Gents,
I'd like to measure voltage of a low impedance source (10mOhm), which is in range of +/-200mV. The ADC cannot handle negative voltages and I can't do the obvious trick with connecting one of the input terminals to, say, VDD/2 and perform differential measurement because the shunt's terminal must be connected to GND. I don't want to go dual supply either, as it introduces more problem than it solves. So the basic idea is to implement a level shifter based on a PNP/PMOS current mirror.
Curiously, even at the Spice level the PNP variant using BC857B simulates poorly, the current waveforms are in sync, but shifted by a detectable offset (the collector current is in the 1mA ballpark). The PMOS variant on BSS84 simulates extremely well. Suspiciously well.
But that's a simulation. What should I expect in practice and how to do it properly? Are the real dual MOSFETs in SOT343/SOT23-6 matched well enough?
Best regards, Piotr