Zrx tuning on ST95HF

[gabriele23]

Posted on November 03, 2017 at 08:15

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.

[Peter Boringer · Accepted solution]

Posted on January 22, 2018 at 17:04

Hello Gabriele,

If you are adjusting the voltage lower than 18Vpp between ac0 and ac1 your design might be secure.

The receiver inputs RX1 and RX2 are typically connected through two external attenuation resistors

or capacitors to avoid saturation of the internal detector

Those 2 serial resistors RRX1, RRX2  have to be adjusted in order to avoid entering the clamping region They are defining the RX path attenuation which might be lower that 9V (Ref Datasheet.) 7V is a secured guarband for prototype trimming considering tuning tolerances.

Regards

PYB