Re: Touchscreen Manufacturers plus EMC


Tom Lucas wrote:
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):

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?


"PeteS" <PeterSmith1954@xxxxxxxxxxxxxx> wrote in message
news:1153763008.998203.258740@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.

My controller is built into the Sharp MCU I use to drive the screen but
it should be fairly well partitioned off from where the CCFL is. The
inverter can be positioned away from most things as well.

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 ;) ]

We don't have any RF stuff so that's one less thing to worry about.

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 :)

Noted about grounding the LCD shield. Our whole chassis is earthed so
perhaps I improve the conductivity to the screws holding on the panel to
help to ground it.

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.

The application is in a boiler room so a little bit of ruggedness is
required. However, it doesn't have to be bombproof and if they manage to
break it then they can put their hand in their pocket for repairs. It's
a touch screen not a jab screen ;-)

Thanks for your detailed reply.


.

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