How to eliminate 50 Hz noise from amplifier’s output

filternoiseoperational-amplifieroscilloscope

My goal is to amplify signals with a frequency from 100Hz above. My circuit is looking as the picture below. With an oscilloscope, I am trying to measure the output of the amplifier.
If the input of the circuit is not connected at all, I can see a small 50Hz signal. Touching the input gives me a huge 50Hz signal which comes from the AC power supply from around the room. But I expected them to be eliminated due to the capacitor added.

I know that the oscillator's probe behaves like an antenna. Maybe it induces the noise into the feedback of the amplifier? But then, I don't understand why touching the input increases this noise.

inverting opamp and high pass filter

Best Answer

Your filter is a very mild one (single pole) and won't have any dramatic effect on the mains-frequency pickup. Also, trying to get a gain of 1000 in one stage is a bit demanding on the op-amp. Here is what the response of your circuit (built with an OPA177) looks like with a 1mV input. The gain is 1000 at 300Hz, and above that the op-amp pole knocks it down.

enter image description here

If you really want to seriously knock down response at 50Hz (rather than reducing it as much as possible, which should be your first thought) without affecting the 100Hz signals much you'll need a much higher order filter.

Ignoring the op-amp rolloff, below is the shape of the response of the filter:

enter image description here

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