In search of a better line driver
From: Chris Carlen (crobc_at_BOGUS_FIELD.earthlink.net)
Date: Thu, 03 Jun 2004 18:40:30 -0700
Here are some of the ways I have driven 50 ohm coax cables with TTL
level logic signals:
1. I have used 74HC14, 74AC14, and 74ACTQ14 devices to drive 50ohm
cables, with reasonably good results. I have tended to use one gate in
the package to drive several others in parallel, with a back terminating
resistor tuned to match the line Z, minus the output Z of the parallel
gates. This tends to produce some bumps after the edges that I'd like
to look better, mainly when driving an open line. Also, unless many
gates are used, lot variations in the output impedances of the gates can
cause substantial variations in edge quality from lot to lot.
I haven't tried using only a 74HC14 package with some parallel gates,
without one gate as pre-driver. Maybe this would be better. Ultimately
I want to not use such a large 14-pin package.
Also, I don't think it would be wise to use one 14-pin package to drive
multiple channels, since the ground and VCC bounces from one channel
feed a little bit into the others, except for very non-critical
2. I have used TC4426A MOSFET drivers as line drivers. They are very
good at this, but too slow for some purposes. I'd like to keep things
at least as fast or faster than HC CMOS.
3. Lately I have tried the Fairchild TinyLogic NC7WZ14 UHS dual Schmitt
inverters. I tied the two gates from one package in parallel, and again
used the back terminating R. I made a really nice layout on a 4-layer
board with a pair of 0.1uF 0805 bypass caps on either side of the package.
Boy this chip works great! It makes simply marvelous edges into even
the unterminated line.
But there is one problem. The thing is so small I can hardly solder it.
It sits on pads just a few tenths of millimeters per side. For the
first and only board on which I have used them, I had another tech.
assemble it who did a lowsy job, giving it back to me after apparently
no inspection. Today I bitched and moaned for an hour while I fixed the
poor solder joints. But they are almost impossible to solder well by
hand. Well, things might be better if I had the proper illuminated
magnifier, but I don't. Also, I was using 0.015" wire solder, which is
still too much to meter out consistently at this size.
Actually, I have a little general purpose line driver board I made using
the TC4426/7 chips, and had 100 of them assembled by an outside
contractor. It's only 0.65" x 0.75" and plugs into any board needing
line drivers, sparing me from re-evaluating the driver design for every
new situation. It simply includes everything: ESD and OVP protection,
LED driver, and the line driver. Expensive, but that's Ok for my lab
environment if it saves me time. These are cool, and I intend to do the
same perhaps with the Fairchild chips.
But if I could find a SO-8 packaged chip with similar specs to the
Fairchild UHS stuff, that would be almost ideal.
Are there any fast, 5V powered line driver chips in SO-8 or a package at
least a little larger than the 2.00mm x 1.25mm package of the Fairchild
Thanks for comments.
-- _____________________ Christopher R. Carlen firstname.lastname@example.org Suse 8.1 Linux 2.4.19