topic Multiple (differencial) antennas on single reader IC in NFC

Multiple (differencial) antennas on single reader IC

oferrer — Sun, 23 Mar 2025 01:13:31 GMT

Hello everyone,

I'm designing the switching between two differential antennas according to the design recommended in AN11314 Epig 4. Differential Design.

I have a question about the schematic shown in Figure 4. Each antenna has four N MOSFETs, two per line. The MOSFETs are in shunt configuration in cascade to ground.

However, the two series MOSFETs on each line have their SOURCE pins connected each other. Is this correct? Shouldn't GND be to SRC2, DRAIN2 to SRC1, and DRAIN1 to TX/RX line? If this is correct, what is the explanation?

Thanks in advance,

Osmany

Re: Multiple (differencial) antennas on single reader IC

Tomas_Parizek — Mon, 24 Mar 2025 07:06:55 GMT

Hello Osmany,

The reason is that the MOSFET´s cointain a "body diode", which is a diode between Source and Drain.

This diode can cause a conductivity/leakage for RF signal even though the MOSFET is in "off" state. Therefore 2 MOSFET´s are used where these two diodes a orianted as antiparallel. See bellow:

Tomas

Re: Multiple (differencial) antennas on single reader IC

oferrer — Thu, 27 Mar 2025 21:06:38 GMT

Hi Tomas (@Tomas_Parizek ),

Thank you for your attention.

Following your explanation, I've been doing some research (as I'm just a fan of the magic of electronics) and have found the back-to-back source common N-MOSFETs configuration applied to bidirectional power switches. When applied to each RF line, it looks like the AN11314 schematic.

I'm sorry to admit that I misunderstood the connections in the image you kindly provided, it suggests me that these two back-to-back configurations are in parallel from RF_P to RF_N.

In any case, I still have a question about Vgs driving, considering that the common source points remain not connected to GND. Is there any public hint to drive Vgs?

I truly appreciate your help, and I send you my best wishes,

Osmany