Hi ESP,
in the picture below i draw what could be the input stage schematic with the PGA113. This time is draw in protel as i use it more frequently than EAGLE.
Please note that despite my comments to shazam, both circuits are substantially better than the original in many ways (constant input impedance, AD/DC modes, precision, size…) and both are designed to be simple to calibrate and to control. The drawback is that both are more expensive than the original one. Maybe the PGA113 is even cheaper as it replaced a lot of circuits (U7, U4, U5) and components.
I hope the integration into a future design could be done as soon as possible, as i’m looking forward to get a dual channel DSO Nano.
Note: there is another input configuration possible in which one of the PGA113 inputs measure AC signal and the other measures only DC component of the signal (low pass filter). This way we have both precision in AC and in DC.
Note2: if PGA113 is difficult to get, you can try with PGA112 (binary gains instead of scope ones -see datasheet-)
