the possible maximum reading distance with ST25R3916

DSai.1 · ‎2022-02-23

We are developing a IC card skipass Reader product for ski resort market.

I checked TRF7961 from TI, but it can provide only 200mW output power.

In the datasheet of ST25R3916, this IC can give up to 1.7W output power.

I wonder these two are same index or not. I would like to know the maximum reading distance with ST25R3916. Could you give me some information or advice?

If possible, we want a distance about 30cm. With TRF7961, about 15cm can be obtained.

Thank you very much for your help in advance.

DSai.1 · ‎2022-03-13

Dear Peter,

I have a new question for obtaining the maximum output power. For STR25R2916, there are power input pins VDD and VDD_TX.

My question is what voltage is need for obtaining the maximum output power. In our application, the power is from a lithium battery which voltage is

about 3.7 ~ 4.0V.

I wonder the maximum output power may be obtained with the voltage lower than 5V for VDD_TX and VDD?

In the system diagram Fig.2 in page 14 of ST25R3916 datasheet, VDD and VDD_TX are connected together. Is it possible to

connect VDD to a 3.3V power and VDD_TX to batter which is from 3.7 to 4.0V?

In our application, the SoC can provide a 3.3V power source for peripherals connected.

thank you in advance for your help.

best regards

Sai

Ulysses HERNIOSUS · ‎2022-03-14

Hi Sai,

please see 4.2.11 of the DS:

"VDD and VDD_TX must be connected to the same power supply."
You may connect VDD_IO to to the 3.3V from your SoC.
Output power of course depends on the voltage. You have also options to bypass the VDD_RF regulator if your supply is stable enough (short VDD_RF and VDD_TX). Please refer here also to symbols IVDD_LDO and IVDD_EXT.

BR, Ulysses

Peter BENSCH · ‎2022-03-14

First of all, it is questionable whether a significant increase in range can be achieved by increasing the power. The overall system includes not only the TX but also the passive tag, which does not transmit itself but can only modulate the surrounding field. For a range increase, it might be more important to increase the RX sensitivity.

Regarding VDD and VDD_TX:

As can be found in the datasheet, section 4.2.11:

VDD is the main power supply pin. It supplies the ST25R3916/7 blocks through two regulators (VDD_A, VDD_D)
VDD_TX is the transmitter supply pin. It supplies the transmitter via two regulators (VDD_RF, VDD_AM). VDD range from 2.4 to 5.5 V is supported. VDD and VDD_TX must be connected to the same power supply.

Does it answer your question?

Regards

/Peter

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DSai.1 · ‎2022-03-14

Dear Ulyssess,

thank you very much for your advice.

Based on your advice, I read the 4.2.11 in datasheet.

It said there are two basic operation modes 3V and 5V.

I wonder if we can use 5V , if we connect VDD and VDD_TX

to a battery that output is 3.7V ~ 4.1V. or if we want to use

5V mode, we must give a power supply that is over 5V?

best regards

Sai

DSai.1 · ‎2022-03-14

Dear Peter,

thank you very much for your help always.

I think maximum output power should be a sufficient condition for a long reading distance.

If the maximum output power can be obtained, we can have more possible adjust margin

to achieve a long distance.

If possible, would you teach me the possible approach to increase the RX sensitivity?

best regards

Sai

Ulysses HERNIOSUS · ‎2022-03-14

Hello Sai,

I think you will want to use 5V supply mode. Refer to bit sup3V bit description and also the electrical operating conditions inside the DS.

BR, Ulysses

Peter BENSCH · ‎2022-03-14

The RX sensitivity can be adjusted within limits via register 3 and 4 (see datasheet, section 4.5.16 and 4.5.17).

If you fixate only on the maximum output power, you will inevitably reach the limit at which tags can be destroyed if the distance is too small.

Essentially, several things affect the maximum tag reading distance: Auto Antenna Tuning (AAT), Dynamic Power Output (DPO) and antenna size. The first two points are already covered by the ST25R3916, so that only the antenna size remains as a factor that can be influenced for the reading distance.

However, I do not assume that the reading distance will be much greater than approx. 20...22cm even with huge Class 1 antennas.

Good luck!

If the problem is resolved, please mark this topic as answered by selecting Select as best. This will help other users find that answer faster.

/Peter

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DSai.1 · ‎2022-03-15

Dear Ulysses,

thank you for your help.

If we provide a power supply less then 5V in 5V mode setting, could the 5V mode work correctly? or the maximum output power will be reduced for lower voltage power source?

Do you have an experiment environment to measure the result?

Best regards

Sai

Ulysses HERNIOSUS · ‎2022-03-15

Hello Sai,

ST25R3916 contains only an LDO for VDD_RF. So if you lower the VDD then it will also subsequently lower VDD_RF (maybe VDD-250mV) and thus also VDD_TX.

If you supply 4V it will still work correctly - but at lower output power.

Regards, Ulysses

DSai.1 · ‎2022-03-15

Dear peter

thank you very much for your detailed explanation.

I will study the datasheet more and try to design our solution. If possible, we would like to use an antenna with about 20cmX40cm size, and try to obtain a 30cm reading distance.

For our reference, could you tell me the size of Class 1 antennas?

best regards

Sai