Abstract: A calibration system for calibrating a plurality of excitation sources of an instrumentation system. The calibration system allows a single calibration adjustment for calibrating all of the excitation sources simultaneously rather than having to independently adjust each excitation source. An adjustable Zener diode coupled to a potentiometer forms an adjustable calibration circuit providing a reference voltage to a plurality of current excitation sources. Each current source preferably includes a buffer and transistor control circuit receiving the reference voltage for controlling current through a precision resistor. In this manner, only the precision resistor is duplicated for each channel, where the calibration adjust mechanism is provided in a single circuit for all channels. This significantly reduces circuitry and cost while also simplifying the calibration procedure to a single adjustment.