I have a NFC chip [Mifare Desfire EV1] which has input capacitance of 17 pF(pico farad).

I got ready-made antenna (Datasheet of Antenna) which has induction of 1.8 uH(Micro Henries). So by simple formulae when i attach chip to antenna it resonates at 28.77 MHz(MegaHertz).

But I need it to resonate at 13.56MHz. So i am thinking of attaching a capacitor.

So my question is :

To resonate at 13.56MHz, Do I need to add 59pF capacitor parallel to Chip and antenna.

Why 59pF :

17pF of chip + 59pF c1 => 76pF which gives 13.56MHz

  • 1
    \$\begingroup\$ Have you checked the reference matching circuit provided in the datasheet? There's more to it than a simple resonant frequency calculation. \$\endgroup\$ – Dmitry Grigoryev Sep 27 '16 at 15:01

This Antenna is good because of challenge to obtain accuracy of L and C.

Note that this loop antenna is designed for two-way or Duplex mode of transmitter (Tx) and Receiver (Rx). Each passive part can affect tuning as well as proximity to ground and connection wire impedance.

Note that the final values of radio filter design are critically affected by physical layout geometry.

C1, C2 below are essential to the operation and should be C0G type or equivalent quality capacitors for temperature stability. Note that a good balanced design is used so the C values are split into two values. Balance makes common mode noise rejection possible.

enter image description here

Originally I said..

Did you use two 33pF for series resonance as specified?

That is the proper mode, not parallel resonance. Ground proximity will affect tuning.

Did you make solder contacts?

The other parts provide EMC notches.

  • \$\begingroup\$ No i havent use any capacitor yet. Are you sure about Series resonance? and yes i made solder contacts. \$\endgroup\$ – Suraj Jorwekar Sep 27 '16 at 4:49
  • \$\begingroup\$ ohhhh now i get it. Thank You Tony Stewart. I will try and let you know the output. Thank you so much \$\endgroup\$ – Suraj Jorwekar Sep 28 '16 at 16:03
  • \$\begingroup\$ so basically i have to add 33pf capacitor at tx1 and tx2 right ? \$\endgroup\$ – Suraj Jorwekar Sep 28 '16 at 16:10
  • \$\begingroup\$ hey i used 33pf cap. in series n parallel but dint work. then i used 56pf cap. and the chip started working but at very less distance. like i literally had to touch the receivers antenna. Any suggestions ????? \$\endgroup\$ – Suraj Jorwekar Oct 26 '16 at 5:17
  • \$\begingroup\$ a lot of unknowns in your description, layout and part numbers \$\endgroup\$ – Tony Stewart EE75 Oct 26 '16 at 14:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.