I am doing THD of my low pass filter.
Theoretically the THD should change with change in input amplitude.
But in my case it is not.
I am using cadence, and the THD formula from calculator.
Can anyone suggest why this may be happening.
Thanks
If it's a linear passive low pass filter than the THD (as a percentage of the fundamental amplitude) is independent of amplitude.
Why do you think otherwise?
If your filter is built with a rats nest of tangled wires on a solderless breadboard then all the rows of contacts and many wires are antennas that pickup all kinds of interference that is probably what you are measuring.
Oh, you do not have an actual circuit, you have only a simulation that is not real, it is just the dream of a circuit. The THD in a dream does not change with the input level.