Electronic – Hand contacting op amp photodiode circuit causes interference – what ist the physic behind it

amplifierbiopotentialinductionoperational-amplifierphotodiode

I am new to analogue electronics and I am still learning about op-amps.
I use the most common circuit for amplifying a photodiode's signal to measure it with an ADC:
Default photodiode amplification

Amplification gain is 1M, the 22pF cap in parallel is for damping oscillating noise when the impulse comes in. I got some 50Hz noise in the signal due to the diode's cable is a .5 meters long flat ribbon cable, that's a problem, but not for this thread (if anyone knows how to filter that I will listen 😉 ).

Now, when I touch the solder joint on the output of the diode (1), the 50Hz noise gets massively amplified. I don't want this to be gone, it will be isolated later, but I want to know why.

My guesses:

a) I'm a big resistor, adding additional resistance to R1 (1M) and so increasing the gain of the amplifier (keyword biopotential)

b) I'm a big antenna, receiving 50Hz noise of my surroundings and the op-amp amplifies that, so the signals do have different sources.

Thanks for any information!

Some background: I'm a computer science master student writing my thesis about hand gesture detection with infrared measurements through the skin, therefore the measured signals are rather small and need amplification.

[edit]

I was able to measure my own body's 50Hz signal with my oscilloscope, thank you all!

Signal of my body

Best Answer

Your option b is correct.

Your body is mostly insulated from ground, so it acts like a plate of a capacitor. The other plate is the rest of the world, or better, the portion of world somewhat near you.

A part of this other plate is, for example, the mains wires in the ceiling or walls. Since the voltage in them oscillates at 50 Hz, and you and them make a capacitor, then some of this voltage is present also on your body. When you touch a high impedance node, such as the cathode of the diode, this voltage is injected in the circuit, amplified and you can see it on the output.

Try to touch the circuit ground node as well, and you will see the noise decrease dramatically.

And use a shielded cable, not a ribbon cable, or even better put the amplifier next to the diode.

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