I want to experiment with receiving ELF signals. In particular 76Hz. The goal is to attach a coil of a few thousand turns of magnet wire, place it on the ground, and read the signal level. My thought is to chain a couple of low noise op amps (OPA211) to take what I would expect is a few microvolts and amplify it and then run it through a switched capacitor filter configured as a very narrow bandpass filter. I would expect another stage or 2 of amplification and then a peak detector to measure the amplitude of the positive going sine wave.
To start this I wired up 2 OPA211 op amps as inverting amps. 100ohm resistors into the inverting input and 3K resistors for feedback. 100 ohm resistors from the non-inverting input to ground. For initial tests, I am using a signal generator to feed a sinewave of 76Hz at 4mV (minimum for my sig gen) into the 100 ohm input resistor and looking at the output of each amp, I get the expected gain of 30 with only a positive peak and clipping of the negative peak of the signal. When I connect the output of the first stage to the input of the second stage, I get no output from the second op amp. I tried to capacitive couple the 2 stages as well but no change in behavior. I'm sure I'm missing something but after much reading and searching for an answer as well as experimenting, I'm getting nowhere. I would appreciate some help at this point.
I have a PDF of my circuit but as this is my first post I didn't see a way to attach it.
Best Answer
Hard to offer advice without a circuit diagram, but my initial "gut feel" guess is that you might want to check that your two inverting input amplification stages aren't both clipping the negative peaks - one in each direction - if they're both clipping to 0V (rather than some V- below 0V) you'd get what you're describing…