In short: I´m working on a battery driven handheld device with capacitive-touch screen. The capacitive-touch works perfect when the PCB connected via USB, but it loses sensitivity when driven by battery after a couple of seconds.
The setup: 10cm dia 4-layer PCB with micorcontroller, radio-module, PCB antenna, and FPC connectors to an 3.5" TFT display, on which FPC the CTP driver IC FT5346 from FocalTech is.
The PCB is mounted in a ABS housing via screws (GND connected). The display is glued on the top of that housing and connected to the PCB via FPC cables.
The phenomen:
While developing, the device is connected via a USB-cable or debugger with the PCB. When I check the capacitive touch functionality on the display, it works perfectly. Every time I touch the display with my fingertip, the capacitive touch sensor triggers an interrupt on INT.
When I unplug all cables from the device and power it only via battery, the touch functionality gets worse over time. The first 5-10s the touch reacts like before, instantly on touch inputs via fingertip, but when I wait at first like 10s and try a touch with my fingertip, there is no reaction, even if I try it repeadly. When I use my whole fingerprint, then it reacts again and directly after that the fingertip works until I wait the 10s again.
My conclusion: As long as I connect the device to an earthed connection (USB, debugger) the capacitive touch works like a charm. When I disconnect the external GND/EARTH, the battery-minus is now the system GND and some "system capacity" is running low or high on time, so my fingertip only produces touches for a short period of time.
My observations so far:
- I tried the same on another PCB with same results.
- I observed the INT signal via oscilloscope and saw that there is really no interupt signal from the FT5346
- I tried to couple the display case directly via cable with the PCB GND; same results
- I tried to increase/decrease the touch threshold in CTP (THGROUP, THPEAK, THCAL) but no (recognizable) change in sensitivity at all
Do you have further tips that Ican try? In the datasheets is something about "auto-calibration" but no information about that anywhere.
The "user experience" is bad if he realizes he needs to put the whole finger on screen sometimes and sometimes the fingertip is enough.
UPDATE_01: I've placed the Device in the middle of a metalplate (stencil 400mm x 280mm) without connection (device-foot of ABS), the problem still persists. Then I connected the PCB-GND with a cable to this metalplate, and the touch works great again. There must be a way to improve the GND on my device to improve this behaviour without metalplate!?
UPDATE_02: I´ve connected the device via usb to the laptop, which is also isolated (run by battery) and the device touch works great. I think the size of the couple-capacity of the device is to small in ratio to the capacitive touch interface. But how do they accomplish this on smart watches or smartphones? Is there a trick in increasing the the GND size on small devices?
