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 · Accepted solution]

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

[ok_pp]

Hello, Carlo.
Thank you very much for your answer. So far the result is negative.
I decided to take a step back and check the modulation on the X-NUCLEO-NFC08A1 board. I want to try external modulation with a Q300 transistor.
But then I had difficulties making changes to the ST25 registers.
In the rfalAnalogConfigCustomSettings[] array, the lines
0x0068,0x20,0x00, /* External Load Modulation Selection (lm_ext) : External Load modulation disabled ; Disable external Load Modulation */
0x0068,0x10,0x10, /* Driver Load Modulation Selection (lm_dri) ; Use internal Load Modulation */

set the modulation mode, but why 0х0068?
How to write registers correctly?
Forgive my ignorance, I just started working with this chip.

Best regards.

[Ulysses HERNIOSUS]

Hi,

hystorically space B registers are mapped into virtual addresses of 0x40|<spaceBaddress>. 0x68 then corresponds to space B regstister 28h. Similarly test regs are mapped into 0x80 address range.

BR, Ulysses

[ok_pp]

Hello, Ulysses.
Thank you, now it's clear.

[ok_pp]

Hello, Carlo.
Your advice increased the tag's reading range. Thank you.

[gamingX]

The main limitation here is not the ST25R3916B sensitivity registers but the antenna environment. A TFT display placed between the smartphone and the antenna significantly attenuates and detunes the NFC field due to metal layers and grounding in the display. This is why the phone struggles to power and read the tag, while reader mode still works at a short distance.

To improve smartphone read range, the focus should be on hardware optimization: increasing spacing between the antenna and display, adding an NFC ferrite sheet to redirect the RF field toward the phone, and re-tuning the antenna under real operating conditions. Smartphone NFC fields are weak, so antenna Q and bandwidth are more critical than register-level sensitivity tweaks.

This is very similar to precision input devices like gaming mice, where accurate performance depends more on sensor placement, shielding, and surface interaction than on software settings alone. Proper physical tuning always has a bigger impact than adjusting sensitivity parameters.

[ok_pp]

Hello, gamingX.
Your thoughts are absolutely correct.
But the design of the device is determined by the case and the TFT display.
In addition, there are no problems in card reader mode.