What is maximum voltage on ST25R3916B output?

[smithderek2000]

Hello, I have a system working that is multiplexing one ST25R3916B to sixteen antennas using relays, but we'd like to replace the relays with something quieter. Looking at either using FETs or an RF switch. However, in order to select the right solution I need to know the maximum voltage. What is the maximum voltage at the antenna? I've copied the X-NUCLEO-NFC08A1 design. Is the signal always positive to ground? (e.g. could I use a PFET?)

Also, have you gotten this to work with either a FET or silicon antenna switch? If so what part number worked for you?

Thanks,

Derek

[Travis Palmer]

Hello Smithderek,

the voltage on the antenna is depending on the used antenna topology (differential, single ended, single ended with cable => AN5592), Q-Factor of the antenna and output power.

Targeting a ISO15693 reader with very high Q-factor, the antenna voltage could reach approx. 100Vpp or more.

A normal reader as our demo boards may have something like 55Vpp.

There are certain requirements for such analog silicon switches:

They must be able to handle the high positive and negative voltages without degrading the system performance and they must have a low on resistance (e.g ~ <=1Ohm). Otherwise there will be an influence on the system Q-Factor.

Another possibility is to build your own analog switch using a nmos and pmos transistor.

BR Travis

[smithderek2000]

Hello,

The antenna switches from Skyworks, pSemi, etc. are all rated for max input power of +30dBm: would those be sufficient?

For the home made version with NMOS and PMOS in series, do you have recommended component values? Or what should I be looking for when selecting the parts? I don't suppose there's an app note for this? I'm already driving all the relays so it would be fairly easy to add a negative drive for the PMOS FETs.

Thanks,

Derek

P.S. Antenna multiplexing would be a good app note

[smithderek2000]

Hello, wanted to follow up on this; I'm still struggling with the best way to multiplex 16 antennas to one reader. Please see questions above.

[Travis Palmer]

Hello Smithderek,

A colleague of mine is looking into this topic. He will start with some simulations and afterwards a prototype to confirm the simulations.

We will keep you posted as soon as we have some results.

br Travis

[smithderek2000]

Thank you. The ways that I've investigated for 16 antennas / 1 ST25R391x IC are:

a) DPDT relays. Working now but expensive, big, and clicky

b) Change to single ended outputs, and each output feeds an 1:8 mux IC. Downside is how to change the circuit for single-ended output, and also changing the code to dynamically switch between output

c) Stay differential but add FETs after the DC block caps to GND. (NXP app note) but this will drag down the bus when you have 16 channels

d) Differential; use pass FETs on each channel, instead of relays - might work but I've never seen it tried. Needs FETS with 60V+ Vgs, and think about gate drive

e) Differential; use a lot of antenna switch ICs (1x DPDT, and then 2x 1:8 mux ICs) - still cheaper than relays but can't test without building a board.

[smithderek2000]

Hello, is there an update on this topic? I'm available for further discussion if it helps.