RE: [SI-LIST] : IC input impedance

Chris Padilla ([email protected])
Tue, 13 Oct 1998 12:41:43 -0700

If you are trying to characterize a pin on an IC, you could fire a TDR at
the pin.
Look at the reflection, subtract out the effects of the transmission lines
(if necessary), and deduce a capacitance from the area under the TDR curve.
The risetime of your TDR will determine the knee frequency and hence the
frequency content of the time-domain signal: 0.5/risetime is a good

You wish to get a first-order approximation (actually, you wanted a simple
but I can't think of one) for the IC, which is a single capacitor. Really,
you do have
other parasitics such as inductance and resistance but for low-speed logic,
this one
cap will do fine. I would defne low-speed logic as a clock below 500 MHz or a
correspondingly 1 ns risetime.

The cap is the only part of the model that stores charge so if we know the
total charge,
we can find the total capacitance. Keep in mind that the TR line also has
a capacitance
associated with it so it must be subtracted off so that you are only left
with the charge
stored in the IC. We should be able to ignore any fringing capacitance, too.

So if we integrate the reflected wave, i.e. the area under the curve, we
get the charge. If
you have the charge, you can get your capacitance.

Of course, this only works if you have a nice TDR handy. They aren't
expensive and
every good lab could use one now and then.

Chris Padilla
EMC Design Engineer
Cisco Systems
>How about the whole range of frequencies over which you have significant
>signals reaching this IC?
>If you choose any one frequency, your results will only be good for that
>frequency as a narrowband solution. If your circuit is wideband or
>digital, the narrowband analysis won't do.

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