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Imaginary Load of ST25R3916B Drivers

Associate II


I am designing an NFC transmitter that uses ST25R3916B-AQWT.


​The coil will be connected to the RFO1/2 pins.


In order to design the coil properly, I will need to know the driver's impedance spec for the various d_res<3:0> multiplier values (Table 78 of the ST25R3916B Datasheet).

I was able to find the resistance values and also measure them using a resistor as the load to calculate the driver output impedance (sum of both RFO1/2). It approximately correlated to Table 126 and Table 78. Results and calculation used attached below. I used an oscilloscope to measure the load voltage.


image (7).pngimage (8).png


What I am unable to measure with this method, and need to know, is the imaginary part of these measurements. In other words, what the capacitive and inductive loads are as well for at least the RFO1 and RFO2 drive output value (i.e. Table 126).


Please let me know if you have these specs or an idea of how to measure. Thanks so much for your help.



ST Employee

Hi Sam,


AN5276 should describe the necessary procedures. According to ch.4 you will require a network or impedance analyzer.


BR, Ulysses

Hi Ulysses,
Thanks for your reply. 
The document AN5276 is helpful for designing our antenna. 
However, the document still doesn't actually present the values or measurement strategy for characterizing the driver itself. Since we need to match for the input impedance of the driver, we need to know what value to match for.

Our antenna's Q is about 100 so we would like to see what guidelines we should follow to ensure that we are matched for this narrow band.
Please advise. Thank you for your help.
Associate II



Any update on this front?

Again, we are just looking for the imaginary load (inductive/capactive, phase) spec of ST25R3916B-AQWT's antenna driver (RFO1/2 pins).

Both the Application Note you sent and the datasheet for the IC only state Zrfo =  2 Ω. Does that mean that the driver load is purely resistive? 


Please let me know what you find out.


Also, is there a way to contact LT's technical support directly? i.e. a phone number or email address?


Thanks so much for your help.




Hi sshafer,


During production we ensure/test that several parameters are within specification and guarantee device operation as specified in the datasheet. 

Unfortunately the imaginary part of the driver impedance is not part of those production tests. 

You could try to measure the impedance via a DC-block connected to the VNA (Without turning on the driver => tx_en = 0).

When the device is outputting VDD_DR level on one RFO and GND_DR level on the other RFO when not beeing active. This is why you need a DC Block.

BR Travis