Re: i want to measure a 100mega ohm impedance with an current source using opamp and using adc . is there any suggestion about that and is this practical with opamp?

Hi Azita:

As described in the CN0349 Circuit Note (Conductivity Sensors Section), in order to measure high impedance (small conductivities) we have to use a cell with smaller constant. For example, 1uS
and 1/cm constant will give 1Mohm impedance, if the constant is 0.01/cm, the
impedance will be 10kOhm which is 100 times smaller. All this is given in the
circuit note.