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The noise increased at 0.5 Hz and is pretty big. This might explain the loose of coherence at low frequencies.
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This is because spectrum analyzer did not plot the real noise spectrum at the first few points at low frequencies. I've remeasured the noise at 1mHz - 3Hz at "output -" (TP9) and compared it to the seismometer signal

The noise seems to be much less then the signal. I've measured the noise several times and once I got a huge amount of noise

I made another measurement in some time and got the low noise again. A circuit might have a bad contact somewhere.
The plan is to change AD620 adjustable resistor (R2) from 5.49kOhm to 500Ohm to increase the gain from 20 up to 200. |