I am working on a data acquisition system project . I have used SMPS power supply and two pressure sensors(-5V to +5V input) and (0-20mV o/p) my analog circuit consists of one op-amp(OP07) and one instrumentation amplifier(AD620) Output of circuit after signal conditioning is given to 16 bit ADC (ads1115). output of sensors show lot of fluctuation so i have used this low pass filter (Image attached) if anyone could please explain how this filter works and what is GAIN of this filter it would be of immense help.

Thank you .enter image description here


1 Answer 1


if anyone could please explain how this filter works and what is GAIN of this filter it would be of immense help

Try looking up sallen key low pass filters. Your circuit is a slight variant on the general theme of these types of filters (R14 and R15 slightly modify the frequency response). To explain how this circuit works requires extensive understanding of 2nd order filters and I think the detail is too much for this Q and A site. The DC gain is 1 + R16/R17 i.e. 2 when R16=R17.

Anyway, if you want to wade through the math take a look at this: -

enter image description here

The upshot of this is that the derived formula has the same "mathematics" as a standard 2nd order low pass filter: -

enter image description here

And, can produce a transfer function bode plot like the following: -

enter image description here

So, depending on the component values you can affect Q (or zeta) and have a smooth pass band or quite a peaky passband.

  • \$\begingroup\$ This is my first pro project and first post on stack exchange !!Thank you so much for the answer ...some follow up questions 1) do 390 ohm and 7.5k ohm need to be used 2) can i replace 180 k ohm with other resistor without altering the working of the circuit . \$\endgroup\$
    – Mr.Sky
    Jun 27, 2016 at 18:20
  • \$\begingroup\$ Please check out the edits I have done. \$\endgroup\$
    – Andy aka
    Jun 27, 2016 at 18:22
  • \$\begingroup\$ I just saw the edit and understood working completely .Thank you..Last and most important Question Q) i am going to use this in India so frequency is 50 Hz will this circuit be useful to minimise fluctuations and minimise noise ? \$\endgroup\$
    – Mr.Sky
    Jun 27, 2016 at 18:36
  • \$\begingroup\$ Impossible even to guess at. \$\endgroup\$
    – Andy aka
    Jun 27, 2016 at 21:16
  • \$\begingroup\$ What you need is a notch filter for 50Hz removal. Not low pass. \$\endgroup\$
    – ammar.cma
    Sep 24, 2016 at 19:37

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