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Below is a Sallen-key type active LPF which filters out all the ac components and integrates(averages) the incoming 1kHz 1V pulse train.

enter image description here

There are tools to design these filters, but my question is about the op amp part:

If I change this op amp with any other single supply op amp in LTspice I get totally different results. I must say Im not into details of op amps and active filters so please answer as to a beginner.

Why is any generic op amp doesn't work for this circuit? What determines us to choose this particular op amp here? What is so special about LM358 here?

Edit:

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  • \$\begingroup\$ You need to put an offset on your input, or use a dual supply op amp \$\endgroup\$ – Scott Seidman Jun 16 '17 at 12:36
  • \$\begingroup\$ What amount of offset and why? \$\endgroup\$ – user1245 Jun 16 '17 at 12:44
  • \$\begingroup\$ Vcc/2, for a start. Look at the data sheet for the op amp you select, and look at the allowable range of common-mode input range. If its an "idealish" rail to rail on the input, the voltage on the input terminals can't go above or below the rails. \$\endgroup\$ – Scott Seidman Jun 16 '17 at 12:48
  • \$\begingroup\$ i was expecting a more informative explanation. \$\endgroup\$ – user1245 Jun 16 '17 at 12:54
  • \$\begingroup\$ Many op amps that are rail to rail on the input only allow inputs that are, say, 150mV of the lower rail. \$\endgroup\$ – Scott Seidman Jun 16 '17 at 13:08
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The LM358 works with inputs that can range from 0 volts (negative supply rail) to Vcc-1.5 volts and, this corresponds with the signal you are feeding the circuit so no problem there. The LM358 output also works pretty much down to 0 volts so there shouldn't be any problem there either.

So, if you tried a different op-amp and it didn't appear to work correctly it's highly likely that its inputs and output do not get close to 0 volts. Not many do but rail-to-rail op-amps can be fairly well relied upon to do what they say on the tin.

Alternatively power the op-amp with a +/- supply and then everything should be fine.

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  • \$\begingroup\$ Lets focus on single supply operation only. "it's highly likely that its inputs and output do not get close to 0 volts." Could you explain what do you mean by that? Does that mean they might have very large input offset voltage? which parameter in data sheet should i look for if i need to replace LM358? \$\endgroup\$ – user1245 Jun 16 '17 at 12:37
  • \$\begingroup\$ As an example I for example tried LT1677 CAN BE USED Single Supply and it is rail to rail Op Amp which doesnt work. It has even much lower input offset voltage than LM358. But I dont get what is happening. \$\endgroup\$ – user1245 Jun 16 '17 at 12:42
  • \$\begingroup\$ Common mode input voltage range is the parameter. Input offset voltage is something different entirely and relates to how slightly different the two inputs are in terms of there being an inherent offset between the two. Output range is the other parameter. \$\endgroup\$ – Andy aka Jun 16 '17 at 12:49
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    \$\begingroup\$ The edit brings up the difference for sure - but it's only relevant for the small input voltages. The circuit doesn't integrate BTW, it filters and without a picture of things there can be no further assistance. All I can say is try a small negative rail to see if things then become clearer and try and find an op-amp that has a lower output swing towards 0 volts (if you need a single supply). \$\endgroup\$ – Andy aka Jun 16 '17 at 13:47
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    \$\begingroup\$ Maybe the signal at the non-inverting input wants to go a little bit below 0 volts in the normal operation of filtering. Check that out. It's hard to say without a proper screen shot of the sim. \$\endgroup\$ – Andy aka Jun 16 '17 at 14:00
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I know this is counter-intuitive, but a low pass active filter requires an op amp with good high-frequency response. You are actively filtering out the high frequency (via C1) and many (most?) newer op amps are low power with correspondingly low bandwidth. These will be poor choices for active filters, when filtering signals with high frequency components (like square waves). Check the gain-bandwidth product (sometimes listed as unity-gain bandwidth).

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