NHD-5.0-800480TF-ATXL#-CTP electrical noise interfering with touch

[Les.Grant]

Hi, Most system with this display work well but we have a small number of systems that are experiencing erratic touch sensing (erratic coordinates) in the presence of electrical noise, typically from an AC motor inverter. Does anyone have a strategy to minimise the erratic behaviour (short of switching to resistive touch!)? I thought of averaging the coordinates but haven't had time to try this yet.
Thanks.
Regards,
Les.

[Paul_B]

Hello Les,

The PCAP touch panel will pick up any ambient radiation. Therefore, when designing a PCAP based system, the first thing to consider is what components in the system are generating noise that the PCAP might pick up. The bezel can act as a shield keeping noise from other system components from reaching the PCAP it must be well grounded to provide the best immunity.

The touch controller of a PCAP touch panel measures capacitances in the low picofarad range, which translates to voltages in the low millivolts range. As with any system that measures low voltages, it is important that the system rails and ground be as noise free as possible. At the system level, utilize a star topology ground design connected to chassis ground or earth ground, if available.

Below are a few tips that should help improve your design and eliminate the ghost touches:
 
•   Ground the TFT's bezel, copper tape comes in handy
•   Utilize flat cable Ferrite core (used for FPC/FFC cable)
•   Utilize EMI shielding foils and film

[Les.Grant]

Hi Paul_B,

Thanks for the pointers. I suspect that some of these systems have a noisy earth. If the machine operator touches the metalwork of the machine with the left hand (equalises to the machine potential) and operates the touch screen with the right hand, it works perfectly. If he doesn't touch the machine, it is erratic. The only solutions I can see are to fix the noisy earth or go to resistive touch. Do you think any of your suggestions would work under these circumstances?

Thanks for your thoughts.

Regards,
Les.

[Paul_B]

Evening Les,

To answer your question yes, most often these disturbances can be eliminated by simply connecting the TFT's bezel to ground. 

Please keep me updated on your progress, hopefully this will eliminate the ghost touching.