Abstract: A relay multiplexer for low level analog signals is disclosed. To provide a low cost system, inexpensive reed type relays are used. Such relays provide a high capacitive coupling from the relay contacts to the relay coil and thereby degrade the AC common mode rejection of the multiplexer. Common mode rejection is enhanced in accordance with the disclosure by (1) using an optical coupling to the relay switch and (2) slaving a floating relay power supply to the guard potential of the selected channel.

Abstract: Herein described is a drift cancellation circuit for signal amplifiers used in multiplexed data monitoring systems. The effect of analog drift is essentially cancelled and the need for an initial or periodic zero adjustment is eliminated. The elimination of the need for drift adjustment is accomplished at the analog to digital A/D converter of the mutiplexer. The A/D converter is encircuited to measure the difference between the measured input channel and a referenced voltage obtained from a sample-hold circuit. Input to the sample-hold circuit is taken from the output to the signal amplifier chain used in processing incoming channel data.