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In several places I have seen that sallen key low filter stages are cascaded together to create multiple order filters. Each opamp connects to the next via resistors. This means that DC voltage is simply passed to the next stage. Shouldn't this be impossible since op amps have offset voltage and this will lead to errors in subsequent stages?

Putting in a capacitor does not seem reasonable since for a low pass filter.

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  • \$\begingroup\$ How small does the offset have to be before it wouldn't concern you? \$\endgroup\$ – The Photon Jul 26 '16 at 23:29
  • \$\begingroup\$ Worst case is you install a servo op-amp that feeds back an inverted version of the error voltage, much like high-performance audio amplifiers do. What little error or drift over time there is gets cancelled out. Final output has virtually zero offset error. \$\endgroup\$ – Sparky256 Jul 27 '16 at 1:03
  • \$\begingroup\$ The sallen key filter actually gets input from Audio DAC and output connects to LM386. I do not think the dc offset matters. But since I want to understand how exactly offset works and when I need to worry about it, I have posted this question. \$\endgroup\$ – quantum231 Jul 27 '16 at 8:01
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No, it's often okay to cascade op-amp filter stages.

  1. Op-amps can be chosen to have very low offset voltages. Input-referred offsets can be on the order of microvolts.

  2. Filters are often designed with low (or unity) dc gain, so the output offset is not much larger than the input referred offset voltage.

  3. The offset voltage from device to device is not correlated (assuming they are not multiple op-amps on the same chip). So the typical offset only grows from stage to stage according to the root-of-sum-of-squares of the offsets of the individual stages.

    Of course, the temperature drift of the offset might all be the same if you build your stages with the same op-amp type. If that's a concern you might want to use an inverting design so that the drift of each two stages is likely to cancel each other.

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  • \$\begingroup\$ I see, so offset voltage is a problem only when we are amplifying signals. Ok. \$\endgroup\$ – quantum231 Jul 27 '16 at 7:59
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    \$\begingroup\$ @quantum, really offset voltage is an issue when the input signal is small or when we need very good accuracy in the output. Whether the circuit is amplifying or not is secondary. \$\endgroup\$ – The Photon Jul 27 '16 at 15:28
  • \$\begingroup\$ thank you sir, could you kindly elaborate what you mean by very good accuracy in this context with an example? \$\endgroup\$ – quantum231 Jul 27 '16 at 23:42
  • \$\begingroup\$ In this context "very good accuracy" means good enough that you need to worry about offset voltage of your op amp. \$\endgroup\$ – The Photon Jul 28 '16 at 0:35

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