AD8236 REF not passing to output

Hi Jan, 

When you say you have nothing on the output, do you mean that the output is at 0V? 

If you haven't tried yet, please disconnect the AD8236 from the AMC1200. Tie both inputs, as well as REF, to Vcc/2. You should read the same (1.65V) at the output.

Separately, with the setup connected, could you probe the inputs of the AD8236 to see what's going into the device?

Thanks, 

Kris

Ok, when Inputs are tied to ref, ref is also on output. In circuit I have HP filter which have resistors to ground. Should I have tie them to the REF?

No, that should not be necessary. Can you post a schematic? It looks like the problem might be in the interface. 

Also, scope shots of the signals coming out of the AMC1200 might help. 

Here I have schematic.

When replacing 8236 to 8226 everything works fine, of course with other RG.

Hi Jan, 

I apologize for misunderstanding your configuration. It is clearer with your schematic. Yes, I think it would be better to tie the resistors of the high pass filter to mid-supply. The problem looks like it could be the input common-mode going into the inputs of the AD8236, it does not go fully to ground. The AD8226 can sense slightly below ground, and that is probably why the setup is working. 

You can also use this design tool to ensure that you operate within the limits of the in amp:

Instrumentation Amplifier Diamond Plot Tool | Analog Devices 

Thanks, 

Kris