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TCAN1462-Q1: Common mode choke placement

Martijn Wams
Prodigy 10 points
Part Number: TCAN1462-Q1

I have a question regarding the placement of a common mode choke; why would the common mode choke be placed between the termination resistors and the transceiver, instead of being between the termination resistors and the transceiver (indicated with red arrow)? The image is taken from the TI SLLA271 application note (https://www.ti.com/lit/an/slla271/slla271.pdf?ts=1704355254549)

 


I think it is preferable to have a high impedance common mode path to board GND for high frequency EMI currents.

This CM choke for example, has a 3000 ohm impedance at 10 MHz: https://www.we-online.com/components/products/datasheet/744235510.pdf, while the common mode impedance of the termination network is about 30 ohm to GND (if we ignore the capacitor). So, moving the CM choke to the cable side increases the impedance for high frequencies significantly. Thanks in advance!

  • 0
    TI__Guru 74252 points

    Martijn,

    Most designs configure the choke this way so that the cable and rest of the CAN bus "see" the termination impedance first. This also allows the CMC to be last line of defense for high frequency noise going into the transceiver, and the first line of defense for emissions out of the transceiver. You analyses are correct though, and I have seen some customers place the CMC on the other side of the transceiver, but the majority have it configured as shown in the diagram.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    Prodigy 10 points

    Thanks! I will place some 0 ohm resistors parallel to the CMC, and to that on either side of the termination so that we can do some experimentation.