filter
I am trying to make a fourth order passive RC low pass filter. I simulate it in Multisim but find that the filter creates THD. And the higher the order of the filter, the larger the THD. I can`t figure out why.
Here is the schematic:
simulate this circuit – Schematic created using CircuitLab
I think, if you assume that the internals of the op-amp contain, in effect, a first order low pass filter, you will have created for yourself what is known as a multiple feedback low-pass filter. I used a great simulation tool from Mister Okawa here to produce this: -
If you look closely at the circuit in the picture above and imagine that R2 and C2 are inside the op-amp, your circuit becomes the same. There is some hand-waving here because I'm taking a stab in the dark about what R2 and C2 actually are and "massaging" them numerically to fit close to producing a peak near 800 Hz.
C2 being 5pF is in the right order for the "conventional" stabilizing stage inside an old-fashioned op-amp like the TL081 and I guess R2 would be in the vicinity of a few kohm.
Anyway I'm convinced this is what is happening!!!
The filter is both a differential filter and a common mode filter. That is why the caps are configured the way they are. Think about both inputs being joined together - the two caps to ground service the common mode filter function.
Best Answer
I'll paraphrase what Dave Tweed says.
If you input a sinewave then there is no misshaping of the signal just an attenuation that is dependent on how high the input frequency is. If you input a square wave, the output will be misshaped because the harmonics (that comprise the square wave) will be attenuated at significantly different levels and you'll likely start to see an output that more resembles a sine wave or triangular wave.
Here's what a 1kHz square wave looks like when it is filtered with the same circuit as the OP has in his question: -
The input is 1Vp-p centred on 3V dc and as you can see the square wave is nearly triangular.