The Model 428 is a true differential amplifier with transformer isolation between input, output, and the power source. An internal dc-to-dc converter provides bipolar voltages for the internal circuitry while accepting power from a single external dc power supply or battery. In addition to the high-performance amplifier, the unit includes an integral excitation power supply. Transformer coupling contributes to high common-mode rejection, and the chopper design results in superior zero stability, both short term and long term.