I built a simple circuit with an instrumentation amplifier (PGA204AP) in order to clean a differential signal from noise. The signal is generated few meters away from the acquiring point and the cable (twisted pairs shielded) pass through a very noisy environment.
The shield of the cable is grounded in the Op.Amp side, and in the other side it is not connected. As you can see, the input of the Op amp is filtered through a differential mode filter (R1 R4 and C2) with the cut-off frequency set to ~60kHz, and a common mode filter (R1-C1 and R4-C3) with the cut-off frequency set to ~1MHz.
At the output of the PGA204 I put a 2nd order low pass filter with the cut-off frequency set to ~500kHz. I tested the circuit with no input, just to see if the noise from the environment would have been attenuated: the noise present in the environment was the following (measured with the scope directly from the cable) :
So clearly there was a very strong noise a 2MHz, but once I connected the instrumentation amplifier, the noise was amplified (more than doubled). So I decided to put this 2nd order filter just to see if this 2MHz noise would have been eliminated. But the result was the same, and this seems very strange to me because two completely independent devices have the same issue. In addition, I tested the device with some noise produced with the signal generator at the input (noise applied at one input, and the other input had it as well thanks to the twisted pairs cable) and the result was the following:
- What I'm not sure about this 10k resistor (R5) between V- input and ground. I've been told to put it for the bias current but doesn't it interfere with the LP filters at the input?
- Is there something else I should pay attention to with these filters? Suggestion for the next tests I should do in order to solve the problem?