6 Replies Latest reply on Dec 9, 2019 4:40 PM by YuMa_1534086

    Regarding advantages and disadvantages of Shield(water proof.)

    YuMa_1534086

      Hello Cypress.

       

      Could you please let us know advantages and disadvantages when below functions are used for water proof?

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

      Enable shield electrode is enabled.

      Inactive sensor connection changes to Shield from GND.

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

       

       

      1) Advantages

       

      <Enable shield electrode is enabled>

      -CSD can reject false touch when there is water between one sensor and mesh pattern.

      Under this condition, CSD can detect finger touch correctly.

      (But CSD can not reject false touch when there is water between multi sensors because inactive sensors connect GND.

        So CSD can not detect finger touch correctly under this condition.)

       

      -Cp can be reduced in comparison with GND mesh pattern.

      So sensitivity improve.( IDAC decreases)

       

      < Inactive sensor connection changes to Shield from GND>

      -CSD can reject false touch when there is water between multi sensors.

      But under this condition, if finger touches this sensor with water, false touch occurs.(All sensor with water become touch-on)

       

      -Cp can be reduced in comparison with GND connection.

      So sensitivity improve.( IDAC decreases)

       

       

      Above is my understanding about advantages.

      Is my understanding correct?

      Please let me know if there are other advantages.

       

       

      2) Disadvantages

       

      <Enable shield electrode is enabled>

      < Inactive sensor connection changes to Shield from GND>

       

      -Need to re-tuning because sensitivity changes.

      -Need additional external capacitor(ctank)

      -Power supply current increases.

      -Emission noise increases.

       

      Above is my understanding about disadvantages.

      Is my understanding correct?

      Please let me know if there are other disadvantages.

       

       

      And I think that any problem occurs if there is delay in actual sensors scan waveform and shield sensor.

      In this case, will it develop into issue of false touch?

      I would like to know what kind of problem occurs specifically.

      What kind of problem can you guess?

      Please let me know if you have any past examples.

       

       

      And there is case that IDAC becomes very small(below 10-20) by using shield.

      I think that any problem occurs when IDAC becomes very small.

      What kind of problem can you guess?

      (for example, false touch, RawCount does not return to original value(baseline) after removing finger-touch ,etc)

      Please let me know if you have any past examples.

       

      Best Regards.

      Yutaka Matsubara

        • 1. Re: Regarding advantages and disadvantages of Shield(water proof.)
          AH_96

          Hello YuMa_1534086

           

          First off, thank you for this detailed comparison description for CapSense shield operation.

           

          1. Advantages -

          < Inactive sensor connection changes to Shield from GND>

          -CSD can reject false touch when there is water between multi sensors.

          But under this condition, if finger touches this sensor with water, false touch occurs.(All sensor with water become touch-on)

          - While this is true, in case there is possibility that the liquid can come in contact with multiple sensors (usually stream of liquid) we generally recommend using a guard sensor. This will trigger whenever there is a stream of sensor and will disable all sensors to prevent this from happening.

           

          2. Disadvantages -

          -Need additional external capacitor(ctank)

          The external capacitor is required only in certain cases, when the parasitic capacitance of the shield pattern is greater than 100pF. In most designs, the external tank capacitor is not required.

           

          Q. And I think that any problem occurs if there is delay in actual sensors scan waveform and shield sensor.

          In this case, will it develop into issue of false touch?

          - When a shield is used, the shield waveform acts as the reference point for the CapSense sensor. Therefore, ideally when there is no finger being touched, both the waveforms will be similar and the field lines being terminated by the shield will be minimal. But if there is delay between actual sensor scan and the shield sensor, the shield hatch might be in ground potential (or close to ground) when the sensor is being scanned and can cause an increased signal. This could result in false touches being registered.

           

          Q. And there is case that IDAC becomes very small(below 10-20) by using shield.

          I think that any problem occurs when IDAC becomes very small.

          - As mentioned in the question, the IDAC will be lowered if shield is used. However, if IDAC is lower than 20, since raw counts are inversely proportional to the IDAC value, CapSense will be extremely sensitive. This causes an increased noise level as well and therefore, the SNR will be lower. Hence it is recommended to tune the sensors such that IDAC is greater than 20.

           

          While using a shield, you also have to make sure that the shield is getting charged and discharged completely. Otherwise, it could result in increased noise as measured by the system. Therefore, it limits the maximum sense clock frequency of the sensor also (since the sense clock frequency set must ensure that shield gets charged and discharged completely).

           

          Thank you,

          Best regards,

          Hari

          • 2. Re: Regarding advantages and disadvantages of Shield(water proof.)
            YuMa_1534086

            Hari-san.

             

            Thank you for your support.

             

            Sorry, I have simple question.

             

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

            But if there is delay between actual sensor scan and the shield sensor, the shield hatch might be in ground potential (or close to ground) when the sensor is being scanned and can cause an increased signal.

            This could result in false touches being registered.

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

             

            If user does not use shield, mesh pattern is connected GND.

            Even in this case, there is voltage difference between sensor drive level and mesh pattern(GND).

            But false touch does not occur.

             

            What is this difference between a case that false touch occurs due to voltage difference by shield and a case that false touch does not occurs even if there is voltage difference between sensor and GND mesh?

             

            Best Regards.

            Yutaka Matsubara

            • 3. Re: Regarding advantages and disadvantages of Shield(water proof.)
              AH_96

              Hello Yutaka-san,

               

              "If user does not use shield, mesh pattern is connected GND.

              Even in this case, there is voltage difference between sensor drive level and mesh pattern(GND).

              But false touch does not occur."

              -> When the hatch pattern is connected to ground, the reference plane (other plate of the capacitor) is at a constant potential. Therefore, when the sensor charges and discharges, the potential of the hatch around does not change and the Cp measured will be  correct.

              Now, when the shield is present with a delay in waveforms, the potential of the reference plane, the hatch pattern, will also change and this change will be from ground potential till Vref (assuming complete charging of shield). This will result in error in Cp measurement since the potential is constantly changing and therefore, will result in a noisy raw count.

               

              Best regards

              Hari

              • 4. Re: Regarding advantages and disadvantages of Shield(water proof.)
                YuMa_1534086

                Hari-san.

                 

                Thank you for your response.

                 

                Sorry, I have one question.

                 

                I think that scan time and process time are not affected by Shield setting(Enabled/Disabled) and inactive sensor setting(GND/Hi-z/Shield).

                Is my understanding correct?

                 

                Or is only process time affected by these setting?

                 

                Best Regards.

                Yutaka Matsubara

                • 5. Re: Regarding advantages and disadvantages of Shield(water proof.)
                  AH_96

                  Hello Yutaka-san,

                   

                  -> I think that scan time and process time are not affected by Shield setting(Enabled/Disabled) and inactive sensor setting(GND/Hi-z/Shield).

                  Is my understanding correct?

                  Yes. The scan time and process time are not affected by inclusion of a shield. The only trade-off is with respect to increased power consumption and increased emissions.

                   

                  Best regards

                  Hari

                  • 6. Re: Regarding advantages and disadvantages of Shield(water proof.)
                    YuMa_1534086

                    Hari-san.

                     

                    Thank you for your response.

                     

                    I understand.

                     

                    Best Regards.

                    Yutaka Matsubara