[This is a re-post. I deleted the first one due to a profile error.]
I am trying to compute the output noise of for a Butterworth filter built using Sallen-Key stages. I reduced the circuit to the following voltage follower to investigate incomprehensible noise results.
R3D = 1.02Meg noise is 83300 nVrmsR3D = 1.05Meg noise is 1.67 nVrms (a significant decrease for a 3% increase in resistance)
So, noise decreases with increased resistance? That's the opposite of what I expected. The noise gain has not changed, so op-amp voltage noise contribution should not change; op-amp current noise should cause higher noise with increasing resistance values; resistor thermal noise should also result in more output noise with a higher R3D.
I attached the .asc file.
6507.Butterworth-8th-Opt-Voltage - D stage noise.asc
Does anybody see what I am missing? The simulator results make no sense to me. Thanks.
Hi Ross: The input is clipped against the V- = Gnd. Give the pulse generator a non-0 start value, so 0.5V or something like that.
Thanks Glen! Spice is new to me. I was not sure how the noise directive works and my search did not turn up anything useful. The numbers make much more sense now.