Why does NFC tag only need one shunt cap for matching

[yzhu12]

Hello, there

  Using NFC antenna tool of NXP, I found that it shows only one shunt capac placeholder for matching is possibly required (actually optional as showed). I don't quite understand the result of this tool for NFC tag if we are tuing the antenna coil matching network to the targeted 50ohm. Why is not extra shunt resistor and serial capacitor for tuning actually required for tag IC?

Hope someone can share some thoughts on this. Thanks.

Regards,

Yuanchen

[KellyLi]

Hello @yzhu12 

Thanks for contacting us and interested in the NXP products. Based on your description it's not clear to us what configurations you are setting. So, please share the following configurations with us.

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Kelly
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NXP Semiconductor

[yzhu12]

In other words, if we use the way of designing a reader antenna coil for that of a tag, do we expect better results?

Thanks,

Yuanchen

[KellyLi]

Hello @yzhu12 

Yes, I understand your needs and I have consulted with a team of experts.
In principle, if you strictly follow the rules of NFC Tool to design, the resulting parameters of antenna do not need to be greatly adjusted. When fine-tuning is required, you must refer to the Smith chart to judge the adjustment direction, and the adjustment direction of capacitive and inductive loads is different. Therefore, your needs depend on the actual situation, and you cannot simply fix the capacitor and adjust other parameters.

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Kelly
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NXP Semiconductor

[yzhu12]

Hello, Kelly

  Thanks for your kindly reply and attachment. Just one last question, what is the rough value of the chip input resistance (RIC) that we shall expect (for NTAG), as mentioned in the attachment? Just a rough range for concept is enough for me. Thanks.

  Regards,

Yuanchen

[KellyLi · Accepted solution]

Hello @yzhu12 

The following equivalent circuits simply describe the characteristics of NTAG integrated circuits as they relate to antenna design. That is to say to design an antenna impedance matching equivalent circuit viewed from LA to LB. The NTAG IC input resistance RIC together with the loss resistance of the antenna and the connection resistance defines the quality factor of the tag. And its value depends on the chip input voltage VLA-LB. RIC is a variable quantity based on the actual application design. For example, the design of the NTAG I2C Plus development board, using the NFC antenna design tool to design the result does not require such an input resistor. So, the input resistance is not a definite value, depending on the size of the antenna, RF field strength. For NATG I2C, as long as you input the corresponding parameters according to your actual application, the final results which is from NFC Antenna Tool will basically meet the requirements. So, don't worry about what the input resistance is.

 Best Regards,

kelly

[KellyLi]

Hello @yzhu12 

There is a document should help you, please check the attachment.

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Please login to our NXP technical support portal:
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Thank you for your interest in NXP Semiconductor products and for the opportunity to serve you.

Best regards,
Kelly
Technical Support
NXP Semiconductor