I'm trying to estimate the SNR of a circuit, which includes measuring the noise power of an analog trace. Right now, I'm attempting to measure the noise with a high bandwidth scope probe, ac coupled, low inductance ground leads, with 1x attenuation. However, I want to attempt to remove the RMS noise induced from the probe itself.

tldr: Is it possible to subtract the RMS noise measured with a shorted scope probe from the RMS noise measured of an analog circuit trace for a more accurate measurement of the actual RMS noise on the trace?

  • \$\begingroup\$ Sure. But noise power does not add/ subtract linearly. You have to use power 2 and Sqrt 2 for noise power. That's why 4 LNAs in parallel have a better snr by x2 than a single LNA. \$\endgroup\$ Aug 2, 2019 at 19:56

1 Answer 1


You need to subtract the squares of the RMS voltages then take the square root to obtain the noise of the source. However, this is only accurate if there is no coherence between sources.

  • \$\begingroup\$ Ah I see, thank you very much Andy! Do you think it is safe to assume the noise captured from the shorted oscilloscope probe is incoherent from that of the PCB trace? I don't see why not but I might be missing something. Thank you! \$\endgroup\$
    – powerboi95
    Aug 2, 2019 at 19:52
  • 1
    \$\begingroup\$ There’s every chance it’s incoherent but I had to mention it. \$\endgroup\$
    – Andy aka
    Aug 2, 2019 at 20:53

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