ST25R95: Resistance value on receiving path

luigiantuono · ‎2024-09-05

I am designing a custom board using your ST25R95 chip. To design the matching network given the antenna parameters, which I show in the following picture:

The tool you recommend (ST25R95_EMI_FILTER_CALCULATION) for the matching network returns me negative capacitance values. To overcome this problem I chose to reduce the value of the resistors on the receiving path. This is something I have not seen done in any other example so I was wondering if it is permissible to do this (changing RRx from 330 Ohm to 1 Ohm). I attach the screenshot of the output of the matching tool:

Thanks for your attention,

Luigi

Brian TIDAL · ‎2024-09-05

Hi Luigi,

I've discussed with our RF experts. From mathematical point of view, adding a 1Ohm serial resistance would work but this is not realistic. They rather suggest to remove 1 or 2 turns to reduce the inductance under 2µH vs 3.7µH currently (which is too high according to their expertise). Having an inductance value under 2µH will give more freedom to adjust the Rx pin voltage under the 9V limit.

Rgds

BT

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luigiantuono · ‎2024-09-09

Hi Brian thanks for your kind answer. I am not able to reduce the inductance of the reader antenna but I have another antenna I can use in order to read the NFC tag we are currently using.

As in the previous question I am going to share with you the characteristics of this antenna:

this is the same antenna we are using for the tag so the two antennas (the reader's one and the tag's one) have a perfect geometric coupling. Unfortunately, because of the plastic between the two antennas, the reader struggles a bit reading the tag and can only do so in some specific relative orientations between the two antennas.

Basically what I am asking to you is if a can maximize the voltage at the receiving pin.

This is the matching network suggested by your excel tool:

First of the solution I found out is related to the placement of a ferrite bead sheet between the PCB and the printed antenna.

Other than this I would also like to connect the Rrx to the antenna pin (between the matching circuit and the antenna pads) in this way I should be able to maximize the voltage at the RX pins of the IC. I know that this can be an issue for the mismatching between the Qtarget and the actual Q. Do you have any guidelines on how to proceed in case I would like to adopt solution #2?

Thanks for your time,

Luigi

Brian TIDAL · ‎2024-09-12

Hi Luigi,

have you followed the various steps described in chapter 5 of AN5248 in particular §5.1 step 3? Also §5.2 provides useful information.

Can you share your smith chart so that it can be reviewed by our RF experts?

Also, have you tried different kind of tag just for comparison? (e.g. credit card sized tags)

Rgds

BT

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