Capacitance reading circuit for nRF51

[Brandt]

Currently, we are trying to have the nRF51 chip working with a type of capacitive pressure sensor. We did quite a lot of work on searching proper circuits of reading the capacitance of the sensor, including capacitance voltage divider, charging time method. However, most of these designs are based on PIC and AVR chips, not the ARM structure that is used for nRF51 chips. I am wondering whether there is any hardware circuit that suits nRF51 chips for it can read the output capacitance of the sensors (not capacitance touching sensor)?

Your attention and time will be appreciated greatly.

Brandt.

[Jørn]

Hello Brandt, you say not capacitance touching sensor, do you by that mean no capacitive sensing as used in place of buttons? What will the purpose of your sensor be? What will change the capacitance?

[swoudlxy]

Hi Jorn, the capacitive sensor is acting as a pressure sensor. The output capacitance will change according to the variation of the thickness of the dielectric sandwiched between two conductive plates. Therefore, we are wondering whether there is a proper circuit that can read the capacitance of the sensor.

[Jørn]

Hello Brandt

We do have a capacitive sensing driver, and a library. Information on the capacitive sensing driver can be found here, and information on the capacitive sensor library can be found here

With the nRF51 it charges the capacitance with an extra output pin, and measures the charge time. Then you can calculate the capacitance from that. With the nRF52 you can use the comparator as a relaxation oscillator (see the link in the driver section above), whose frequency will be dependent on the capacitance.

We also have a couple of examples for capacitive sensing, these examples can be found at [SDK folder]/examples/ and are called csense and csense_drv. You can find the example documentation here and here. Examples for the nRF51 series is not available in SDK13, but you can find them in SDK12.2

Best regards

Jørn Frøysa

[Brandt]

Thanks a lot for your advice, Jorn. I'll look into the information and give a try.

[Ehsan]

What would be the best way to measure the charge time? Also, what peripherals would be required?