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chongjian
chongjian's picture
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Last seen: 3 years 1 month ago
Joined: Oct 25 2017 - 2:05am
741 amplification factor doesn't change

Hello, 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(http://www.kynix.com/Detail/113507/UM741MJG.html), I seem to be getting a bit rounded square wave on the oscilloscope, so I believe it's working:
https://i.stack.imgur.com/HnIuD.png
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?
Thanks

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