ST25R3916B in tag mode - how to increase sensitivity?

[ok_pp]

I am using ST25R3916B with differential antenna without varicaps.
The antenna is configured to receive maximum signal  from the smartphone on the RFI1.
There is a TFT display between the antenna and the smartphone. The smartphone reads the tag with great difficulty even when pressed against the display.
The ST25R3916B reads cards in card reader mode at a distance of about 2 cm from the display (when setting up the antenna to get the maximum signal from smartphone).
Question: How can I increase the tag's reading range with my smartphone? ST25R3916B has sensitivity control registers, but there is no method for setting these parameters.
Best regards.

[ok_pp]

The design of the device is approximately as follows

 

[Carlo Corlattoni]

Hello,

assuming that 3916B is correctly configured for card mode, I would suggest to first check the antenna tuning. When using 3916B in card mode, it is strongly recommended to match the antenna impedance at a higher target frequency than 13.56MHz, eg. 15-20Mhz. This will reduce the loading effect on the reader device.
Next, check the passive target modulation register 29h, SpA. The largest load modulation can be achieved by setting reg 29 to F0. ptm_res and pt_res sets the RFO resistance during modulated and non-modulated state, and F0 giving the largest modulation. Finally, the RF peer detection threshold setting also influences the performance, because the device only response when a field is detected.

Best regards