US Patent 7489125 Calibrating a tester using ESD protection circuitry

An apparatus includes a circuit element that requires calibration, a calibration circuit for use in calibrating the circuit element, and a damping diode electrically connectable in a first path that includes the calibration circuit and electrically connectable in a second path that excludes the calibration circuit. The first path is for electrically connecting the calibration circuit and the circuit element, and the second path is for use in protecting the apparatus from electrostatic discharge. A switching circuit is used to switch the clamping diode between the first path and the second path.