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AD8220 Input impedance


We built this circuit. When at the "Vin1" pint I apply 100mVpp, 100Hz sinus (output impedance HighZ on signal generator), on pin7 OUT I have the same sinus wave with approx  100mVpp amplitude, which seems to be OK.

If I apply to point "Vin2" (200Mohm in series with voltage generator) the same 100mVpp sinus, I have no signal on the output, just noise.

If I measure with a multimeter (measure current ~0.5uA) between pin 4 (IN+) and pin6 (REF - connected to GND) I measure 1,3 MegaOhm (my expectation was 500Mohm). At this measurement   the input signal generator was removed, Vin1 and Vin2 float. IN- was connected to GND. If IN- was connected to Vout, than on VOUT was a +/- 2.5V, approx 1 MHz square wave (I guess self oscillating).

From this measurement it seems for me, that IN+ to Vref input impedance is in range of 1,3Mohm, but from datasheet this values shall be in range of 100 GOhm.

Best regards,


  • Hi Attila,

    Sorry if we missed this question.

    May I know what are you trying to do with the AD8220?

    AD8220 is an instrumentation amplifier and we don't normally connect its output to the negative inputs. Negative feedback usually for opamps products.

    If you can tell me more about what do you want to design, discover, or applications that you are getting into, I can help you looking for more suitable parts/circuits.

    Best regards,