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Range on Panel Resistance

Andy Liu (BMS)
Guru 52115 points

This quetsion is from on touch panel vendor in China.  Their customers usually require that the panel resistance should be like Rx-x =200 – 500 Ohms, Ry-y= 600 – 100Ohms.   This vendor wants to know why this kind of requirement is needed and whether it is needed for TI resistive touch controllers. Besides, what will happen if the panel resistance goes up to 2k or 3k Ohms?  

  • 0
    TI__Prodigy 220 points

    it is strange that the customer is requesting certain panel resistance from panel manufacturer. Normally

    it is the other way around. The panel manufacturer is telling customer what is the value and what variation the panels have. If the panel resistance change the touch location should not change much, because of the ratiometric measurements. Lets say you have a panel which is 10cm in X direction and 2 panels one with 400 Ohm x panel resistance and one with 3k Ohm x panel resistance. Supply voltage is 1.8V. And you have a touch at 4 cm in x direction. For panel 1 the X voltage would be: 160Ohm/400Ohm*1.8V=0.72V. For panel 2 the X voltage would be: 1200Ohm/3000Ohm*1.8V=0.72V. That means the touch point is similar.

    Different panel resistance will have different power consumption when there is a touch. The lower the resistance the higher the power consumption.

    In the past we have seen that panel with higher resistance have higher line resistance. Maybe the wires to the
     X and Y pins have higher resistance. That would reduce the voltage applied to the touch pins at the TSC. For panel 2 if you have 100 Ohm line resistance on the X+ and X- lines then the voltage range would be not 0 - 1.8V but (0V + 100Ohm/3200Ohm*1.8V) to (1.8V - 100Ohm/3200Ohm*1.8V).

    In an application you always should have a panel calibration before using the customer device. That means even if the panel manufacturer produces panels like above mentioned, calibration will help to eliminate those differences.

    Bottom line is I don't understand why customer has those requirements? Maybe some system requirements we have not seen before.