Re: Touchscreen Manufacturers plus EMC

Tom Lucas wrote:
Well that's my hols over with for another year :-( I can highly
recommend Bulgaria by the way. However, back to work....

Does anyone have any experience of different manufacturers of resistive
touchscreens? I'm almost at the time where I have to decide which make
to go for and I have three main choices (others may be considered but
only if they are obtainable by the people who will be sticking them to
the LCDs they will be selling us):

3M - Already in use on the development system and have been good and
reliable (not their capacitive screen though - shafted me good and
proper <grrr> ) but they are not cheap.

DMC - About half the price of 3M and appeared to allow more light
through when I saw one demonstrated. Might have just been a brighter LCD
though.

eTurboTouch- Roughly the same price as DMC but I haven't seen one in
action.

Also does anyone have any experience with EMC on touchscreens. I imagine
that, being conductive, they are good at blocking emissions from the
hole cut in the box for the LCD but could prove sensitive to receiving
noise. How much of a problem am I likely to face?

I've used both glass and polycarbonate touch panels ( I currently use a
poly panel in a shipping product), but my supplier is a fairly small
outfit in the south of England (I'll get the name once I'm back at work
in the morning).

EMC - Use a fully differential driver and ADC system. I use an
integrated solution with AC97 (because it was on the initial dev board
and the software group bitched when I asked about changing it). If I
had started the whole thing from scratch I am not sure I would have
used it.
Anyway:
Wolfson WM9712
http://www.wolfson.co.uk/products/touchscreen/adcs/WM9712/

This device is rather sensitive to the CCFL drive when it's in close
proximity, (as most do one suspects) so shield it and get one with a
nice big analog ground pad underneath. If you end up using it, let me
know and I'll email you the details of what it took to get it really
clean. If you're using another controller, you'll probably still end up
with the same issues I did. Every product is different, of course, but
a lot depends on the positional relationships of the internal signals
and HV wires.

This device is in close proximity to the CCFL drive, LCD signalling,
GSM/GPRS modem, Bluetooth unit, 802.11, barcode scanner-imager and an
RFID reader and works just fine. [That's no different from a lot of
products, of course; just mentioning it so you know the amount of
interfering radiation I had to worry about ;) ]

The unit is used for signature input (amongst other things) and that
works fine during GPRS sessions (the GPRS antenna is perhaps 3/4 inch
behind the touch panel and the one side of the radiation pattern is
aimed directly at the touch panel (but with the LCD in the way,
thankfully). Ditto for BT, RFID and 802.11.

The panel itself doesn't really absorb too much of the energy passing
through - a few dB at most - and that was actually an issue at EMC
compliance testing. The best advice I can give is to make sure your LCD
is solidly grounded to it's perimeter shield all the way around. That
also means that the panel shouldn't be particularly susceptible to
external interference; certainly mine isn't, but see the note about
fully differential drive :)

Poly absorbs more light than glass based units, but it's far more
rugged (and easier to get safety approval on). Depends on your
application, of course, and panel size. It's also more expensive.

Hope that helps

PeteS

.

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