Signal acquisition circuit

[Muna Hamdan]

Hello, Im working on a design for signal acquisition circuit, the simulation works fine, but when i built the circuit, it seems that it's highly affected by any movements neraby, the output doesnt show up on the oscilliscope, but when i take a cetain position or move my hand over it, it does! then disappears as soon as i move again. -when it does show up, im sure it's my signal, because i can contol its frequency and amplitude from the function generator. my question is how to stabilize the output?

Im using instrumentation amplifier in the pre-amplification stage, then another op-amp for band-pass filtering, then a third op-amp for the main amplification, and the problem seems to be in the last stage cuz i can detect the signal fine at the previous ones.

 

[dorke]

Welcome to EP.
Can you please post a full diagram of the circuit.

 

[Muna Hamdan]

here it is

 

[dorke]

1. What frequencies do you expect to pass through capacitor C3?

2. What about biasing U3A as you did with U2A ?