Toby,
I rarely have to look this up, but it is my understanding that the data sheets usually (if not always) list the input and output impedance of the pins. It's based on a whole bunch of things within the part and the lead size itself. My thought would be to check there if that is what you need. But more importantly, check to see if the part (signals) specifies a specific impedance needed on the traces themselves.
It takes a lot of energy to charge a transmission line, and very little of that energy is needed at the load. There is a big impedance to that extra energy once it reaches a load, and that's what causes the positive reflections. In effect, the load says "thanks but I don't need all that energy, so please go back where you came from". If the parts/signals are slow, or have have large operating limits as to what they can handle, then an impedance may not be specified for the traces. If the signals are faster, then the trace impedance may well be specified and in that case, that impedance should be kept as uniform as possible across the entire length of the trace. But other factors may be an issue too, like long trace length or traces changing layers to find a different impedance there. In that case, you may need to add termination devices to absorb that extra energy and keep it from reflecting back and forth messing up your signals.
All this means that there is almost always an impedance mismatch between the input and output impedances of the parts and the trace impedances on the boards. It's what (if anything) that needs to be done about it that is the question. All this has to do with "normal" high speed boards. Once you get into screaming fast signals there are even more things to consider.
Susy
---- Jack Olson <[log in to unmask]> wrote:
> Unless you are talking about something more advanced that I don't know
> anything about,
> I don't think you have to calculate the impedance at the chip level. In
> fact, I think digital
> devices are almost always shooting for 50ohms, but if not it should say on
> the data sheet.
> (For example, lots of Op-Amps are more like 10ohms)
>
> What component are you worried about? Or is this just a general question?
> Jack
>
>
> .
> On Tue, Apr 20, 2010 at 8:39 AM, Toby Carrier <[log in to unmask]>wrote:
>
> > Hi all,
> >
> > This may be a repetitive question but, is there a rule of thumb or a good
> > way to calculate the input and output impedance of an IC? I know there are
> > measurement techniques, but how can you get a proper trace impedance if
> > this
> > info is not know from the start? I appreciate the help.
> >
> > Toby, CID
> >
> >
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