MOSFETs are provided to connect the sensor input terminals of a ratiometric output sensor to a pair of power terminals, and the gate of each MOSFET is coupled to the opposite power terminal so that both MOSFETs are rendered conducting to power the sensor when a supply voltage of a predetermined polarity is connected across the power terminals but one of the MOSFETs is rendered non-conducting when a voltage of the opposite polarity is so applied. The MOSFET that is rendered non-conducting is oriented so that any internal source-drain diode does not bypass current around the MOSFET when voltage of the opposite polarity is applied. Optionally, over-voltage protection is provided by an input voltage sensor controlling the other MOSFET through a third MOSFET.