Re: In search of a better line driver

From: John Larkin (jjlarkin_at_highSNIPlandTHIStechPLEASEnology.com)
Date: 06/04/04


Date: Fri, 04 Jun 2004 14:26:22 -0700

On Thu, 03 Jun 2004 18:40:30 -0700, Chris Carlen
<crobc@BOGUS_FIELD.earthlink.net> wrote:

>Hi:
>
>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
>applications.
>
>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
>UHS device?
>
>
>Thanks for comments.
>
>
>Good day!

My favorite is the Onsemi NL37WZ16US, a triple buffer, using all three
sections in parallel. If you run it from 6 volts and use, say, 10 ohms
in series, it puts almost 5 volts into 50 ohms in about 500 ps. Or
source terminated, powered from 5, it will deliver about 4.5 at the
open end.

But it's a US8 package. To hand solder, wet the pins with liquid flux,
blob a tiny bit of solder on your iron, and touch each pin; solder
will wick from the tip to the pin and pad.

John