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very low frequency oscillation with OPA1654

Other Parts Discussed in Thread: OPA1654

Hi,

my circuit has a strange behaviour that I can't explain (and get rid of) but I think there is a chance that the source of trouble is the circuit around an OPA1654.

The circuit is a sensor signal amplifier with the sensor signal being an about 10 mV 4Hz sine wave. See the schematics for details. The signal is then rectified and low pass filtered ie a DC signal is generated from the amplitude.

The strange behaviour I see is a sinusodial variation in amplification of the first opamp with a period of 125sec, which is 8 milli Hz.resulting in a sine wave on the DC output signal. This is very small, just a few 1e-3 of the DC value but enough to be too much for my purpose.

I couldn't find any reason why the sensor should oscilate with this frequency nor which external cause could influence the circuit in this way. So I suspect the opamp - but need some help to find a reason here too ;-)

Any idea is welcome, thanks a lot

Martin