Capacitance-to-digital modulator with sensor failure-mode detection

• US 7,236,113 B1
• Filed: 01/26/2006
• Issued: 06/26/2007
• Est. Priority Date: 01/26/2006
• Status: Active Grant

• Current Assignee: Fisher-Rosemount Systems Incorporated (Emerson Electric Company)
• Original Assignee: Emerson Process Management LLP (Emerson Electric Company)
• Inventors: Wang, Rongtai
• Primary Examiner(s): Nguyen; Khai M.
• Application Number:
  US11/340,363
• Time in Patent Office: 516 Days
• Field of Search: None
• US Class Current: 341/143
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor
• CPC Class Codes: H03M 3/378 Testing
• CPC Class Codes: H03M 3/43 the quantiser being a single bit one
• CPC Class Codes: H03M 3/438 the modulator having a higher order loop filter in the feedforward path

Abstract

A pressure transmitter having a capacitance-to-digital modulator produces an output as a function of sensor capacitance and a reference capacitance. Transmitter also includes a sensor failure mode detector produces an output signal and identifies failure modes of the sensor capacitance and reference capacitance.

Claims

1. A capacitance-to-digital (CD) modulator for converting a sensor capacitance C_S and a reference capacitance C_R to a pulse code modulated (PCM) signal, the CD modulator comprising:

a first stage sigma-delta integrator for selectively forming charge packets in C_S and C_R during a first phase, and integrating charge from C_S and C_R during a second phase to produce a first stage output V_OUT1, wherein the first stage sigma-delta integrator includes a feedback capacitor for having a capacitance C_F1;

a second stage sigma-delta integrator for sampling the first stage output V_OUT1, and integrating the sampled first stage output to produce a second stage output V_OUT2;

a quantizer for converting the second stage output V_OUT2 to the PCM signal; and

a sensor failure mode detection (SFD) circuit for deriving from V_OUT1 an SFD signal that identifies failure modes of C_S and C_R, wherein the SFD signal is representative of a capacitance ratio C_R/C_F1.

2. The CD modulator of claim 1 and further comprising:

a CD control unit for selectively providing an excitation signal to the sensor capacitance C_S and reference capacitance C_R as a function of the PCM signal.

3. The CD modulator of claim 2, wherein the first stage sigma-delta integrator comprises:

an integrator input node;

an amplifier having a first input, a second input and an output;

an auto-zero capacitor C_Z1 connected between the integrator input node and the first input of the amplifier; and

wherein the feedback capacitor having capacitance C_F1 is connected to the output of the amplifier.

4. The CD modulator of claim 1, wherein the SFD signal is pulse code modulated.

5. The CD modulator of claim 1, wherein the SFD circuit comprises:

an SFD integrator for sampling V_OUT1 and producing an output V_OUT3;

an SFD quantizer for converting V_OUT3 to the SFD signal; and

an SFD control unit for providing an SFD excitation signal to the SFD integrator as a function of the SFD signal.

6. The CD modulator of claim 1, wherein the SFD circuit comprises an over-sampling analog-to-digital converter for sampling V_OUT1.

7. The CD modulator of claim 6, wherein the SFD circuit is active when C_R is connected to the first stage sigma-delta integrator.

8. A pressure transmitter comprising:

a sensor capacitance;

a reference capacitance;

a primary capacitance-to-digital (CD) modulator for producing a pulse code modulated signal PCMP representative of sensed pressure, the primary CD modulator comprising;

a first stage sigma-delta integrator for selectively forming charge packets as a function of sensor and reference capacitances, and integrating the charge packets to produce a first stage output V_OUT1;

a second stage sigma-delta integrator for sampling the first stage output V_OUT1, and integrating the sampled first stage output to produce a second stage output V_OUT2;

a quantizer for converting the second stage output V_OUT2 to the PCMP signal; and

a secondary modulator for sampling V_OUT1 and producing a pulse code modulated signal PCMR indicative of whether the sensor capacitance and the reference capacitance are in a failure mode, wherein the signal PCMR is representative of a capacitance ratio C_R/C_F1, where C_R is the reference capacitance and C_F1 is capacitance of a feedback capacitor of the first stage sigma-delta integrator.

9. The pressure transmitter of claim 8, wherein the first stage sigma-delta integrator comprises:

an integrator input node;

an amplifier having a first input, a second input and an output;

an auto-zero capacitor connected between the common node and the first input of the amplifier; and

wherein the feedback capacitor which is connected to the output of the amplifier.

10. The pressure transmitter of claim 8, wherein the secondary modulator comprises:

an integrator for sampling V_OUT1 and producing an output V_OUT3;

a quantizer for converting V_OUT3 to the signal PCMR; and

a control unit for providing an excitation signal to the integrator as a function of the signal.

11. The pressure transmitter of claim 8, wherein the secondary modulator is active when the reference capacitance is connected to the first stage sigma-delta integrator.

12. The pressure transmitter of claim 8, and further comprising:

a microprocessor for determining whether a failure mode exists based upon the signal PCMR.

13. A capacitance-to-digital (CD) modulator for converting a sensor capacitance C_S and a reference capacitance C_R to a pulse code modulated (PCM) signal, the CD modulator comprising:

a first stage sigma-delta integrator for selectively forming charge packets in C_S and C_R during a first phase, and integrating charge from C_S and C_R during a second phase to produce a first stage output V_OUT1;

a second stage sigma-delta integrator for sampling the first stage output V_OUT1, and integrating the sampled first stage output to produce a second stage output V_OUT2;

a quantizer for converting the second stage output V_OUT2 to the PCM signal; and

a sensor failure mode detection (SFD) circuit for deriving from V_OUT1 an SFD signal that identifies failure modes of C_S and C_R, wherein the SFD circuit comprises;

an SFD integrator for sampling V_OUT1 and producing an output V_OUT3;

an SFD quantizer for converting V_OUT3 to the SFD signal; and

an SFD control unit for providing an SFD excitation signal to the SFD integrator as a function of the SFD signal.

14. The CD modulator of claim 13 and further comprising:

a CD control unit for selectively providing an excitation signal to the sensor capacitance C_S and reference capacitance C_R as a function of the PCM signal.

15. The CD modulator of claim 14, wherein the first stage sigma-delta integrator comprises:

an integrator input node;

an amplifier having a first input, a second input and an output;

an auto-zero capacitor C_Z1 connected between the integrator input node and the first input of the amplifier; and

a feedback capacitor C_F1 connected to the output of the amplifier.

16. The CD modulator of claim 15, wherein the SFD signal is representative of a capacitance ratio C_R/C_F1.

17. The CD modulator of claim 16, wherein the SFD signal is pulse code modulated.

18. The CD modulator of claim 13, wherein the SFD circuit comprises an over-sampling analog-to-digital converter for sampling V_OUT1.

19. The CD modulator of claim 18, wherein the SFD circuit is active when C_R is connected to the first stage sigma-delta integrator.

20. A capacitance-to-digital (CD) modulator for converting a sensor capacitance C_S and a reference capacitance C_R to a pulse code modulated (PCM) signal, the CD modulator comprising:

a first stage sigma-delta integrator for selectively forming charge packets in C_S and C_R during a first phase, and integrating charge from C_S and C_R during a second phase to produce a first stage output V_OUT1;

a second stage sigma-delta integrator for sampling the first stage output V_OUT1, and integrating the sampled first stage output to produce a second stage output V_OUT2;

a quantizer for converting the second stage output V_OUT2 to the PCM signal; and

a sensor failure mode detection (SFD) circuit for deriving from V_OUT1 an SFD signal that identifies failure modes of C_S and C_R, wherein the SFD circuit comprises an over-sampling analog-to-digital converter for sampling V_OUT1, and wherein the SFD circuit is active when C_R is connected to the first stage sigma-delta integrator.

21. The CD modulator of claim 20 and further comprising:

a CD control unit for selectively providing an excitation signal to the sensor capacitance C_S and reference capacitance C_R as a function of the PCM signal.

22. The CD modulator of claim 21, wherein the first stage sigma-delta integrator comprises:

an integrator input node;

an amplifier having a first input, a second input and an output;

an auto-zero capacitor C_Z1, connected between the integrator input node and the first input of the amplifier; and

a feedback capacitor C_F1 connected to the output of the amplifier.

23. The CD modulator of claim 22, wherein the SFD signal is representative of a capacitance ratio C_R/C_F1.

24. The CD modulator of claim 20, wherein the SFD signal is pulse code modulated.

25. A pressure transmitter comprising:

a sensor capacitance;

a reference capacitance;

a primary capacitance-to-digital (CD) modulator for producing a pulse code modulated signal PCMP representative of sensed pressure, the primary CD modulator comprising;

a first stage sigma-delta integrator for selectively forming charge packets as a function of sensor and reference capacitances, and integrating the charge packets to produce a first stage output V_OUT1;

a second stage sigma-delta integrator for sampling the first stage output V_OUT1, and integrating the sampled first stage output to produce a second stage output V_OUT2;

a quantizer for converting the second stage output V_OUT2 to the PCMP signal; and

a secondary modulator for sampling V_OUT1 and producing a pulse code modulated signal PCMR indicative of whether the sensor capacitance and the reference capacitance are in a failure mode, wherein the secondary modulator comprises;

an integrator for sampling V_OUT1 and producing an output V_OUT3;

a quantizer for converting V_OUT3 to the signal PCMR; and

a control unit for providing an excitation signal to the integrator as a function of the signal.

26. The pressure transmitter of claim 25, wherein the first stage sigma-delta integrator comprises:

an integrator input node;

an amplifier having a first input, a second input and an output;

an auto-zero capacitor connected between the common node and the first input of the amplifier; and

a feedback capacitor connected to the output of the amplifier.

27. The pressure transmitter of claim 25 wherein the secondary modulator is active when the reference capacitance is connected to the first stage sigma-delta integrator.

28. The pressure transmitter of claim 25 and further comprising:

a microprocessor for determining whether a failure mode exists based upon the signal PCMR.

29. A pressure transmitter comprising:

a sensor capacitance;

a reference capacitance;

a primary capacitance-to-digital (CD) modulator for producing a pulse code modulated signal PCMP representative of sensed pressure, the primary CD modulator comprising;

a first stage sigma-delta integrator for selectively forming charge packets as a function of sensor and reference capacitances, and integrating the charge packets to produce a first stage output V_OUT1;

a second stage sigma-delta integrator for sampling the first stage output V_OUT1, and integrating the sampled first stage output to produce a second stage output V_OUT2; and

a quantizer for converting the second stage output V_OUT2 to the PCMP signal; and

a secondary modulator for sampling V_OUT1 and producing a pulse code modulated signal PCMR indicative of whether the sensor capacitance and the reference capacitance are in a failure mode, wherein the secondary modulator is active when the reference capacitance is connected to the first stage sigma-delta integrator.

30. The pressure transmitter of claim 29, wherein the first stage sigma-delta integrator comprises:

an integrator input node;

an amplifier having a first input, a second input and an output;

an auto-zero capacitor connected between the common node and the first input of the amplifier; and

a feedback capacitor connected to the output of the amplifier.

31. The pressure transmitter of claim 29 and further comprising:

a microprocessor for determining whether a failure mode exists based upon the signal PCMR.