Re: [SI-LIST] : Zo for HCT

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From: Mike Degerstrom (degerstrom.michael@mayo.edu)
Date: Fri Oct 22 1999 - 06:37:35 PDT


Johnni,

On Oct 22, 2:37pm, jfr.gaab@velux.com wrote:
> Subject: [SI-LIST] : Zo for HCT
>
> Hi all,
>
> Can one find Zo of the 74HCT245 from: HIGH (typ): (Vcc-Voh)/Ioh =
(4.5-4.22)/6m
> = 30 ohm,
> LOW (typ): Vol/Iol = 0.16/6m = 26.67 ohm ?

I think the general answer is that your equations listed above
are not valid. However, I also think your impedances are fairly
close to what I would define for the 'effective' output impedance
for most '245 parts.

When I want to use a buffer such as the '245 in a design, I order
a few parts, put one on a small pcb with good quality ground AND
power planes (with a decoupling cap). Next I add a small high frequency pot
as an adjustable series resistor between a '245 output pin and
a short section of 50 ohm coax. The coax is then put directly into
a high impedance channel of an oscilloscope. I then adjust the
pot to get the waveform to switch as cleanly as possible from
GND to VDD.

Looking back through my notes, it appears that we were actually
testing a 74LVT244 part. The pot was adjusted to 32 ohms for the cleanest
output into a 50 ohm coax. With the pot at 32 ohms there was a
little undershoot on the LOW to HIGH transition and a little overshoot
on the HIGH to LOW transition meaning that the HIGH state impedance
was higher than the LOW state impedance.

One could say that the effective output impedance of the '244 is
about 50-32=18 ohms. However, I would be suprised if these
outputs have a linear impedance response with respect to the
characteristice impedance of the interconnect.

> Is it possible to use the Zo of the 74HCT245 as source terminator for a trace
of
> 30 ohms ? Is it wise ?
> Should one choose an other trace impedance ?
> Tracks are on a daughter card and will be both point to piont and point to
> multipoint (databus),
> the CPU on the mainbord is running 33 MHz clock.
>

You can use a lower impedance (than 50 ohms) to match your '245 but
you have to use the technique I mentioned above to determine the
matching impedance. If you have IBIS models, then you can simply
simulate using trial and error to determine the best Zo to achieve
the cleanest signals.

> Is there a better way to terminate tracks (74HCT245 can't drive an end
> termision) ?
> - e.g. choose an other device family and use end termination.

You can still get clean signals using the approach that I mentioned
above. End termination will collapse the swing and I assume your
inputs were design for full-swing voltage.

Mike

>
>
> Regards,
>
> Johnni
>
>
>
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>
>-- End of excerpt from jfr.gaab@velux.com

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