11 Replies Latest reply on Jul 10, 2018 12:03 AM by brvi

    Regarding MBR signal level and series resistance.

    matsubara-y_1534086

      Hello.

       

       

      Q1)

      Below is description of “Getting Started with CapSense”(AN64846).

      Do the following contents also apply to MBR3?

       

      In other words, does MBR3 signal level become lower if series resistance increased from 560ohm?

       

      But MBR3 tuning mode is smartsense.

      Smartsense automatically tunes parameters such as Fsw etc by MBR3 itself.

      Since parameters are auto-tuned, will not user feel too much sensitivity reduction even if series resistance increased?

      I think that MBR3 can drive enough sensor-pad if MBR3 uses slower Fsw by auto-tuning even if series resistance becomes bigger.

      In this time, I think that signal level does not become lower.

       

      Document description

      ==========================================

      If the series resistance value is set high, the slower time constant of the switching circuit suppresses the emission but limits the amount of charge that it can transfer.

      Thus the sensors may not get charged and discharged completely.

      This lowers the signal level, which in turn lowers the SNR.

      Smaller values are better, but are less effective at blocking RF emissions and interference.

      ==========================================

       

       

      Q2)

      Below is description of “Getting Started with CapSense”(AN64846).

      Do the following contents also apply to MBR3?

       

      Could you please let us know recommended series resistance(min/max) of MBR3?

      560ohm to 4.7kohm?

       

      Document description

      ==========================================

      The recommended series resistance for CapSense input lines on general copper PCBs is 560 Ω.

      An ITO panel already provides a high resistance; one may not have too much flexibility in the value selection (range 100 Ω–1 kΩ).

      Series resistors are generally used in the range of 560 Ω–4.7 kΩ for EMC purpose.

      The actual maximum value of the series resistor that can be used varies from device to device.

      This depends on multiple factors such as the resistance of the GPIO used as sensor, the switching frequency used to scan sensors, and the SNR required.

      ==========================================

       

       

      Q3)

      When does MBR3 decide Fsw?

       

      Does MBR3 decide Fsw every scan timing?

      (So MBR3 Fsw changes from scan to scan. MBR3 Fsw is not constant value.)

       

      Or does MBR3 decide Fsw when power-up or reset?

      (So MBR3 Fsw is constant value until power-up or reset)

       

       

      Best Regards.

        Yutaka Matsubara