Bridged capacitor sensor measurement circuit

An industrial process control transmitter arranged to be coupled to a central station by a two-wire communication link, the transmitter including:

• 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.

A process for measuring a process variable comprising steps of:

• 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.