NFC Antenna With DC Blocking Capacitors

[Kyle]

Hi,

Reading the nRF52 product specification, I see:

"If the antenna is exposed to a strong NFC field, current may flow in the opposite direction on the supply due to parasitic diodes and ESD structures.

If the battery used does not tolerate return current, a series diode must be placed between the battery and the device in order to protect the battery."

Can anyone comment on whether it's acceptable to use two DC blocking capacitors (~10nF) between the NFC pins on the nRF52 and the NFC antenna?

[bjorn-spockeli]

Adding two DC blocking capacitors between the NFC pins and the NFC antenna is not something that we recommend as this will most certainly affect the NFC functionality in a negative manner.

I recommend staying within the nRF52 Product Specification,i.e add a series diode between the battery and the device if the battery does not tolerate return current.

-Bjørn

[Kyle]

Thank you, Bjørn.

Upon further consideration, I also realized that blocking caps are unlikely to actually prevent reverse current flow. The NFC antenna is already an AC-coupled device, so blocking caps are redundant.

I'm assuming that a strong NFC field can generate sufficient AC voltage to forward bias the ESD diodes connected between the NFC pins and VDD inside of the nRF52 such that they begin to rectify the AC voltage and generate DC current flow back into the power supply.