ST25R3916 Max Allowed Voltage Level on RFI

[LltWc]

We've had issue with reading NTAG215 cards with some devices, presumably due to the large spread-out antenna, so I'm working on a better understanding of the power output so that we can properly calibrate devices out of manufacturing.

Currently, I do not have any RFO resistors enabled, so Tx power should be at its maximum. The datasheet says that the maximum allowed voltage level on an RFI is 3 Vpp, but on devices functioning optimally, I see up to 3.2 Vpp from the output of a Measure Amplitude command, particularly when an NTAG215 card is in the device's field. 

Does this signify that something in hardware or firmware is implemented incorrectly? Should I be setting the RFO resistance so that > 3 Vpp is never seen on an RFI pin? Does it matter at all? The electrical characteristics section says that in reader mode the recommended signal level is 2.5 Vpp, so should I be optimizing for that?

Thanks for your help.

[Ulysses HERNIOSUS · Accepted solution]

Hi LltWc,

yes 2.5V is recommended to have some head-room if detuning causes higher RFI voltage. 3V is the maximum limit, you should not exceed this ever.

Quite likely it will survive somewhat higher voltage for some time. But nothing guaranteed!

BR, Ulysses

[LltWc]

Thanks—that answers my question, I'm now setting the RFO resistance to 2x so that the typical measured RFI voltage is ~2.6 V.

The actual solution to the problem, though, ended up being in hardware. A pin was left unsoldered (floating) in the antenna connection pathway, resulting in losing a portion of both the transmitted and received signal. Soldering that back on fixed the issue.