I've simulated an adjustable gain instrumentation amplifier circuit using two AD8040 that appears to work very well. I'm curious if there is an instrumentation amp version that is functionally equivalent? Perhaps one based on the 8040? See pic...
I'm curious as to why you chose the AD8040. Does your application require a high bandwidth?
If you do not require so much bandwidth, the AD8275 might be a good option for your first stage. One concern with using discrete resistors is that it brings down your gain accuracy and CMRR - with 1% tolerances at your resistors, you would end up with a worst case CMRR of 30dB. Using the AD8275 would guarantee you a minimum CMRR of 80dB since it is an integrated solution, and it would also provide the attenuation you need.
You could also use the AD8250/1/3 in your second stage to provide variable gains for your signal. This would provide excellent gain accuracy to your system. It would also be a good choice if you do need some bandwidth for your application.
I hope this helps!
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