Fig 15. Optimized Matching Circuit; Damping Resistors; Parallel Capacitor - NXP Semiconductors PN7150 Hardware Design Manual

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Fig 15. Optimized matching circuit

7.2.1 Damping resistors

R
q
Indeed, a too high quality factor will negatively impact the generated signal shaping.
However, in an embedded environment, the presence of metal (battery, PCB tracks,
electronic components...) tends to significantly reduce the antenna quality factor.
Therefore, although they can be needed on an open-air board environment, it is very
unlikely that these 2 resistors are required for embedded equipment integration.
Proof point:
Once the customer has measured its NFC antenna within its final environment he should
calculate the resulting quality factor as explained in the PN7150 Antenna Design Guide
[4]. If the quality factor is below or equal to 35, R
See circles 1 in above Fig 15.
7.2.2 C
2
Two serial capacitors are used in parallel with the antenna to tune it to 13.56MHz.
The reason to have 2 capacitors in parallel in that the peak to peak voltage at antenna
ends can reach more than 50V depending on the antenna geometrical characteristics
(size, number of turns...)
However, if for a given antenna, the maximum peak to peak voltage measured at
antenna ends is lower, then a single parallel capacitor on C
Its value needs then to be divided by 2 compared to the ones used in case of 2 serial
capacitors solution.
Proof point:
As the voltage generated at antenna ends for a given external field power will be strongly
linked to the antenna environment, this measurement must be performed within the final
product.
AN11756
Application note
COMPANY PUBLIC
damping resistors are used to lower the antenna quality factor when it is above 35.

parallel capacitor

All information provided in this document is subject to legal disclaimers.
Rev. 1.2 — 15 January 2018
resistors can be safely removed.
q
347612
AN11756
PN7150 Hardware Design Guide
can eventually be used.
p
© NXP B.V. 2018. All rights reserved.
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