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ADS7843: resistive screen controller data readings affected by varying pressure

Robert Poor1
Prodigy 180 points
Part Number: ADS7843

On my ADS7843 based systems, I notice that the raw X value changes with pressure.  Specifically, near the left edge of the screen, increasing pressure causes the raw X value to decrease, when near the right edge of the screen, increasing pressure makes the raw X value increase.  (Similar comments for Y / top / bottom.)

(Note that this is using finger pressure -- the client requirement precludes using a stylus or a capacitive screen.)

I've pored over the data sheets and Burr Brown / TI app notes and haven't seen any mention of varying pressure introducing errors.  Any insights as to cause (and cure) are welcome!

  • 0
    TI__Mastermind 21810 points

    What percent change do the settings change with application of pressure?

    Generally pressure changes resistance between X and Y. . If the Input impedance is much Larger then this resistance then pressure should not inpact X or Y Value.

    I suggest the following test on touch panel without the chip. take a 3.3v DC Supply across X+ and X- on the Touch Pad. Put Meter on Y+ and  X-. 

    and read Move finger on X on panel and read voltage on meter. Now change pressure  and read the voltage at the corners.

    This Test shall demonstrate the role of the touch panel in this observation you have and is independant of the chip

  • 0 in reply to Sanjay Dixit
    TI__Mastermind 21810 points

    You should read a DC Voltage from 0v to 3.3v as you move your finger. At the extremities press harder to see if the reading deviates.

  • 0 in reply to Sanjay Dixit
    Prodigy 180 points

    What I've found is that with a stylus or fingernail the readings are solid.  Pressing with a full fingertip (a much larger area) is what causes a change in x, y.  Has this been your experience as well?  (I have not tried disconnecting the touch panel yet -- it's tightly integrated into the controller board.)

  • 0 in reply to Robert Poor1
    TI__Mastermind 21810 points

    It may be that the fingertip creates a larger and varying series impedance that connects into the ADC. At the edge the fingertip may be making inadequete contact with the touch screen. This resiatance  may be creating a voltage divider into ADC Input impedance.

    Does the value measured dip as compared with the expected value at the corner? 

    Also if its hard to disconnect the touch screen you can take X+ X- and Y+ on an Oscilloscope. When X+ is High and X- is low the Y+ should be giving a voltage of x data that is being input into the chip. This should be going from 0v-3.3v. it would help to see what is happening to this as you touch the finger at the corner..

    if its the finger having improper contact you can possibly think of scaling slightly the values at the edges in software.