CR95HF transceiver antenna tuning - conflicting information/measurements regarding voltage on ST_R0 and RX1 resp. RX2 Pin

[teysachaux]

Hi there,

I made a design with the CR95HF and it's working. The design of the EMI filter and matching circuit is done like on the X-NUCLEO-NFC03A1 NFC card reader board, see [UM1799] page 12.

I'd now like to tune the components on my design to increase the reader performance. In [AN4327] on page 15, there is a point saying: "Measure the DC voltage in the ST_R0 pin: adjust the ZRX component value to limit the voltage measured on ST_R0 below 7 V." I guess it's to make sure, the differential voltage between RX1 and RX2 stays within the allowed range, because if it is above 7 V one should adjust ZRX.

If I'm measuring the voltage between GND and ST_R0, I measure an average value of 8.6 V in my design (see hhd-gnd_str0.png). On the X-NUCLEO-NFC03A1, I measure 9.8 V (see nucleo-gnd_str0.png). Both voltages are over the recommended 7 V from [AN4327].

However if I'm measuring the voltage between pin RX1 and RX2, I'm far away from the maximum differential voltage of 18 V mentioned in [CR95HF] page 47 (see hhd-rx1_rx2.png and nucleo-rx1_rx2.png).

My questions:

What does the voltage on pin ST_R0 indicate?

Is it no problem to have a voltage above 7 V @ ST_R0 pin?

If it is no problem, why is it mentioned the way it is in [AN4327]?

If it is a problem, why is it around 10 V on the X-NUCLEO-NFC03A1 NFC card reader board?

Best regards

teysachaux

===

[UM1799] Getting started with X-NUCLEO-NFC03A1 NFC card reader board based on CR95HF for STM32 Nucleo - https://www.st.com/content/ccc/resource/technical/document/user_manual/6b/a3/ef/49/6b/75/43/04/DM00125142.pdf/files/DM00125142.pdf/jcr:content/translations/en.DM00125142.pdf

[AN4327] CR95HF tuning circuit with an EMI filter - https://www.st.com/content/ccc/resource/technical/document/application_note/8e/99/77/a7/4f/58/4d/d6/DM00089926.pdf/files/DM00089926.pdf/jcr:content/translations/en.DM00089926.pdf

[CR95HF] 13.56-MHz multi-protocol contactless transceiver IC with SPI and UART serial access - https://www.st.com/resource/en/datasheet/cr95hf.pdf

[Henry Crane · Accepted solution]

​Hi,

DC level on ST_R0 is a picture of the RX1-RX2 RF signal magnitude.

An electrical path exists between ST_R0 pin and RX1/RX2 pins so use of this pin is restricted. It shall be decoupled with a 1nF capacitor. Adding a resistor on this pin is equivalent to put a resistor between RX1 and RX2 with the result that the overall Q factor of the antenna circuit decreases and the voltage on RX1/RX2 pins too.

Dedicated protection circuits prevent IC I/Os to be damaged by overvoltages. Voltage on RX1 and RX2 pins (referred to GND) shall not exceed 9V. Over 9V protection circuit slightly enters in action with a possible clamping of the signal on RX1 and RX2. 9V maximum on RX1 and RX2 leads to a maximum peak to peak differential voltage between RX1 and RX2 of 18V.

The attached plot show probing of VRX1, VRX2 and VST_R0 using three 10-20pF oscilloscope probe (similar to those you use with picoscope) on a NUCLEO-NFC03 . You can see the relationship between voltages in the system. Because of the high parasitic caps of probes, the antenna circuit is detuned and voltage on ST_R0 is lower than in reality. However, as you can see, your measurements hdd_RX1_RX2 and nucleo_RX1_RX2 are lower than mine.

I do confirm VST_R0 close to 10V on NUCLEO-NFC03 which is above the recommendation. There is no risk of IC damage but a smight clamping of signal on RX pins may occur which could lead to a small loss of performance reading tags at large distance. 8.6V in your system.

You can see that there is rougly 1V between RX amplitude and ST_R0 amplitude. As a consequence, recommendation of 7V on ST_R0 in AN4327 is a bit conservative and could be extended to 8V. 

feel free to contact us for further details.

best regards,

Henry Crane, NFC/RFID support team.