Well, one of the less obvious things is what does your system do to
detect a true mouse pad contact? Do you have denounce circuitry on the input
of the touch pad (or keyboard lines), do you sample a couple of times to see if
the input stays there (thumbs are still much slower than ESD pulses). If the
touch pad does provide a ground path for the ESD currents how does your
equipment handle that current to insured it doesn't get into your electronics?
The touch pad can certainly be the culprit and about the only way to tell is
buy physically examining the pad to see how they are grounding things and using
your judgment to see if its adequate. But even a well done touch pad can appear
to be a problem if you have not protected the input lines, or checked for hung
input states etc.
Over time, touch pads also become more sensitive to the intended
tapping function. After a few months I have to use and run a program that
suppresses taps altogether. If I don't and the mouse pointer happens to be over
the send button, for example, I will often get partially completed e-mails sent
out when the old fat thumbs happen to lightly touch the touch pad itself.
Telling the prospective vendor that you won't be using their touch pad
and why, may offer some relief as well. Even if it does look at how well you
are protecting your system from the vagrancies of ESD from the pad or a
keyboard. Considering what keyboards and touch pads sell for you won't find
those folks putting a whole bunch of money into hardening them.
Good luck
Gary
-----Original Message-----
From: Chris Maxwell [mailto:chris.maxw...@nettest.com]
Sent: Wednesday, January 02, 2002 1:47 PM
To: EMC-PSTC Internet Forum
Subject: Touch-Pad ESD immunity
Dear Ann Landers,
I've always had trouble with peripherals. Keyboards and mice that were
CE marked and looked like such good prospects have mostly turned out to
be fickle.
Well, I've been involved with a touchpad for about five months now.
When I first bought it, we were so happy. Whenever we were together it,
it could read my mind. A tap of my finger and it knew just what to do.
And then this ESD gun comes along. One zap and BOOM! The touchpad
turns its back on me. It won't respond at all! I tried talking to
it...but it just gave me the cold shoulder. I suggested
counseling...still no response. I threatend to go and get a mouse...no
response. Well, I finally had to just take a deep breath and go
through with it. I cycled power.
Well it now responds to me... but I don't know if I'll ever trust it
around an ESD gun again. I don't know if our relationship will ever be
the same.
Signed "Out of touch in New York"
OK OK
The real question is... does anybody have some words of advice
regarding touchpads. I am testing a unit which consists of a
keyboard/touchpad combination. The touchpad is approx 1.5" x 1.5" and
is able to sense a sliding or tapping finger. The touchpad is used to
perform all of the functions that a mouse typically performs.
I am assuming that it has some sort of capacitive sense circuit which
can tell when your finger slides across the pad or taps on the pad.
I have one that gets all out of whack with 8KV ESD. i.e. the touchpad
becomes unresponsive and it stops software execution in our host system.
Unfortunately, this is one of those instances where we don't build the
keyboard/touchpad; so my bag of fix tricks is limited. Probably limited
to seeing if another manufacturer produces a keyboard/touchpad with
better performance.
Or, am I slamming my head against the wall on this one? The
keyboard/touchpad is already CE marked by its manufacturer. Is this
typical? Are all touchpads (even CE marked ones) ESD sensitive? Do I
just live with it? Am I over-testing this touchpad?
Overall... I have had REALLY bad experiences with CE marked keyboards
and mouses. Now I have trouble with our first touchpad. We typically
use a capacitive filter on our inputs and we typically put a ferrite on
the cable...yet still trouble. Is this typical of what others see?
Any words of advice, experience... would be appreciated.
Chris Maxwell | Design Engineer - Optical Division
email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797
8024
NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA
web www.nettest.com | tel +1 315 797 4449 |
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