ST25R3911B - Calculating coil inductance

[Gualbert Maas]

Posted on April 10, 2018 at 15:53

Hi all, I am looking at the ST25R3911B-DISCO board and I want to see what the inductance is of the coil they have on the eval kit. I can't seem to find the formulas for this specific application where you have 2 coils shorted to GND as it is done on the DISCO board.

Also, I would like to understand why ST, on the DISCO board, have 2 coils shorted to GND rather than just 1 coil connected between RFO1 and RFO2?

On the eval kit the distance from coil to circuitry is approximately 10mm but what would the general safe distance be from the coil. I know 'more is better' but I am a bit crammed for space in my design.

Any insights to the above is greatly appreciated

[Travis Palmer · Accepted solution]

Posted on April 13, 2018 at 11:24

Dear Gualbert,

The antenna parameters and how to gain them can be found in AN4974 page 16 which is available on st.com 

The used antenna consists of two antennas which are put in series and share a common ground connection.

This allows the user to play around with the 'single ended' antenna feature of the chip. In this mode only one antenna will be driven by only one RFO channel. In this way the user can experience the difference between single ended driven antenna and differential driven antenna by changing only a few bits in the register. 

In fact there are three possible combinations:

1.) differential using RFO1 and RFO2

2.) single ended using RFO1

3.) single ended using RFO2

A metal inside the antenna will cause losses. Due to the magnetic field produced by the antenna eddy currents will flow inside the metal. The distance between antenna and metal will determine how big your losses are.

The reader will work even if there are only a few mm distance between antenna and metal but the performance will be decreased. You could compensate this for example by increasing the output power. I would recommend you to make your antenna as big as possible - according to you geometry constraints - and then optimize the electronics inside to get as much as distance between metal and antenna.

BR Travis