Abstract: Disclosed is a two-wire transmitter which transmits information over a pair of wires having an external supply connected thereto. The transmitter receives its power from the pair of wires and stores a portion of this power during spaced apart first time intervals. This stored power is used to excite an external sensor during second time intervals that are substantially shorter than the first time intervals and which lie between respective ones of the first time intervals. The excitation response from the sensor is sampled and held in synchronization with this excitation. A variable resistance device is included for insertion in series with the pair of wires. This device is responsive to the held excitation response from the sensor to allow a current representative thereof to flow through the pair of wires.

Abstract: The disclosed transmitter operates in a two wire process control system to modify a loop current through the two wires that are representative of pressure drop through an orifice into a second loop current through the two wires that are representative of flow rate through the orifice. To accomplish this, the transmitter generates a first voltage that is representative of the first loop current. Then, it generates a second voltage proportional to the square root of the first voltage, and generates a third voltage proportional to the difference between the first and second voltages. The transmitter then sinks an additional current in the two wires proportional to the third voltage. The sum of the first loop current and the additional current equals the second loop current.