The motive for implementing an external amplifier was 2-fold. First. there was a rectangular hole in the front panel left vacent because the micro SD drive I originally put there could not be used since the SD library uses so much memory there was insufficient left for the display, which needs 1K byte.The solution for this was to connect an existing external transmitter to the DIN connector, which required re-allocating the pin which was intended for external analog input.
Secondly, I had an analog volt-ammeter made decades ago which already had a jack across the meter.
It has 18 ranges: DC voltages from 0.8 to 200, AC voltages from 8 to 800, and DC currents from 20 uA to 800 MA. This needed a differential input because the AC voltage output has the 200 mv DC measurement imposed on much larger AC pulses on two outputs relative to the negative input terminal. The input resistance needs to be high because it is in parallel with the meter’s 5000 ohms.
A CMOS op-amp was used.
It seemed advisable to isolate the external meter ground from the Scientech Meter ground sometimes to avoid ground loop errors, but to connect them in other cases.
The amplifier first stage therefore needs accurately-matched resistors and capacitors.
Because of the AC input the first stage low-pass filter is augmented by a second-order filter in the second stage.