[sdiy] measuring pulse width with ns accuracy...

[harrybissell]

Hej Magnus (et al)

who wrote:

> Like all tools, it has its limits and you really must understand these and
> be sure they don't stab you in the back. As for the 4069 example, you must use
> the correct Spice model. If you use a model which has sufficient detail about
> the innards, i.e. with the FET-transistors, Spice would probably be fairly
> accurate. If you only use a digital gate model you would be out in the blue.
>

Absitively !!!  You can, in many cases... draw a model using the transistor level
schematic of the CMOS part. You don't KNOW what the actual transistor parameters
are, and they won't tell you... but it is WAY better than the normal CMOS models
which are strictly behavorial and do NOT interface as, or with real analog
circuitry.

I have some good circuits that work in real life... and spice says they won't.
Usually
the models are no good.

That said... here's a tip in the other direction.  I use a "three way voltage
summer"
as an opamp.  Just invert one input and give an output gain of whatever (10KV/V)...
It has infinite input impedance, zero output impedance, and zero offset etc...  Not
real
accurate, but fast as hell in simulation.  I use it when I don't CARE about the
nitty details
(like a buffer, or good-enough gain stage....)

Try it.

H^) harry