Abstract: A switching circuit for selectively switching either of two input signals to an output circuit employs electrically isolated separate input circuits with independent grounds to prevent drift of the output signal delivered by the output circuit. Circuit isolation and independent grounds are made possible by the use of a single relay employing separate poles acting with two pairs of contacts respectively coupled with the input circuits. The switching circuit may be employed in a temperature control system for alternately coupling temperature and calibration signals to a temperature controller.