when the supply rails collapse (i.e. the power supply to the amplifier is
down/switched off and all the rails are at the same voltage)? Datasheet doesn't
specify the input impedance when the device has collapsed power rails (we have
had problems in the past where inputs are shorted through a low impedance when
the supply rails are collapsed).
For the AD620, input impedance when the input voltage is beyond the supply
rails can be as low as 400 ohms to each input. In other words 200 ohms common
mode or 800 ohms differential. Since presumably the input voltage will beyond
the supplies when the supplies are both 0V, then you can use these numbers.
In practice the input impedance will vary some with input voltage, but the 400
ohms should be the lower bound, since these are physical resistors in the
When voltages beyond the supplies rails will be encountered, we recommend using
external resistors on the input to limit the current to 6 mA.
If you need something with higher input impedance when the device is powered
off, we recommend the AD8226 or AD8227. These devices have internal current
limiting devices that limit the input current to less than a mA during an
overvoltage condition. The current flow into one input vs. input voltage shown
in the typical performance curves for the 2.7V single supply case. This should
you a good approximation for your +/-0 V case. (Just shift the positive
voltages over 2.7V. For example: use 2.7V on the X axis for your 0V, 7.7V on
the X axis for your 5V, etc.) This is figure 16 in the AD8226 datasheet.