Excitation circuit for compensated capacitor industrial process control transmitters

• US 6,509,746 B1
• Filed: 06/04/2001
• Issued: 01/21/2003
• Est. Priority Date: 06/04/2001
• Status: Active Grant

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Wang, Rongtai
• Primary Examiner(s): Le, N.
• Assistant Examiner(s): He, Amy
• Application Number:
  US09/873,668
• Time in Patent Office: 596 Days
• Field of Search: 324/60, 324/60.C, 324/60.R, 324/678-679, 324/607, 324/706, 324/711, 341/143, 341/155, 737/18, 737/24, 737/80, 340/870.37
• US Class Current: 324/678
• 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: G01N 27/228 Circuits therefor measuring capacitance per se G01R27/26
• CPC Class Codes: H03M 3/32 with special provisions or arrangements for power saving, e.g. by allowing a sleep mode, using lower supply voltage for downstream stages, using multiple clock domains, by selectively turning on stages when needed
• CPC Class Codes: H03M 3/43 the quantiser being a single bit one
• CPC Class Codes: H03M 3/456 the modulator having a first order loop filter in the feedforward path
• CPC Class Codes: H03M 3/474 using time-division multiplexing

Abstract

An industrial process control transmitter has a capacitive sensor having at least three outputs each responsive in differing amounts to a process condition and to sensor hysteresis. An excitation circuit charges the sensor and pumps the charges to a sigma-delta capacitance-to-digital converter. The excitation circuit includes a charge amplifier coupling one of the sensor outputs to a summing node for summing with a charge on at least one other output. An auto-zeroing switch re-sets the charge amplifier on each cycle of operation of the charge amplifier.

Claims

1. An excitation circuit for charging capacitors of a sensor and for transferring representations of charges on the capacitors to a capacitance-to-digital converter, the sensor having at least one sensing capacitor and at least one compensation capacitor each responding differently to a condition and to error, the excitation circuit including:

a coupler selectively coupling an excitation source to the sensor to charge the capacitors;

a first inverting charge amplifier circuit having;

a first inverting amplifier having an inverting input coupled to the at least one compensation capacitor and having an output,

a first feedback capacitor coupled between the inverting input and output of the first amplifier, and

a first auto-zeroing switch selectively coupled in parallel with the first feedback capacitor;

a first summing node coupled to the output of the first inverting amplifier and to at least one sensing capacitor; and

a switch logic circuit for selectively operating the coupler.

2. The excitation circuit of claim 1, including a programmable gain control coupled between the output of the first inverting amplifier and the first summing node.

3. The excitation circuit of claim 1, wherein the excitation circuit is on a single circuit board, and the first inverting charge amplifier, first summing node and capacitance-to-digital converter are on a single chip.

4. The excitation circuit of claim 1, wherein the first inverting charge amplifier circuit further includes:

an offset capacitor coupled to an input of the first inverting amplifier, and

the coupler couples the first offset capacitor between the first auto-zeroing switch and the excitation source to charge the offset capacitor while coupling the excitation source to the at least one compensation capacitor.

5. The excitation circuit of claim 1, wherein the sensor includes first and second sensing capacitors and first and second compensation capacitors, and the coupler comprises:

a first switch circuit selectively coupling the excitation source to the sensor to charge the first sensing capacitor and the first compensation capacitor, and

a second switch circuit selectively coupling the excitation source to the sensor to charge the second sensing capacitor and second compensation capacitor.

6. The excitation circuit of claim 5, wherein the first summing node is coupled to the output of the first inverting amplifier and to the first sensing capacitor, and the excitation circuit further includes:

a second inverting charge amplifier circuit having;

a second inverting amplifier having an inverting input coupled to the second compensation capacitor and having an output,

a second feedback capacitor coupled between the output and the inverting input of the second inverting amplifier, and

a second auto-zeroing switch selectively coupled in parallel with the second feedback capacitor;

a second summing node coupled to the output of the second inverting amplifier and the second sensing capacitor;

an output node; and

the coupler includes a third switch circuit selectively coupling the first and second summing nodes to the output node,

the switch logic circuit operating the first, second and third switch circuits to charge the first sensing capacitor and first compensation capacitor and to charge the second sensing capacitor and second compensation capacitor during respective first phases, and to couple the first and second summing nodes to the output node during respective second phases.

7. The excitation circuit of claim 6, wherein the second phase has a longer time duration than the first a phase.

8. The excitation circuit of claim 6, wherein the first and second inverting charge amplifier circuits each further includes an offset capacitor coupled between the respective compensation capacitor and the inverting input of the respective inverting amplifier for compensating for voltage offset of the respective inverting amplifier, the respective auto-zeroing switch coupling the respective offset capacitor to the output of the respective inverting amplifier while the respective first and second switch circuits couples the excitation source to the respective compensation capacitor.

9. The excitation circuit of claim 8, including

a fourth switch circuit coupling the respective offset capacitor between the respective auto-zeroing switch and the excitation source.

10. The excitation circuit of claim 5, wherein the coupler further includes:

a third switch circuit selectively coupling the first and second compensation capacitors to the inverting input of the first inverting amplifier, selectively coupling the first and second sensing capacitors to the first summing node and selectively coupling the output of the inverting amplifier to the first summing node,

the switch logic circuit operating the first, second and third switch circuits to charge the first sensing capacitor and first compensation capacitor and to charge the second sensing capacitor and second compensation capacitor during respective first phases, and to couple the first compensation capacitor to the inverting input of the inverting amplifier and couple the output of the inverting amplifier and the first sensing capacitor to the first summing node and to couple the second compensation capacitor to the inverting input of the inverting amplifier and couple the output of the inverting amplifier and the second sensing capacitor to the first summing node during respective second phases.

11. The excitation circuit of claim 10, wherein the first inverting charge amplifier circuit includes an offset capacitor coupled between the third switch circuit and the inverting input of the inverting amplifier for compensating for voltage offset of the inverting amplifier, the auto-zeroing switch coupling the offset capacitor to the output of the inverting amplifier while the respective first and second switch circuits couples the excitation source to the respective compensation capacitor.

12. The excitation circuit of claim 11, including

a fourth switch circuit coupling the offset capacitor between the respective auto-zeroing switch and the excitation source.

13. The excitation circuit of claim 1, including a transmitter output circuit coupled to receive a digital output from the capacitance-to-digital converter to generate a standardized transmitter output adapted for coupling to a remote receiver.

14. The excitation circuit of claim 13, wherein the standardized transmitter output is selected from the group comprising 4-20 mA, FieldBus and fiber optic.

15. The excitation circuit of claim 1, further including an open lead detection circuit coupled to the output of the first inverting charge amplifier.

16. The excitation circuit of claim 1, wherein the sensor includes first and second sensing capacitors and first and second compensation capacitors, and the excitation source supply supplies at least first, second and third excitation voltage levels, the third excitation voltage level being intermediate the first and second excitation voltage levels, and the coupler comprises:

a first switch circuit selectively coupling the first and second excitation voltage levels to an input side of each of the first and second sensing capacitors and the first and second compensation capacitors,

a second switch circuit selectively coupling the third excitation voltage level to an output side of the first and second sensing capacitors and the first and second compensation capacitors, and

a third switch circuit selectively coupling the output sides of the first and second sensing capacitors to the first summing node and selectively coupling the output sides of the first and second compensation capacitors to the inverting input of the first inverting charge amplifier and selectively coupling the output of the first inverting charge amplifier to the first summing node;

the first inverting charge amplifier having a non-inverting input coupled to the third voltage level, and the switch control logic operates

the first and second switch circuits during a respective first phase to charge the first sensing capacitor and first compensation capacitor,

the third switch circuit during a respective second phase to couple the output of the first sensing capacitor to the first summing node and to couple the output of the first compensation circuit through the first inverting charge amplifier to the first summing node,

the first and second switch circuits during a respective first phase to charge the second sensing capacitor and second compensation capacitor, and

the third switch circuit during a respective second phase to couple the output of the second sensing capacitor to the first summing node and to couple the output of the second compensation circuit through the first inverting charge amplifier to the first summing node, the respective first and second phases being mutually exclusive.

17. The excitation circuit of claim 16, wherein the second phase has a longer time duration than the first phase.

18. An industrial process control transmitter for transmitting an output representative of a process condition comprising:

a sensor having at least one sensing capacitor and at least one compensation capacitor each responding differently to the process condition and to error;

a capacitance-to-digital converter for providing a digital output based on an analog signal;

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

an excitation circuit for charging the sensing and compensation capacitors and for transferring an analog signal representative of the process condition to the capacitance-to-digital converter, the excitation circuit including;

a coupler selectively coupling an excitation source to the sensor to charge the capacitors;

a first inverting charge amplifier circuit having;

a first inverting amplifier having an inverting input coupled to the at least one compensation capacitor and having an output,

a first feedback capacitor coupled between the inverting input and output of the first amplifier, and

a first auto-zeroing switch selectively coupled in parallel with the first feedback capacitor;

a first summing node coupled to the output of the first inverting amplifier and to at least one sensing capacitor; and

a switch logic circuit for selectively operating the coupler.

19. The industrial process control transmitter of claim 18, wherein the sensor includes first and second sensing capacitors and first and second compensation capacitors, and the first summing node is coupled to the output of the first inverting amplifier and to the first sensing capacitor, the excitation circuit further includes:

a second inverting charge amplifier circuit having;

a second inverting amplifier having an inverting input coupled to the second compensation capacitor and having an output,

a second feedback capacitor coupled between the output and the inverting input of the second inverting amplifier, and

a second auto-zeroing switch selectively coupled in parallel with the second feedback capacitor;

a second summing node coupled to the output of the second inverting amplifier and the second sensing capacitor;

an output node; and

the coupler includes

a first switch circuit selectively coupling the excitation source to the sensor to charge the first sensing capacitor and the first compensation capacitor,

a second switch selectively coupling the excitation source to the sensor to charge the second sensing capacitor and the second compensation capacitor, and

a third switch circuit selectively coupling the first and second summing nodes to the output node,

the switch logic circuit operating the first, second and third switch circuits to charge the first sensing capacitor and first compensation capacitor during a first phase and to charge the second sensing capacitor and second compensation capacitor during respective first phases, and to couple the first and second summing node to the output node during respective second phases.

20. The industrial process control transmitter of claim 18, wherein the sensor includes first and second sensing capacitors and first and second compensation capacitors, the coupler includes

a first switch circuit selectively coupling the excitation source to the sensor to charge the first sensing capacitor and the first compensation capacitor,

a second switch selectively coupling the excitation source to the sensor to charge the second sensing capacitor and the second compensation capacitor, and

a third switch circuit selectively coupling the first and second compensation capacitors to the inverting input of the first inverting amplifier, selectively coupling the first and second sensing capacitors to the first summing node and selectively coupling the output of the inverting amplifier to the summing node,

the switch logic circuit operating the first, second and third switch circuits to charge the first sensing capacitor and first compensation capacitor and to charge the second sensing capacitor and second compensation capacitor during respective first phases, and to couple the first compensation capacitor to the inverting input of the inverting amplifier and couple the output of the inverting amplifier and the first sensing capacitor to the first summing node and to couple the second compensation capacitor to the inverting input of the inverting amplifier and couple the output of the inverting amplifier and the second sensing capacitor to the first summing node during respective second phases.