TRF7970A: Detecting MIFARE DESFire EV2 cards

Wilmer Suarez

Part Number: TRF7970A

Hello,

I'm having a fair amount of success with the detection and reading of MIFARE DESFire EV2 cards; but I'm running into a weird issue. Is there a reason why the edges of the tag are better detected than the center. When placing the center of the card near the antenna the detection rarely works. Placing the edges of the tag never fail. I cannot find a reason why this might be the case.

Thank you

over 5 years ago

0 Helfried Vollbrecht over 5 years ago

TI__Mastermind 18265 points

Hello Wilmer,

at the edges of the antenna you have a higher field-strength which explains why the card detection is better.

I would recommend to check the antenna parameters and the impedance matching (chapter 7.3 in the datasheet).

Maybe you will find the document "Antenna Design Guide" also helpful:

http://www.ti.com/lit/an/sloa241b/sloa241b.pdf

Best regards,

Helfried

0 Ralph Jacobi over 5 years ago

TI__Guru*** 133335 points

Hello Wilmer,

To add, DESFire tags are notorious for being off tuned from 13.56MHz. I've even had some tuned to 16-17 MHz. This is done to reduce the read range.

It could explain this behavior as you may be able to read the offtuned tags better away from the center of the antenna where the 13.56MHz field would be the strongest.

0 Wilmer Suarez over 5 years ago in reply to Ralph Jacobi

Intellectual 371 points

I was considering switching to MIFARE DESFire Light cards as I only need a single application and it consumes less power. Do you know if this issue is prevalent in those cards?

0 Ralph Jacobi over 5 years ago in reply to Wilmer Suarez

TI__Guru*** 133335 points

Hello Wilmer,

I haven't tested DESFire Light tags. If you have access to a network analyzer you could test the resonance for the tags. We have steps for checking resonance in documentation for our dynamic transponder, you can reference that to check the resonance for basically any tag, look at Section 7.2: http://www.ti.com/lit/an/sloa217/sloa217.pdf - The small test tool (Figure 17) would work well.

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TRF7970A: Detecting MIFARE DESFire EV2 cards