Three-phase excitation circuit for compensated capacitor industrial process control transmitters

• US 6,684,711 B2
• Filed: 08/23/2001
• Issued: 02/03/2004
• Est. Priority Date: 08/23/2001
• Status: Active Grant

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Wang, Rongtai
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Jenkins, Jermaine L
• Application Number:
  US09/939,079
• Publication Number:
  US 20030056597A1
• Time in Patent Office: 894 Days
• Field of Search: 737/00--56, 361/283--28
• US Class Current: 73/724
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor

Abstract

A capacitor industrial process control transmitter includes a three-phase excitation circuit to charge a sensing capacitor and a compensation capacitor of the transmitter and transfer charges to an integrator. The sensing capacitor is charged during the first phase. During the second phase, the voltage to the sensing capacitor is reversed, and the charge on the sensing capacitor is pumped to the integrator. Also, the compensation capacitor is charged with the reversed voltage during the second phase. During the third phase, the voltage to the compensation capacitor is changed, and the charge on the compensation capacitor is pumped to the integrator.

Claims

1. A pressure sensor having first and second capacitors each responsive to pressure and to sensor hysteresis, the sensor comprising:

a charging circuit having a first charging level having a first polarity, a second charging level having a second polarity opposite the first polarity and third charging level that is intermediate the first and second charging levels;

a sensing circuit that is a sigma-delta capacitance-to-digital circuit providing a digital output representative of the pressure; and

a switch circuit selectively coupling the first and second capacitors to the charging circuit to charge the first and second capacitors, and selectively coupling the first and second capacitors to the sensing circuit to transfer charge from the first capacitor based on the first and second charging levels and to transfer charge from the second capacitor based on the second and third charging levels.

2. The pressure sensor of claim 1, wherein the switch circuit includes:

a first switch operable to couple an input of the first capacitor to the first charging level;

a second switch operable to couple an output of the first capacitor to the sensing circuit and to couple inputs of the first and second capacitors to the second charging level;

a third switch operable to couple the input of the second capacitor to the third charging level and to couple an output of the second capacitor to the sensing circuit; and

a switch control responsive to the sensing circuit for operating the first, second and third switches in sequence.

3. The pressure sensor of claim 2, wherein the charging circuit supplies a fourth charging level that is midway between the first and second charging levels, the first switch being further operable to couple the output of the first capacitor to the fourth charging level.

4. The pressure sensor of claim 3, wherein the second switch is further operable to couple the output of the second capacitor to the fourth charging level.

5. The pressure sensor of claim 3, wherein the sensing circuit includes:

an integrating amplifier having a first input connected to the fourth charging level, a second input and an output,

a feedback capacitor,

the first switch being further operable to couple the feedback capacitor between the amplifier output and the amplifier second input,

the second switch being further operable to couple the feedback capacitor between the amplifier output and the first capacitor, and

the third switch being further operable to couple the feedback capacitor between the amplifier output and the second capacitor.

6. The pressure sensor of claim 5, wherein the third charging level is between the first and fourth charging levels and the charging circuit supplies a fifth charging level that is between the second and fourth charging levels, the sensor further including a fourth capacitor,

the switch control is responsive to the sensing circuit to operate the first, second and third switches during respective first and second sets of phases,

the first switch is operable during a first phase of the first set to couple the input of the first capacitor to the first charging level and is operable during the first phase of the second set to couple an input of the third capacitor to the second charging level,

the second switch is operable during a second phase of the first set to couple the output of the first capacitor to the sensing circuit and is operable during the second phase of the second set to couple an output of the third capacitor to the sensing circuit and to couple the fifth charging level to an input of the fourth capacitor, and

the third switch is operable during a third phase to couple the feedback capacitor between the amplifier output and the fourth capacitor.

7. The pressure sensor of claim 6, wherein

the second switch is coupled to the feedback capacitor to couple the feedback capacitor between the amplifier output and the first capacitor during the second phase of the first set and to couple the feedback capacitor between the amplifier output and the third capacitor during the second phase of the second set, and

the third switch is coupled to the feedback capacitor to couple the feedback capacitor between the amplifier output and the second capacitor during the third phase of the first set and to couple the feedback capacitor between the amplifier output and the fourth capacitor during the third phase of the second set.

8. The pressure sensor of claim 1, including third and fourth capacitors responsive to pressure and to sensor hysteresis, and wherein the charging circuit has a fourth charging level that is intermediate the first and second charging levels, the switch circuit selectively coupling the third and fourth capacitors to the charging circuit to charge the third and fourth capacitors, and selectively coupling the third and fourth capacitors to the sensing circuit to transfer charge from the third capacitor based on the first and second charging levels and to transfer charge from the fourth capacitor based on the first and fourth charging levels.

9. The pressure sensor of claim 8, wherein the switch circuit includes:

a first switch operable to couple an input of the first capacitor to the first charging level;

a second switch operable to couple an output of the first capacitor to the sensing circuit and to couple inputs of the first and second capacitors to the second charging level;

a third switch operable to couple the input of the second capacitor to the third charging level and to couple an output of the second capacitor to the sensing circuit;

a fourth switch operable to couple an input of the third capacitor to the second charging level;

a fifth switch operable to couple an output of the third capacitor to the sensing circuit and to couple inputs of the third and fourth capacitors to the first charging level;

a sixth switch operable to couple an input of the fourth capacitor to the fourth charging level and an output of the fourth capacitor to the sensing circuit; and

a switch control responsive to the sensing circuit for operating the first, second, third, fourth, fifth and sixth switches in sequence.

10. The pressure sensor of claim 9, wherein the charge circuit supplies a fifth charging level that is midway between the first and second charging levels, the third charging level being between the first and fifth charging levels, and the fourth charging level being between the second and fifth charging levels, and the first switch is operable to couple the output of the first capacitor to the fourth charging level, and the fourth switch is operable to couple the output of the second capacitor to the fifth charging level.

11. The pressure sensor of claim 10, wherein the second switch is further operable to couple the output of the second capacitor to the fifth charging level, and the fifth switch is operable to couple the output of the fourth capacitor to the fifth charging level.

12. The pressure sensor of claim 1 coupled to an industrial process control transmitter, further including a transmitter output circuit receiving the digital output and generating a standardized transmitter output adapted for coupling to a remote receiver.

13. An industrial process control transmitter comprising:

a pressure sensor having first and second capacitors each having a capacitance responsive to pressure and to sensor hysteresis;

a charging circuit having a first charging level having a first polarity, a second charging level having a second polarity opposite the first polarity and third charging level that is intermediate the first and second charging levels;

a sigma-delta capacitance-to-digital circuit providing a digital output representative of an analog input;

a switch circuit selectively coupling the first and second capacitors to the charging circuit to charge the first and second capacitors and selectively coupling the first and second capacitors to the capacitance-to-digital circuit to transfer an analog signal from the first capacitor based on the capacitance of the first capacitor and the first charging level and to transfer an analog signal from the second capacitor based on the capacitance of the second capacitor and the second and third charging levels; and

a transmitter output circuit responsive to the digital output to generate a standardized transmitter output adapted for coupling to a remote receiver.

14. The industrial process control transmitter of claim 13, wherein the switch circuit selectively couples the first capacitor to the capacitance-to-digital circuit so that the analog signal transferred from the first capacitor is based on the first and second charging levels.

15. The industrial process control transmitter of claim 14, wherein the switch circuit includes:

a first switch operable to couple an input of the first capacitor to the first charging level;

a second switch operable to couple an output of the first capacitor to the capacitance-to-digital circuit and to couple an input of the second capacitor to the second charging level;

a third switch operable to couple the input of the second capacitor to the third charging level and to couple an output of the second capacitor to the capacitance-to-digital circuit; and

a switch control responsive to the sensing circuit for operating the first, second and third switches in sequence.

16. The industrial process control transmitter of claim 14, wherein the pressure sensor includes third and fourth capacitors responsive to pressure and to sensor hysteresis, and wherein the capacitance-to-digital circuit has a fourth charging level that is intermediate the first and second charging levels, the switch circuit selectively coupling the third and fourth capacitors to the charging circuit to charge the third and fourth capacitors, and selectively coupling the third and fourth capacitors to the capacitance-to-digital circuit to transfer an analog signal from the third capacitor to the capacitance-to-digital circuit based on the first and second charging levels and to transfer an analog signal from the fourth capacitor to the capacitance-to-digital circuit based on the first and fourth charging levels.

17. The industrial process control transmitter of claim 16, wherein the switch circuit includes:

a first switch operable to couple an input of the first capacitor to the first charging level;

a second switch operable to couple an output of the first capacitor to the capacitance-to-digital circuit and to couple inputs of the first and second capacitors to the second charging level;

a third switch operable to couple the input of the second capacitor to the third charging level and to couple an output of the second capacitor to the capacitance-to-digital circuit;

a fourth switch operable to couple an input of the third capacitor to the second charging level;

a fifth switch operable to couple an output of the third capacitor to the capacitance-to-digital circuit and to couple inputs of the third and fourth capacitors to the first charging level;

a sixth switch operable to couple an input of the fourth capacitor to the fourth charging level and an output of the fourth capacitor to the capacitance-to-digital circuit; and

a switch control responsive to the sensing circuit for operating the first, second, third, fourth, fifth and sixth switches in sequence.

18. A process of operating a pressure sensor having first and second capacitors each responsive to pressure and to sensor hysteresis, the process comprising steps of:

(a) charging the first capacitor to a first polarity during a first phase;

(b) pumping a charge on the first capacitor to a sigma-delta capacitance-to-digital circuit during a second phase that is mutually exclusive from the first phase;

(c) charging the second capacitor to a second polarity opposite the first polarity during the second phase; and

(d) pumping a charge on the second capacitor to the sigma-delta capacitance-to-digital circuit during a third phase that is mutually exclusive from the first and second phases.

19. The process of claim 18, wherein the sigma-delta capacitance-to-digital circuit includes an amplifier, and an offset capacitor coupled to an input of the amplifier to adjust for amplifier offset, the process further including the step of:

(e) charging the offset capacitor to represent the amplifier offset during the first phase.

20. The process of claim 18, wherein pressure sensor further includes third and fourth capacitors responsive to pressure and to sensor hysteresis, and steps (a)-(d) are performed during a first set of first, second and third phases, the process further comprising steps of, during a second set of first, second and third phases:

(e) charging the third capacitor to the second polarity during the first phase,

(f) pumping a charge on the second capacitor to the sigma-delta capacitance-to-digital circuit during the second phase,

(g) charging the fourth capacitor to the first polarity during the second phase, and

(h) pumping a charge on the fourth capacitor to the sigma-delta capacitance-to-digital circuit during the third phase.

21. The process of claim 20, wherein the sigma-delta capacitance-to-digital circuit includes an amplifier, and an offset capacitor coupled to an input of the amplifier to adjust for amplifier offset, the process further including the step of:

(i) charging the offset capacitor to represent the amplifier offset during the first phases.