When the positive terminal of the oscilloscope probe touches my skin, I see a 60Hz sine wave. As far as I understood this is the electromagnetic buzz coming from the AC wires in the wall and is being picked up by my body.

I can also see this wave if I grab the ADC input of my ATMEGA328 with my fingers (when the input is floating/not grounded), but it also picks up any hand movements close to it, so as far as I understood I need to ground it as well, but then the signal appears so weak when it's grounded, that the ADC is not picking it up.

I decided to try to amplify the signal with UM741, I seem to be getting a bit rounded square wave on the oscilloscope, so I believe it's working:

enter image description here

But now I face a problem with amplification factor:

I read that the op-amp can amplify the signal up to 80,000x and that the:

amplification rate = 1 + (R2/R1)

But when I replace the R2 with 1M resistor, I would expect for the amplification factor to jump from 2x to 27x, but the output is exactly the same.

PS: My negative input terminal on the op-amp is grounded to the power supplies negative terminal, is this correct or should it be grounded to my body since I'm taking a reading from it?

  • \$\begingroup\$ 60kHz? Shouldn't that be 60Hz? \$\endgroup\$
    – Andrew
    Jan 25, 2017 at 10:21
  • \$\begingroup\$ @Andrew sorry my mistake, thanks for pointing it out, fixed \$\endgroup\$
    – Kristian
    Jan 25, 2017 at 10:22
  • 1
    \$\begingroup\$ Your body probably doesn't look like a source of a constant voltage, so you're not going to get easily explicable results like this. If you're trying to learn about op-amp gain, feed in a signal from a signal generator or an audio amplifier (headphone output on phone?) \$\endgroup\$
    – user1844
    Jan 25, 2017 at 10:24
  • 3
    \$\begingroup\$ "My negative input terminal on the op-amp is grounded to the power supplies negative terminal" - surely you have it connected as per your diagram. BTW the 741 isn't recommended for supplies lower than +/- 10 volts. It's a piece of crap too! \$\endgroup\$
    – Andy aka
    Jan 25, 2017 at 11:40
  • 2
    \$\begingroup\$ @Andyaka I wonder whether we can have an automatism that checks for a few ancient Opamp types on question submission and automatically alerts the asker that "hey, unless you want to ask about a historic circuit..." as well as displaying an (unobtrusive, yet clearly visible block) below the question informing potential future readers that they should probably not recreate the circuit. \$\endgroup\$ Jan 25, 2017 at 12:25

1 Answer 1


Here is your schematic:

This shows a + and - supply, and the left end of R1 grounded. But they you say:

My negative input terminal on the op-amp is grounded to the power supplies negative terminal

There two statements contradict each other, so now we don't know what you really have connected to what.

It should be connected as your schematic shows. You want the input signal amplified about ground, which should also be near the center of the power supply range. The scope trace you show hints that the amp is clipping on the negative portion of the square wave:

This shows no noise ripple at the bottom level, but does at the top.

Further discussion is pointless while you're contradicting yourself. A better answer requires a better question.


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