Electronic – How to a voltage follower op amp amplify

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)

I went to a break out session titled "Top 10 Op Amp Mistakes" at a conference not long ago, given by an Analog Devices FAE and this scenario came up. The first option with bypass caps from both power and the ground plane to the virtual ground will be better. Ideally, you'll want to have several caps each, say 10uF, 1uF, 0.1uF, even better, in progressively smaller packages. Of a 0603 and 0402 cap of the same value, the 0402 cap will have the higher SRF. The idea is to extend the SRF of the total capacitance out significantly further than any signal of interest.