Electrical – Simple inverting op-amp problem

Looks like mains hum. (I'm assuming the annotations on the graph show you are changing the input gain during acquisition, so that the sinewave is not continuous).

Now we don't know:

• the period of the observed "sine" wave
• the ADC sampling rate
• your mains frequency but you do, so you can work out if that's a possible hypothesis.

For example if you are sampling at 50Hz in a 50Hz mains country, and seeing a frequency less than 0.5Hz, chances are that you are aliasing mains hum down to the observed frequency.

And given the high input impedance opamp inputs, they look like a target for electrostatic coupling, rather than magnetic.

Now, on which input?

If you remove the sensor and short the input to ground, does the noise disappear? Then the sensor connection must be more carefully screened. Or you need a buffer amp at the sensor to reduce the sensor's output impedance.

If the noise remains: reduce R21 to 100k and R66 to 100 ohms. Did that attenuate the noise tenfold? If so, it is the negative input that's picking up. More likely both inputs are picking up noise, since they are both fairly high impedance points.

You can reduce electrostatic pickup by screening : experimentally, surround the amp with tinfoil (and ground the tinfoil to the amp's ground)

There is no reason to use an op-amp just to buffer the input- use a non-inverting configuration, for example with a JFET input op-amp.

If you want to use low GBW op-amps, you can just use more stages with less gain each.

The big problem may be that an input capacitance of only 10pF will have an impedance of about 100K at 150kHz, assuming a sine wave (worse if there are harmonics). There are techniques to get down way sub-pF using discrete parts and bootstrapping.

Minimizing the amplifier cost may not be ideal since cheap fast-ish op-amps tend to want dual supplies. Looking at the problem at a system level may yield a more optimal solution, but for cheap consider jellybean JFET amplifiers.