ST25R3911B SINGLE-ENDED CONFIGURATION EXAMPLES AVAILABLE?

[Danyon Chu]

Posted on February 20, 2018 at 02:22

The ST25R3911B datasheet, page 14 Figure 3, shows a minimum configuration of antenna connection components. There are provided schematics and layouts for the differential antenna configuration, however are there any recommended component values/layout available for the single ended configuration?

Datasheet: 

http://www.st.com/content/ccc/resource/technical/document/datasheet/group3/ba/60/79/5c/bb/a9/41/76/DM00321525/files/DM00321525.pdf/jcr:content/translations/en.DM00321525.pdf

 

Application note: 

http://www.st.com/resource/en/application_note/dm00347152.pdf

 #st25r391x #st25r3911b

[Travis Palmer · Accepted solution]

Posted on March 01, 2018 at 16:20

Hi Danyon,

The ST25R3911B is capable of driving one differential antenna (using RFO1 and RFO2) or switching between two single ended antennas (one connected between RFO1 and GND the other to RFO2 and GND).

Which RFO/RFI is used by the chip can then be selected by flipping the 'rfo2' register.

These antennas could either be normal antennas (like the differential one) or 50Ohm antennas.

50Ohm antennas are already foreseen with damping resistor, series and parallel capacitor to be tuned to 50Ohm at 13.56MHz. They can then be plugged using a 50Ohm coax cable to the NFC reader unit. In addition the NFC Reader unit requires a matching circuit to match the IC's load impedance seen on RFO1/2 to 50Ohm.

The minimum load impedance for differential use is Zload = 8Ohm. (Datasheet Table 100) Therefore the minimum load impedance for single ended use would be Zload = 4Ohm.

Since the target (50Ohm) is fixed, the matching values do not have to be changed when changing the antenna:

For example:

4Ohm to 50Ohm matching network:

LEMC = 240nH

CEMC = 330pF

Cseries = 680pF

Cpara = 270pF

8Ohm to 50Ohm matching network:

LEMC = 330nH

CEMC = 220pF

Cseries = 470pF

Cpara = 180pF

For conventional antennas (not matched to 50Ohm) the approach of finding the proper matching network is similar to what is described in the AN4974.

You have to first measure the antenna parameter.

Measured Antenna Parameters:

Inductance @ 1MHz:    929nH

DC resistance @ 1MHz:    329mOhm

Parallel resistance @ SRF: 1500Ohm

SRF:             62.667MHz

To get the proper values for the single ended antennas you have to input twice the Antenna Inductance, DC resistance and Parallel resistance. The Self Resonance Frequency stays the same.

The inputs into the 'ST25R Antenna Matching Tool' would then be:

Inductance @ 1MHz:    1858nH

DC resistance @ 1MHz:    658mOhm

Parallel resistance @ SRF: 3000Ohm

SRF:             62.667MHz

Since the tool will calculate the circuitry for a differential matching network, we will have to input also twice the target impedance.

The 'Matching inputs' would look like this:

EMC Inductor: 270nH

DC Resistance: 500mOhm

EMC filter: 11750kHz

Target matching Z: 16Ohm (this will give us 8Ohm for single ended use)

Target antenna Q: 17

After hitting 'calculate matching values' you will get reasonable results.

Of course the matching inputs are subject to change - according to the AN4974 -

Antenna

matching

for

ST25R3911B/ST25R391x

devices

.

BR Travis

[AShir.10]

Hello Travis, I want to use standard 50 Ohm antenna with two outputs. What cable should I use for connection? Is coax cable is ok when using two outputs in differential mode or should i use shielded twisted pair?

[Travis Palmer]

Hello Anatoly,

Ideally a coax cable should be taken for single ended applications and shielded twisted pair with a defined characteristic impedance should be used for differential applications.

BR Travis

[AShir.10]

Thanks for the answer, Travis!

[EMeur]

Hello Travis,

Could you provide us with a schematics (pdf) of the suggested implementation?

Thanks and BR,

Eric