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it may sound a bit stupid, but the thing is that I want to cancel the noise caused by AC frequencies in a circuit. I know I have to use a low pass filter, but which cut-off frequency should I use?

I'm confused because the DC current does not suppose to have any frequency right? Then the lower the cut-off value is the better, but until what value?

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    \$\begingroup\$ How is it getting picked up in the first place? And how much? \$\endgroup\$ – Trevor_G Jan 25 '18 at 10:40
  • \$\begingroup\$ agree Trevor_G it is better to eliminate rather than attenuate \$\endgroup\$ – JonRB Jan 25 '18 at 10:45
  • \$\begingroup\$ There's a decent question here, but this isn't it. How much noise do you have, and how much would be acceptable? That tells us the attenuation (reduction) you need. What range of frequencies do you need to preserve? That tells us how steep the filter needs to be. Add that information to the question. \$\endgroup\$ – Brian Drummond Jan 25 '18 at 10:46
  • \$\begingroup\$ Thanks for answering. The thing is that I have a current source that provides current up to 200mA and I want it to be very low noise. I have been trying to measure the noise in an oscilloscope, but it is very confusing, so I don't know how much noise I have. Any idea about how to measure it? \$\endgroup\$ – Martin Jan 25 '18 at 11:07
  • \$\begingroup\$ @Martin that sounds like a totally different question to the one above. \$\endgroup\$ – Andy aka Jan 25 '18 at 11:20
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which cut-off frequency should I use?

Well if you are in the USA you would use 60 Hz and in other parts of the world it would be 50 Hz or 60 Hz but, that doesn't mean that the induced interference is at 50 or 60 Hz - it could be double this if it comes from the output of a bridge rectifier feeding (say) an amplifier. So, you need to establish what precisely the interfering frequency is.

Nobody here can tell you what that frequency is, only you can.

I know I have to use a low pass filter

Normally this is done using a notch filter because it targets the specific frequency.

But, unfortunately it doesn't end here and it could be more complex than just filtering a single frequency. There could be harmonics (multiples of the AC or rectified AC frequency) and these can cause borader band interference.

Bottom-line: analyse what the problem is then work out where the noise source is then take remedial action with filters, shields, cancellation techniques and increased spacing/segregation.

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  • \$\begingroup\$ +1 Assuming the noise is real and not just a scope artifact or lack of ground. \$\endgroup\$ – Trevor_G Jan 25 '18 at 11:15

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