A 4 20 mA interface circuit is disclosed having a resistor in series with a DC/DC converter, and further having a current bypass circuit in parallel to the series combination of the resistor and DC/DC converter. In this way, a 4 20 mA analog current at a low frequency may be bypassed around the series combination, whereas a high-frequency current carrying a digital communications signal may be presented with the resistor as its only effective input impedance. In this way, the 4 20 mA current sees an input impedance which allows the 4 20 mA current to be used in supplying power to an external device, whereas the high-frequency current sees an impedance which allows a reliable, low-distortion reading of the digital communications signal therefrom. The high-frequency current may be superimposed on the 4 20 mA, low-frequency current. Portions of the 4 20 mA current not required to power the external device may be shunted to ground, or may be driven back through the DC/DC converter, to thereby present a negative impedance to the 4 20 mA current.