ST25R3911B capacitive wakeup guidance

[MrPhilFred]

I am finding it difficult to get clear direction on best practices for how to do the layout for CSI and CSO.  App note AN4985 says that "parasitic capacitance to ground should be made as small as possible", and then recommends using a coupling capacitor up to 2.7pF (C_COUP, between the 2 electrodes, I think... not sure).  But, on the other hand, the reference design (Discovery Kit ST25R3911B-DISCO) has no C_COUP shown in the schematic -- maybe because there is enough capacitance already between the electrodes? --  and it has an RC of 1k and 12pF on both CSI and CSO, without explanation.  It is not clear why we want to minimize parasitic capacitance to ground, but then we deliberately add 12pF of capacitance to GND.  Some clarity on the design goals for CSI and SCO would really be appreciated.  thanks!

[Travis Palmer · Accepted solution]

Hello MrPhilFred,

The ST25R3911B-DISCO has the GND plane between the capacitive plates. It is the opposite what is recommended in the Wake-up application note. This design has been chosen not having capacitive wake-up as its main focus.

The X-NUCLEO-NFC05A1 is more like the recommendation in the wake-up AN. Therefore please have a look at the NFC05A1.

The mentioned coupling capacitor is the capacitance adjusted when using the direct command Calibrate Capacitive Sensor:

(DS11793 - Rev 8)

So no external MLCC coupling capacitors are required.

The capacitive sensing on the ST25R3911B and ST25R3916 has a very basic implementation and can be influenced by external noise sources. This is the reason why ST did not put this feature in new devices (ST25R3916B, ST25R200/100, ST25R300/500).

Automotive customer tend to implement capacitive sensing via MCU and external circuits. This way they can post-process the measurement and can do their own implementations / algorithms.

Nevertheless, the basic building blocks are present in the ST25R3911B / 16. 

Please let us know if you have further questions.

BR Travis

[MrPhilFred]

thanks, that is very helpful!