NFC Amplitude limited not working? Significant noise on Vdd when Vdd =1.8V (as opposed to 3V)

[GilesHutchison]

Hi, We are using the nRF52840 in a 3V battery powered product. We have an on board 1.8V LDO regulator that supplies Vdd and Vdd_IO. We have a problem with the NFC antenna causing significant 13.56 MHz noise +/- 200mV on the Power Supply , I suspect because there is > 3V of induced voltage on the NFC input pins being much larger than Vdd. We are using a Nokia Android phone to interact with the nRF52840. As a result we are seeing device crashes.  We have tried increasing the 4.7uF bulk capacitance (to MLCC 10uF and 47uF) , next to the Vdd pins, along with usual  100nF / 100pF decouplers.  Has this issue been seen before? Vdd is very quiet at all other times.

I note that Vswing is not defined in the data sheet, and would be interested to know how the NFC amplitude limiter works?

Thanks in advance for any help.

[Turbo J]

I don't thinks its just the pins - it is probably the "high power" NFC field coupled directly.

Check the LDO feedback path in the PCB layout - it must be properly shielded from the 13.56 MHz. Otherwise the LDO might do stupid things.

The NFC high frequency could also affect the oscilloscope probe (or the GND return path), so I don't really trust any measurement here.

[GilesHutchison]

Hi Turbo,

Thanks for tyour reply.

The LDO is carefully layed out, and follows the data sheet layout guidelines, and has stitched ground plane on top and bottom layers.

I have a very similar set-up using the EVK running at 3V with 1/10th the amount of noise. I will see if I can configure the EVK to run from 1.8V Vdd / Vdd_IO and provide more feedback.

Regards

Giles