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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.

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  • \$\begingroup\$ Not sure how complicated your schematic is, but for schematics that are really not too complex like this one, a screenshot uploaded to an image host works well. Once you hit 11 rep (which should be soon, it's basically to ensure that you're not a spammer) you can upload images to our dedicated hosting provider by pressing ctrl-g (for 'graphic') or by clicking the image button in the editor. We've toyed with the idea of attaching PDFs or schematic files, but so far screenshots have proven the most flexible and readable. \$\endgroup\$ – Kevin Vermeer Mar 12 '12 at 2:18
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    \$\begingroup\$ 76Hz can be sampled directly after the initial amplification and an EMI LPF with a µC (built-in 10-12bit ADC), then one may perform bandpass filtering and peak detection in software. \$\endgroup\$ – tyblu Mar 12 '12 at 3:33
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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…

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    \$\begingroup\$ Thank you bigiain. I thought of this earlier and wired the second amp as a non-inverting amp but it didn't help. After reading your post I just did it again and it worked great. I'm guessing I mis-wired something when I tried it earlier today. Your nudge got me to give it another try so thanks. The help is much appreicated. \$\endgroup\$ – FrJ Mar 12 '12 at 2:56
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Perhaps the signal is less than the 125µV input offset voltage. A chopper/auto-zero amp has very low VOS. Also, perhaps the drive strength is much lower than the input impedance of the amplifier circuit. This can be increased with proportionally larger resistors at the expense of thermal noise, or by using a higher impedance configuration amplifier circuit (buffers, instrumentation amp., etc.).

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  • \$\begingroup\$ Thanks for the reply. I will investigate your suggestions further and give this thought. Much appreciated. \$\endgroup\$ – FrJ Mar 15 '12 at 17:59

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