How to calculate the voltage divider capacitor values for reading in ST25R3911B?

[PPill]

I am trying to calculate the matching network for my project. In the Antenna Matching document it is mentioned that the incoming signals at RFI pins must be limited to 2.5Vpp. So my question is how to design this voltage divider network? Tool gives me 10pF and 150pF values, but how can I confirm this?

It would be very helpful if someone can post a detailed answer about the voltage divider network.

Also, the matching network I calculated comes out to be 210pF and 41pF whereas the tool gives me 199pF and 41pF. Is this tolerance acceptable?

Thanks in advance!!!!!!!! :)

[Travis Palmer]

Hello Prashanth,

The capacitive voltage divider is to feedback a fraction of the voltage on the antenna to the receiver input pins of the ST25R3911B. V_RFI is calculated like this:

(Assuming a capacitive voltage divider 10:150)

There are now two easy ways to get your V_ANT.

1.) You use the build in simulation in the "ST25R Antenna Matching Tool" (Save & Simulate).

There you can also adjust the capacitive voltage divider and see its influence.

2.) You assemble the estimated matching components to your reader board and measure the voltage across the antenna.

When doing the calculation of the matching components, have you considered the capacitive voltage divider? It is put in parallel of the parallel tuning capacitor and therefore adds additional 9 to 10 pF. (according to the ratio)

Our tool already takes this into account and therefore your and our calculation would fit quite well.

BR Travis