I've attempted to create a high-speed (200kHz) strain-gauge (Wheatstone bridge) amplifier system through a simple amplifier scheme using an AD8421. The input has a maximum voltage of 10mV, so I chose an amplification of 991 to achieve +/- 10V output signals. I made a simple drawing of the schematic as I have it:
I expected this design to have some flaws, but to basically function. However, I cannot get this to work.
If I solder everything as in the scheme above, and leave the V+ and V- leads unconnected, I get a couple of mV over the inputs. At the AD8421 this already increases to approximately 400mV, leaving the output voltage (between V_ref and V_out to saturate to a little above 10V). Thinking I might have soldered something incorrectly, I disconnected the inputs to the AD8421. I checked that indeed the input voltage (pin 1 - 4) is exactly 0V. Still, the output fluctuates heavily and is around 600mV.
I use a transformer, converting 230V AC to +/-12V. I have referenced the grounds in the schematic to the 0V pin of the transformer (DC side). I also tried referencing to the ground of the AC side, without effect.
I don't have much background in these matters, so I might have overlooked something trivial, but to me this is not the behavior I wanted to achieve. Can anyone advice me on what I'm doing wrong, or how I can proceed in finding the mistake?