A 4-20 mA interface circuit for use, for example, in a valve controller, communicates
with a two-wire loop. The interface circuit includes 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 a related device, the power extraction
circuit including a DC-to-DC converter that generates the DC operating voltage
as a voltage having a smaller magnitude than a voltage between the two wires of
the two-wire loop. The circuit also includes 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. The digital communications circuit is 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 includes an impedance controller
having an operational amplifier connected to control an impedance presented to
the two-wire loop by the interface circuit.
