AnsweredAssumed Answered

Zrx tuning on ST95HF

Question asked by bettoni.gabriele on Nov 3, 2017
Latest reply on Jan 22, 2018 by Peter Boringer

Dear sirs,

we have produced more than 10000 pcs of NFC readers based on your ST95HF the design is based on your reference design  EVAL-ST95HF.

We experience on a small batch of boards a problem in the reading distance, I have already tried to measure the field of the antenna ad it seems good, even the tag detection procedure starts correctly, but we are not able to read or write the TAG at the maximum distance.

As the field seems good, in my opinion it seems a problem in the RX side, so I have tried to reduce the damping resistor Zrx from the default 330Ohm to 220Ohm and now all seems to work fine.

I have found some references in the AN4327 that the values should be defined to reduce the Vrx1-Vrx2 voltage below 7V and it seems that this voltage could sampled at pin ST_R0, with the new Zrx value this voltage reaches 9V, and the datasheet says that the maximum limit is 18V. So I suppose to be on the safe side, but  I don't understand why this level should be tuned to less than 7V, and I would like to be shure that this modify should not introduce other side effects.