The output of the load cell, the force sensing part of the instrument, is a +/- 10 V analog signal available at the chart recorder output. Prior to getting there it goes through a series of demodulation and gain stages that I will elaborate on later. The Arduino Due can only accept a 0 - 3.3 V signal so I had a choice of either rescale the signal or clip the signal to levels that are safe for the microcontroller. I chose that latter because there is a final amp with selectable gain that give me plenty of leeway to change the scale, I just need to make sure the signal doesn't damage the Arduino.

To clip the output I'm using the circuit shown in the figure. The positive output is clipped by the 3.3V zener diode with a 1K resistor to limit the current. One more diode was required because the forward voltage drop of the zener actually exceeded the allowable negative voltage of the Arduino.