MPR121 Electrode Design.

[a_alia]

Hi.

I´m making electrodes for my MPR121 and I have a problem. For what are the resistors R1 and R2 tha you can find in KITMRP121EVM_SCH.pdf (pages 17 or 23), because those paths goes to the MC9S08Q68, but I don´t undertand It, and how to find the resistor values I need.

Sorry, I have a bad english and thanks for helping me.

Thanks

[Joshevelle]

Those resistors are directly connected to ELE_ID1 and ELE_ID2 on jumper 1 (J1) pins 29 and 27.

Those pins are connected to pins ADP6 and ADP7 of the MC9S08QG8 thru the header HDR_2X15_M (see page 4 of the schematic) . The resistors are working as voltage dividers, you can notice that the resistors values are different from one electrode board to another, i.e. in the 6 LED 6 ELECTRODE board (page 17) the values are 1K82 and 825 respectively, while in the 12 ELECTRODES board (page 23) those resistors values are 1K82 and 1K21.

Hence the voltage level in the voltage divider connected to ADP6 and ADP7 will be different from one board to another. This allows the microcontroller to configure the MPR121 properly, those resistors do not affect the electrodes sensing capability.

I would highly recommend you to take a look at the following application note:

Designing Touch Sensing Electrodes:

http://cache.freescale.com/files/sensors/doc/app_note/AN3863.pdf

Hope it helps =)

Regards,

Josh

[scacpiacenza]

I have readen the AN 3863 I am wondering how to improve sensing with a capacitive guard ring.

Thanks for reply