Design questions around ST25R3911B

[Nicolas Goy]

Posted on June 19, 2018 at 19:10

I need to integrate an NFC reader for medical device pairing. I already designed a tag for the sensor which works with my phone.

The next step for me is to add an NFC reader to the portable monitor. The biggest antenna I can fit into the back of the monitor is about 15cmx10cm and as it is quite big, I picked the ST25R3911B to drive it because it has good power. I need a read range of about  20-30cm. Apparently, the best standard to achieve such range is ISO15693. My current tag uses ISO14443/B but I can easily swap the IC with something like the MLR24LR.

The sensor has a zigbee chip, and I only need the NFC to exchange the encryption key (about 128bytes with all metadata), so datarate is not important.

Some info:

- Tag is 45mmx35mm, using TI RF430CL330, but if ISO15693 is preferred, I can swap the IC and redesign the PCB.

- The max antenna I can fit inside my monitor (NFC reader) is 15cmx10cm.

- I need to pass 128bytes of data. (1kbit) Read only (only written by the sensor microcontroller using SPI)

- Read distance should be minimum 20cm, 30 or even 50cm preferred.

I have a few questions:

- Is ISO15693 the best choice for my application?

- I picked the ST25R3911B, is it good, or another chip?

- If MLR24LR the right choice for ISO15693 for the tag?

- I downloaded the antenna matching tool, I understand the antenna parameters inputs, but I wonder about the 'matching inputs', there are presets called 'VHBR, EVMCo and General purpose', but I am not sure I understand how those impacts the system.

- In AN4974 the antenna is attached to the ground at the middle, is this necessary for differential design (using both RFI0/1 and RFO0/1 connections, design of figure 3 in DS11793 page 14)?

#nfc

[Travis Palmer]

Dear Nicolas,

Your calculation seems feasible. i would try using 10/330pF and then adjust the capacitive voltage divider based on the measured V_rfi voltage.

In order you want to enable AAT you have to replace the trim capacitor by a capacitive voltage divider similar to the RFI capacitive voltage divider. The capacitance of these voltage dividers should have the values 56pF. 27pF, 12pF and 5.6pF - similar to the current trim capacitance values. If necessary you can increase or decrease the trim range afterwards.

The target voltage should be <= 20V.

BR Travis

[SGeor.15]

Dear Travis,

We are also trying to achieve up to 25 cm reading distance by using the ISO15693 standard.

I would like to replicate your setup, for demonstration purpose. Unfortunately, we do not have an VNA that goes that low in frequency. In that case can you reveal what type of wire you used as reader antenna, thickness and length? Do you have these values available to share?

Best regards,

Sindre Georgsen

Development engineer

Informasjonskontroll AS

Norway

[Travis Palmer]

Dear Sindre,

I used an "Insulated copper wire" with 0.5mm diameter.

I can recommend you a device called “miniVNA pro�?. It can be used for matching the antenna to a real ohmic impedance for a quite reasonable price.

BR Travis

[rg12]

Hi Travis,

Apologies for resurrecting an old post and I recognize you may no longer have the details, however I was looking for some help regarding increasing the read range of the ST25R3911B-Disco. However I am unsure of the procedure you followed: which further changes were necessary from changing the antenna and soldering the jumper on J303?

[Travis Palmer]

​Dear rg12,

All nessesary changes have been described in my first response.

The design flow would be the following:

1.) Do a antenna 14x10 cm (wire wound)

2,) Measure the antenna parameters as descirbed in the "AN4974 Antenna matching for ST25R3911B/ST25R391x devices"

3.) Adapt the matching inputs in the ST25R Antenna matching tool according to your application

4.) Simulate and verify the calculated matching component values.

5.) Adjust the capacitive voltage divider. V_RFI should not exceed 3.0Volt

6.) if nessesary (Powerconsumption in constant wave > 200mA) bypass the internal regulator = shortcut J303

Please let me know if this answers your questions.

BR Travis

[rg12]

Hi Travis,

Thank you very much for describing your process flow. I was unsure if hardware changes were required on the Disco-Board, and I see now that it is.

Regarding V_RFI, is this adjusted by first measuring the output voltage of V_RFO and then adjusting the capacitive voltage divider accordingly or is the value of V_ RFO determined during simulation.

Cheers

Rg12

[Travis Palmer]

​Hello Rg12,

Both approaches are valid.

I would recommend to adjust the RFI value in simulaton between 2.5 and 2.9V and after assembly crosscheck and finetune it. Be aware, the ST25R Antenna matching tool does not calculate the capacitive voltage divider. It chooses a default value of 10:150pF. In simulation you can then alter this value according to your need.

br Travis