instrumentation amplifier negative feedback

[Lincoln]

Hi, I am trying to reject the 50Hz noise by bandpassing the output and feeding it back to the negative input of an instrumentation amplifier. This doesn't seem to work at all. The output is always saturated with 50Hz signal after I power it up. The gain of the IA is set to 50. And I used an narrow 50Hz bandpass filter with a gain of 10. Anybody has any idea why? Thanks in advance.

 

[MikeMl]

I cannot figure out what you are trying to do based on your verbal description. Post a schematic.

 

[crutschow]

Sounds like you are trying to build a 50Hz notch filter with feedback from a bandpass filter. Why not just build a notch filter instead?

 

[Lincoln]

Thank you for your prompt reply. I am in fact trying to pick up ECG signals using capacitive plating. This will introduce a lot of 50Hz noise and the noise level is much higher than the ECG signal. I try to feed it back because the noise will sometimes saturate the output and clip the ECG signal off. I have attached a simple schematics in my previous post. But it simply doesn't work.

 

[MrAl]

Hello,

What does the very output of "IA" look like?

 

[Lincoln]

It's a square wave of 50Hz and 9V when I don't give any input. And it remains the same whatever input i give to the positive terminal.