I would like to use the ADC-20 to make low-level current measurements (nA...uA) by measuring the voltage drop across a shunt resistor. The shunt is likely to have such a large value that the input impedance of ADC-20 may affect the measurement.
The input impedance of ADC-20 is specified to be 1 Mohm. How accurate is this? Can the excact value be checked by a multimeter?
As I am measuring a current into earth, could one use the input impedance directly as a shunt? With 1 Mohm one would get a scale of 1V/1uA. Is this realistic?