I am using two AD8220 in a closed loop system. First InAmp has the gain of 32 and it amplifies the output of a Wheatstone bridge and then the output of this InAmp it is fed to the second InAmp which has the gain of 2 and the output of this InAmp provides the supply voltage of the bridge.
When the bridge is in the balanced condition everything is fine and I am getting the proper results. But the problem is when I increase the input voltage of the 1st InAmp (G=32) which is still closed to the balanced condition. At some values the output of the InAmp (G=32) oscillates!
According to the datasheet I just added 47pF capacitor in parallel with the bridge to prevent ringing at the output of the InAmp (G=2) but this did not solve the problem and I still have that oscillation. So I am increasing the input of the InAmp (G=32) and the output increases but at some point it starts to oscillate! the oscillation signal is a pure sine wave with a fixed frequency around 1.3 KHz and variable amplitude somehow proportional to the increase at the input.
These are the spec. of the circuit:
Single Supply= 16V
Vref = 1V
RFI filter @input of both InAmps according to the datasheet
Thanks for your help.