I have a small voltage drop (about 20-25 mV, DC) on a sense resistor (0.180 Ohm) and I need to amplify this with an op-amp to reach 2-3 V (DC). The signal is not clean and the noise, with spikes, amplified generates oscillations making the circuit not working properly. Spike and noise Spike and noise2
How can I reduce or filter the spikes and have a more linear signal?
Thanks in advance for your help.

  • 1
    \$\begingroup\$ First single-trial ... sense resistor with a paralleled capacitor (something as 100nF -> 1 uF ?). Followed by R-C filter. \$\endgroup\$
    – Antonio51
    May 14 at 10:26
  • \$\begingroup\$ How fast is your (useful) signal changing? Can you tolerate a huge filter wich averages e.g. 1s of the waveform? \$\endgroup\$
    – akwky
    May 14 at 21:13
  • \$\begingroup\$ This signal, after amplification, must be compared (continuously) with a V ref by LM393 comparator and in case it will be greater the source is interrupted. The spikes amplified disturbs the circuit and false trigger the comparator. A circuit that filter and averages the waveform could be a solution, but I do not know how to build it \$\endgroup\$
    – philfs
    May 15 at 11:30

1 Answer 1


If you are not doing so already, you can use an instrumentation amplifier to buffer the signal from the sensor. Instrumentation amplifiers are low-noise and have a high CMRR. Also, determine the bandwidth of the signal that you wish to capture and design a corresponding low-pass filter. If the spikes are due to RFI then an RFI filter can be added as well.

  • \$\begingroup\$ Instrumentation amplifier seems to be an interesting choice. Keep searching about that, I found that a sense resistor to high-side with a In-amp could reduce the noise instead of having the R sense to low side (with ground interferences). So + input and - input of in-amp go on Rsense and it makes the difference between two voltages (Rsense drop), then it amplifies this, right? Now I have to try this and see how it works. \$\endgroup\$
    – philfs
    May 15 at 11:47

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