Patent ID: 7167537

Claim:
A valve controller comprising: a 4–20 mA interface circuit having terminals for connection to a two-wire loop; a processor in data communication with the interface circuit and operable to generate a control signal for controlling a valve position; and a control device operable to control the valve position in response to the control signal from the processor; wherein the 4–20 mA interface circuit comprises: a power extraction circuit connected to the two-wire loop and operable to generate a DC operating voltage for use in powering the interface circuit and the processor; a current sensor connected to the two-wire loop and operable to generate a measure of an analog current through the two-wire loop, the measure being used in controlling a device associated with the interface circuit; and a digital communications circuit connected to the two-wire loop and operable to inject a digital transmission signal on to the two-wire loop and to extract a digital reception signal from the two-wire loop, the digital communications circuit including an impedance controller having an operational amplifier connected to control an impedance presented to the two-wire loop by the interface circuit.