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Re: Isn't Slew Rate a bit like THD?

Mfc, I think that there is some confusion as to what TIM is. It can be related to harmonic distortion, BUT harmonic distortion can also be generated by a non-linear transfer function, including crossover distortion and voltage sensitive capacitance built into every bipolar transistor and FET. Graduate study of nonlinearity goes into what happens when you operate solid state devices above 100KHz or so. Under these conditions, the nonlinear capacitances between the base-emitter-collector, for example, are dominant sources of distortion. Spice was invented at UCB at the time I took courses on non-linear effects like this, by the same professors who taught it to me, longhand. The Spice model will need to incorporate these effects, in order to be accurate at higher frequencies. Before Spice, we used ECAP, developed by IBM. It would not simulate non-linear caps, and was therefore much more limited in assessing distortion in active circuits. I am not an expert in Spice modeling, but I am reasonably sure that it is generically accurate, at least.
Now back to TIM. TIM is an op-amp dominated kind of distortion. To get significant TIM, you have to have a dominant pole within a feedback loop that is significantly lower than the working bandwidth of the system. Therefore, if you have an open loop bandwidth (dominant pole) of 1MHz, then you won't have any TIM distortion below 1MHz. However, you can have plenty of harmonic distortion that is frequency sensitive, due to non-linear cap effects. Is this making sense to you? This is off the top, so there may be some missing pieces that I might need to fill in. Keep up on doing your measurements. I'm glad to see some folks measuring or modeling something around here ;-)

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