Capacitance sensing circuit

Abstract

A circuit that senses a process variable, comprises a voltage divider that includes first and second capacitances. At least one of the capacitances is varied by the process variable. Divider ends receive modulated potentials, and a divider center tap coupled to a detector. The detector has a detector output representing carrier frequency range components. A control circuit controls a baseband envelope of the modulated potentials as a function of the detector output.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a circuit that is useful for capacitive sensing of a process variable.

FIG. 2 illustrates an example of baseband and carrier frequency ranges.

FIG. 3 illustrates an example of a clock generator circuit.

FIGS. 4-5, taken together, illustrate an example of a detector circuit.

FIG. 6 illustrates an example of a control circuit that controls a baseband potential.

FIG. 7 illustrates an example of a linearity correction circuit.

FIG. 8 illustrates an example of a scaling circuit and zero adjusting circuit.

FIG. 9 illustrates an example of a span-setting circuit.

FIG. 10 illustrates an example of a regulator circuit.

FIG. 11 illustrates a graph of percentage error as a function of process variable for a pressure sensing circuit at room temperature.

FIG. 12 illustrates a graph of percentage error as a function of process variable for a pressure sensing circuit over an operating temperature range.

Claims

1. A circuit sensing a process variable, comprising: a voltage divider that includes first and second capacitors, at least one of which has a capacitance that is varied by the process variable in a baseband frequency range; the divider having divider ends that receive modulated potentials that have modulation in a carrier frequency range and that have a baseband envelope; the divider having a center tap connection that provides a detector input;a detector with a detector output representative of carrier frequency range components of the detector input; anda control circuit that controls the baseband envelope of the modulated potentials as a function of the detector output, the control circuit providing a process variable output in the baseband frequency range.

2. The circuit of claim 1 wherein the control circuit controls at least one of the modulated potentials as a non-linear function of the detector output, compensating the process variable output for a non-linearity with respect to the process variable.

3. The process variable sensing circuit of claim 1 wherein the envelopes of the modulated potentials have variations at the baseband frequencies that combine with the capacitance variation at baseband frequencies to generate the detector input in a way that tends to rebalance toward a balanced condition when the process variable is stable within the baseband.

4. The process variable sensing circuit of claim 1 wherein the detector input includes baseband noise, and the detector filters out the baseband noise.

5. The process variable sensing circuit of claim 1 wherein the detector comprises: a filter having a filter output and having a bandpass that includes the carrier frequency; anda synchronous demodulator receiving the filter output and demodulating the filter output to provide the detector output.

6. The process variable sensing circuit of claim 5, comprising: a clock generator that generates clock outputs; andthe synchronous demodulator demodulating as a function of at least one of the clock outputs.

7. The process variable sensing circuit of claim 6, comprising: switches controlled by at least one of the clock outputs that modulate the modulated potentials.

8. The process variable sensing circuit of claim 1 wherein the filter comprises a multistage amplifier.

9. The process variable sensing circuit of claim 1 wherein the first capacitor comprises a first electrode deposited on a first substrate, and the second capacitor a second electrode deposited on the first substrate.

10. The process variable sensing circuit of claim 9 wherein the first and second capacitors include electrode connections to the detector input.

11. The process variable sensing circuit of claim 10 wherein the electrode connections to the detector input are free of connections to switches.

12. The process variable sensing circuit of claim 1 wherein the first capacitor comprises a process variable sensing capacitance, and the second capacitor comprises a capacitance that is substantially insensitive to the process variable.

13. The process variable sensing circuit of claim 1 wherein the carrier frequency is a fixed carrier frequency.

14. The process variable sensing circuit of claim 1 wherein the baseband frequencies are in a frequency range of less than 10 hertz, and the carrier frequency is greater than 1 kilohertz.

15. The process variable sensing circuit of claim 14 wherein the process variable sensing capacitance is responsive to baseband process variable variations up to at least 400 hertz, and the process variable output is responsive to process variable variations up to at least 400 hertz.

16. A method of sensing a process variable, comprising: modulating modulated potentials at a carrier frequency;providing the modulated potentials to first and second capacitors that couple to a detector input;varying a capacitance of one of the first and second capacitors by applying a varying process variable at baseband frequencies;detecting, at the carrier frequency, a detected representation of the detector input; andcontrolling baseband envelopes of the modulated potentials as a function of the detected representation.

17. The process variable sensing method of claim 16, and comprising: controlling at least one of the modulated potentials as a non-linear function of the detected representation to compensating the detected representation for a non-linearity of the capacitance as a function of the process variable.

18. The process variable sensing method of claim 16 and comprising: varying the amplitudes of the modulated potentials at the baseband frequencies in order to combine with the capacitance variation at baseband frequencies such that the detector input tends to balance when the process variable is stable within the baseband.

19. The process variable sensing method of claim 16 wherein the detector input includes baseband noise, and filtering out the baseband noise in the detecting.