Sigma-delta analog to digital converter for capacitive pressure sensor and process transmitter
First Claim
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1. A process pressure transmitter having hysteresis compensation, comprising:
- an output circuit configured to provide an output related to a sensed pressure;
a pressure sensor comprising;
a pressure cell having an interior to form a main cell cavity;
a deflectable diaphragm which deflects in response to an applied pressure having a first side and a second side configured to divide the main cell cavity into first and second cavities;
first and second electrodes in the first cell cavity proximate the diaphragm configured to form respective first and second capacitors therewith;
third and fourth electrodes in the second cell cavity proximate the diaphragm configured to form respective third and fourth capacitors therewith;
an analog to digital converter coupled to the first, second, third and fourth electrodes, comprising;
a sigma delta converter having a first input coupled to the first electrode and a second input coupled to the third electrode and having an output related to the applied pressure; and
at least one charge inverter amplifier including a switch capacitor to couple the second electrode to the first input of the sigma delta converter.
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Abstract
A pressure transmitter includes a pressure sensor comprising a pressure cell having an interior to form a main cell cavity and a deflectable diaphragm which deflects in response to an applied pressure. The pressure sensor includes electrodes to compensate for hysteresis. A charge inverter amplifier includes a switch capacitor such that it may be implemented in a single integrated circuit.
268 Citations
26 Claims
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1. A process pressure transmitter having hysteresis compensation, comprising:
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an output circuit configured to provide an output related to a sensed pressure;
a pressure sensor comprising;
a pressure cell having an interior to form a main cell cavity;
a deflectable diaphragm which deflects in response to an applied pressure having a first side and a second side configured to divide the main cell cavity into first and second cavities;
first and second electrodes in the first cell cavity proximate the diaphragm configured to form respective first and second capacitors therewith;
third and fourth electrodes in the second cell cavity proximate the diaphragm configured to form respective third and fourth capacitors therewith;
an analog to digital converter coupled to the first, second, third and fourth electrodes, comprising;
a sigma delta converter having a first input coupled to the first electrode and a second input coupled to the third electrode and having an output related to the applied pressure; and
at least one charge inverter amplifier including a switch capacitor to couple the second electrode to the first input of the sigma delta converter. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21)
a differential amplifier having an inverting input coupled to the third electrode, non-inverting input coupled to a reference and an output;
a feedback capacitor coupled between the output and the inverting input of the differential amplifier;
a switch coupled across the feedback capacitor and between the output and the inverting input of the differential amplifier.
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4. The process pressure transmitter of claim 1 wherein the charge inverter amplifier has a programmable gain.
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5. The process pressure transmitter of claim 4 wherein the charge inverting amplifier includes a plurality capacitors selectively couplable in series with the charge inverting amplifier and the gain is a function of the capacitance value of the capacitors.
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6. The process pressure transmitter of claim 1 including an open lead detector configured to detect an open electrical connection in the pressure sensor.
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7. The process pressure transmitter of claim 1 including a second charge inverter amplifier to couple the fourth electrode to the second input of the sigma delta converter.
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8. The process pressure transmitter of claim 1 wherein an output from the charge inverter amplifier is inversely related to a signal from the second electrode applied to an input of the amplifier.
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9. The process pressure transmitter of claim 1 wherein the charge inverter amplifier includes an auto-zero phase.
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10. The process pressure transmitter of claim 1 wherein the charge inverter amplifier further couples the fourth electrode to the second input of the sigma delta converter.
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11. The process pressure transmitter of claim 10 further including a first and a second switch configured to selectively couple the second and fourth electrodes to the charge inverter amplifier.
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12. The process pressure transmitter of claim 1 including switch control logic configured to operate the switch capacitor in the charge inverter amplifier.
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13. The process pressure transmitter of claim 1 wherein the sigma delta converter includes a first stage integrator.
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14. The process pressure transmitter of claim 1 including a first clock and a second clock configured to operate the charge inverter amplifier.
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15. The process pressure transmitter of claim 14 wherein the first and second clocks have differing phases.
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16. The process pressure transmitter of claim 1 wherein the sigma delta converter and the charge inverter amplifier are fabricated on a single integrated circuit.
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17. The process pressure transmitter of claim 1 including a clock having a Φ
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1 phase and an Φ
2 phase.
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1 phase and an Φ
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18. The process pressure transmitter of claim 17 wherein the Φ
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1 phase and the Φ
2 phase have substantially equal durations.
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1 phase and the Φ
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19. The process pressure transmitter of claim 17 wherein the Φ
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2 phase has a duration which is greater than a duration of the Φ
1 phase.
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2 phase has a duration which is greater than a duration of the Φ
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20. The process pressure transmitter of claim 1 including a sample and hold circuit configured to hold an output from the charge inverter amplifier.
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21. The process pressure transmitter of claim 20 wherein the sample and hold comprises a capacitor and a switch.
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22. An apparatus for measuring pressure, comprising:
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a diaphragm configured to deflect in response to applied pressure;
first and second main capacitors having capacitance related to deflection of the diaphragm;
first and second compensation capacitors having capacitance related to deflection of the diaphragm;
a sigma delta analog to digital converter coupled to the first and second main capacitors; and
a shared charge inverter amplifier coupling the first and second compensation capacitors to the sigma delta analog to digital converter. - View Dependent Claims (23, 24, 25, 26)
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Specification