Airpax A175
The Airpax A175 is an SPDT break-before-make chopper with a coil designed for approximately 6.3 VAC at 60 Hz. It is used in the 067-0596-00, 067-0543-99, and the 067-0502-00, and 067-0502-01.
The use of a chopper to make precise DC measurements is a well known and trusted technique, using instruments that are sensitive to AC, but not capable of drift-free DC performance.
The chopping technique can be used for calibration, comparing a device under test with a reference. In this case, the result of chopping is viewed on an oscilloscope, possibly with a preamplifier between the output of the chopper and the input of the oscilloscope. The operator adjusts the device under test to minimize the amplitude observed on the oscilloscope. The vertical sensitivity, DC accuracy, and DC drift performance of the oscilloscope and preamplifier are not critical if the goal is simply to calibrate the device under test for optimal match with reference. However, if the objective of the calibration is to provide assurance that the error is less than some specific value, then the gain of the preamplifier and oscilloscope matters. Still, the gain of the preamp and oscilloscope only affect the accuracy of the tolerance, which is typically around 10%, (e.g., +/- 0.01% or +/- 0.3%).
