other capacitors; and

controlling the auxiliary capacitor digital-to-analog converters (DACs) based on the accumulated digital signal.

14. The method as recited in claim 13 further comprising canceling a difference in capacitance between the sense capacitors and the other capacitors in the bridge circuit using the auxiliary capacitor DACs.

15. The method as recited in claim 14 further comprising;

controlling a feedback capacitor circuit coupled to the bridge circuit using the digital signal to cancel a residual error caused by incomplete cancellation by the auxiliary capacitor DACs of the difference in capacitance between the one or more sense capacitors and the other capacitors in the bridge circuit.

16. The method as recited in claim 14 further comprising using a digital delta sigma modulator coupled to the digital accumulator to generate signals for use in controlling the auxiliary capacitor DACs.

17. The method as recited in claim 13 further comprising generating a scaled average of a predetermined number of samples based on the digital accumulator and providing the scaled average as an output signal corresponding to the difference in capacitance between the one or more sense capacitors and the other capacitors in the bridge circuit.

18. The method as recited in claim 13 further comprising accumulating the signal supplied from the bridge circuit as offset by the auxiliary capacitor DACs using an analog integrator circuit of the ADC, the analog integrator circuit including an amplifier and switched capacitors coupled to the bridge circuit.

19. A capacitance-to-digital converter comprising:

a capacitor bridge circuit including one or more first capacitors that vary according to a sensed parameter and second capacitors, the bridge circuit creating an error signal corresponding to a difference in capacitance between the one or more first capacitors and the second capacitors;

one or more auxiliary capacitor digital-to-analog converters (DACs) coupled to the capacitor bridge circuit to offset the difference in capacitance sensed by the capacitor bridge circuit;

an analog-to-digital converter (ADC) coupled to the capacitor bridge circuit to supply a digital signal corresponding to the difference in capacitance sensed by the capacitor bridge circuit as offset by the one or more auxiliary capacitor DACs;

a feedback capacitance coupled to the digital signal from the ADC to the capacitor bridge circuit to cancel a residual error caused by incomplete cancellation by the one or more auxiliary capacitor DACs of the difference in capacitance between the one or more first capacitors and the second capacitors in the bridge circuit;

a digital feedback circuit that includes a digital accumulator to accumulate the difference in capacitance sensed by the capacitor bridge circuit as offset by the one or more auxiliary capacitor DACs and supply an accumulated signal indicative thereof, the accumulated signal corresponding to the difference in capacitance between the one or more first capacitors and the second capacitors; and

wherein the digital feedback circuit is further configured to control the one or more auxiliary capacitor DACs to offset the difference in capacitance between the one or more first capacitors and the second capacitors based on the accumulated signal; and

wherein the digital feedback circuit is further configured to supply an output of the capacitance-to-digital converter based on the accumulated signal.

20. The capacitance-to-digital converter as recited in claim 19 wherein the digital feedback circuit further comprises a digital delta sigma modulator coupled to the digital accumulator and configured to supply an output for use in controlling the one or more auxiliary capacitor DACs.

21. The capacitance-to-digital converter as recited in claim 19 wherein the digital feedback circuit further comprises:

fast settling logic coupled to receive the digital signal from the ADC and responsive to a predetermined number of consecutive bits of the digital signal having an equal value, to supply a step value to the digital accumulator to improve settling time of the capacitance-to-digital converter; and

a digital accumulate and dump circuit coupled to the digital accumulator to compute a scaled average of a predetermined number of samples of the digital accumulator and provide the scaled average as an output signal of the capacitance-to-digital-converter, the output signal corresponding to the difference in capacitance between the one or more first capacitors and the second capacitors in the bridge circuit.