Skip to main content
B
Broman3100
Graduate
November 21, 2022
Solved

Problems with EMI calculation tool

  • November 21, 2022
  • 1 reply
  • 1112 views

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

0693W00000WJ4E8QAL.png 

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.0693W00000WJ4IoQAL.pngUsing online impedance calculator (https://www.translatorscafe.com/unit-converter/en-US/calculator/inductor-impedance/) I got 67.36ohm

0693W00000WJ4KQQA1.pngHaving 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.0693W00000WJ4QnQAL.pngAdmittedly 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.

    This topic has been closed for replies.
    Best answer by Henry Crane

    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:

    0693W00000WJSNTQA5.pngAs 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

    1 reply

    H
    Henry CraneAnswer
    ST Employee
    November 24, 2022

    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:

    0693W00000WJSNTQA5.pngAs 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

    Terms & ConditionsAccessibility statement