Stephen Casey wrote:

> Hello Rene,
>
> Indeed I agree with you. The behaviour was that a previously low distortion
> (<-90dB THD and IMD) op-amp based circuit, when re-laid out on a new PCB was
> displaying distortion at higher than -35dB. Removal of the power rail
> decoupling caps fixed the THD, but is not a very ideal solution! The problem
> was due to the ground returns for the caps being of too high an impedance,
> and the current spikes were injecting garbage into the signal path. I tried
> to simulate the behaviour but the macromodels don't show the distortion that
> I was getting, so I wanted to see if a transistor level model would work
> better for this. It was more of a question of interest than a need to solve
> a problem. The problem solution is easy - one plane for cap returns, and one
> plane for signal ground, joined at one place. That has fixed it.

You'd probably have to construct models for the power supply and ground
environment.  Probably a series resistor and inductor, maybe similar components

for the lead frame of the chip, too.

Jon

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