Question
The output range in my application is not as wide as the specification says.
What is wrong?
Answer
The AD8422 is a modified version of the 3 op-amp instrumentation amplifier,
which means that internal nodes, such as the pre-amplifier outputs and the sub
tractor op-amp inputs, can saturate. Because the common-mode voltage is not
removed until the sub tractor stage, the pre-amplifier outputs of the AD8422
experience a combination of the common-mode signal (minus a diode drop, see
schematic) and the gained differential signal. Therefore, the output swing is
dependent on the common-mode input voltage as well as the gain and reference
pin voltage. Figures 10 through 15 in the AD8422 data sheet show these limits
for some common configurations. See the following video for a more thorough
general explanation of these limits: http://www.youtube.com/watch?v=VSf31DaXWUE