Problems with EMI calculation tool

[Broman3100]

Good day everyone,

I'm trying to make my own NFC antenna for ST25R95. This is my first time making antenna and I got errors in EMI calculation, probably because of bad values on components since I couldn't find much on internet to reference from.

First of all I got the inductance 790.63nH

 

Then, using online calculation tool ( https://www.allaboutcircuits.com/tools/trace-resistance-calculator/ - can't insert url for some reason ) I got antenna resistance of 0.736ohms.Using online impedance calculator (https://www.translatorscafe.com/unit-converter/en-US/calculator/inductor-impedance/) I got 67.36ohm

Having all the antenna values, I started filling out the EMI calculation tool. I used values of components I have at hand for low pass filter, 2.2uH for inductors and bunch of pico-nano capacitors as well as variety of resistors for receiving part, I still keep getting error for the matching circuit calculations.Admittedly even after reading ST's and other available sources (AN3394, AN5248...etc.) I still dont fully understand the component values I should be aiming for. The EMI manual said to input the EMI inductor/capacitor values but didn't mention if I can use any. Similar for the receiving resistors...

If anyone can share their insight on component selection, it would be much appreciated.

[Henry Crane · Accepted solution]

Hello,

the issue comes from the fact you entered a value in the Za cell while this cell content is calculated automatically from the Ra and La values. Here is the result with the component values you chosen:

As you can see, the calculation results with negative C11/C12 capacitances.

I suggest you to decrease the L0 value to move the EMI filter resonant frequency above 13.56MHz; the calculated solution will become valid.

1KOhm resistors in the RX paths is a maybe to high. Maybe you can reduce their value to few hundreds kOhms.

feel free to ask for more if necessary.

Best regards,

Henry Crane,

RFID/NFC application engineer