Bridged capacitor sensor measurement circuit
First Claim
1. Apparatus for measuring a process variable comprising:
- a first sensing capacitor having a capacitance, C1, based on the process variable;
a second sensing capacitor having a capacitance, C2, based on the process variable;
a bridge node coupling a first side of the first sensing capacitor to a first side of the second sensing capacitor;
an excitation source supplying at least first and second voltage levels; and
a switch circuit selectively coupling a second side of each of the first and second sensing capacitors to the excitation source to derive a representation of C1-C2 at the bridge node.
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Accused Products
Abstract
A pair of sensing capacitors, each having a capacitance C1 and C2 respectively, based on a process variable, are coupled to a bridge node which is coupled to a summing node. A reference capacitor, coupled to the summing node, has a capacitance CREF greater than an expected maximum difference between the capacitances of the pair of sensing capacitors. Switches selectively couple the sensing capacitors and the reference capacitor to at least first and second voltages to derive charges representative of C1-C2 and CREF. In one embodiment the sensing capacitors are operated to charge and discharge during respective first and second phases of first cycles and the reference capacitor is operated to charge and discharge during respective first and second phases of second cycles. In another embodiment, the reference capacitor is operated to charge and discharge during alternate phases of the first and second cycles and the sensing capacitors are operated to charge and discharge during respective first and second phases of all cycles.
8 Citations
25 Claims
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1. Apparatus for measuring a process variable comprising:
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a first sensing capacitor having a capacitance, C1, based on the process variable;
a second sensing capacitor having a capacitance, C2, based on the process variable;
a bridge node coupling a first side of the first sensing capacitor to a first side of the second sensing capacitor;
an excitation source supplying at least first and second voltage levels; and
a switch circuit selectively coupling a second side of each of the first and second sensing capacitors to the excitation source to derive a representation of C1-C2 at the bridge node. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
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15. An industrial process control transmitter arranged to be coupled to a central station by a two-wire communication link, the transmitter including:
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a transceiver coupled to the communication link for transmitting information to the central station and for receiving information from the central station;
a processor coupled to the transceiver for processing information;
a process variable sensor comprising;
a first sensing capacitor having a capacitance, C1, based on the process variable a second sensing capacitor having a capacitance, C2, based on the process variable, and a bridge node coupling a first side of the first sensing capacitor to a first side of the second sensing capacitor;
an excitation source supplying at least first and second voltage levels;
a switch circuit selectively coupling a second side of each of the first and second sensing capacitors to the excitation source to derive a representation of C1-C2 at the bridge node; and
a delta sigma converter coupled to the bridge node for supplying digital information signals to the processor representative of the difference between the capacitances of the first and second capacitors. - View Dependent Claims (16, 17, 18, 19, 20, 21, 22)
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23. A process for measuring a process variable comprising steps of:
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applying the process variable to first and second sensor capacitors to derive first and second capacitances, C1 and C2 respectively, based on the process variable;
providing a reference capacitor having a reference capacitance, CREF, larger than a difference between the capacitances of the first and second capacitors;
deriving a first charge representative of C1-C2;
deriving a second charge representative of CREF; and
integrating the first and second charges. - View Dependent Claims (24, 25)
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Specification