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TSC2007: Capacitor C4-C7 at Touch Screen Input

Hideki Hayashi1
Genius 10100 points

Part Number: TSC2007

Hi team,

I saw TSC2007EVM schematic and find C4-C7 located between the input of touch screen and ground. I would like to ask about this.

1, Why are the capacitors placed?

2, Does these always have to be placed?

3, Is there any limitations for selection of capacitors such as maximum capacitance?

Regards,

Hideki

  • 0
    TI__Guru**** 182881 points

    Hello Hideki,

    The capacitors you circled are actually not installed (NI) on the TSC2007EVM, so that hopefully answers question 2.  For question1, think of them as the 'C' portion of an RC low-pass filter, where the resistance of the touch screen itself is the 'R'.  For Q3, the value of the capacitor depends on the amount of noise in the system and how much settling time you have to accurately capture the the touch coordinates.  Here is an application note that might help explain a bit more:

    https://www.ti.com/lit/an/sbaa155a/sbaa155a.pdf

  • 0 in reply to Tom Hendrick
    TI__Genius 10100 points

    Hi Tom,

    I appreciate your comment.

    The capacitor is surely NI, but is there any problem to place only one or two capacitors on schematic? For exxample, there are bigger noise on X+ line than others, can the customer populate only C4? while the others are not populated. 

    Or should they be balanced such as the capacitance must be the same?

    Regards,

    Hideki

  • +1 in reply to Hideki Hayashi1
    TI__Guru**** 182881 points

    Hideki,

    Generally, the caps would be the same values.  The X and Y plates of the touch screen can vary, so there could be a case where X+/- inputs had a slightly different value than the Y+/- inputs.  Hope that resolves your question!