ST25R3911B Field Strengh Ratio

[SnoZek]

Hello,

I'm working with ST25R3911B and in my application i want to control the field strengh.

I relied on the documentation at this chapter : 1.3.40 RFO normal level definition register page 80 in the Datasheet - ST25R3911B - High performance HF reader / NFC initiator with 1.4 W supporting VHBR and AAT

And, if i understand well if i write 0x00 in the 0x27 register it will enable all Ohms segments, it will make 510 ohms.

If i write 0x01 510-256=254 right ?

But when I test my application with a field detectorit it doesn't seem to react as expected. 

Am I doing something wrong ?

Best regards,

Yvan

[Ulysses HERNIOSUS · Accepted solution]

Hi Yvan,

the segments of the drivers are parallel. Thus you also need to apply calculation of parallel resistors, not series resistors (check e.g. https://en.wikipedia.org/wiki/Resistor). 

Regards, Ulysses

[SnoZek]

Hello Ulysses,

Thanks for help

If i understand well the value of the resistance depending on the register looks like this ? 

Regards,

Yvan

[Ulysses HERNIOSUS]

Correct.