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TCAN1042V-Q1: CANH and CANL has no ringing without CMC but has ringing with CMC

Amelie Zheng
Genius 10865 points
Part Number: TCAN1042V-Q1

Hi Team,

My customer now use TI's TCAN1042V-Q1 and found a really strange thing.

When customer has CMC in their board, the output of TCAN1042 has ringing as following:

However, when customer doesn't use CMC in their board, the output is very beautiful, as following:

Here is the SCH:

And the PCB layout is also as following:

The CMC which they used is as attached:

murata-DLW43SH510XK2.pdf

What customer really concerned is whether to add the CMC or not...

May I ask the following questions:

  • Why does it ring when CMC is added?
  • Do we recommend to add CMC or not? What most customers choose, with/without CMC?

  • 0
    TI__Guru 60022 points

    Amelie,

    Thanks for bringing this to our attention.

    • The ringing in the signal may be caused by the introduction of the inductance from the CMC to the CAN bus. This, with the capacitance can cause ringing due to resonance. Where on the schematic is this measurement being taken? And is this device on a bus or being measured alone? Is it possible for the customer to run a test by removing C1007 and C1002 and see if it lessens or goes away? Adding more capacitance is an option for this test too.

    • In most applications, especially automotive, a CMC is present because it helps with RF emissions and immunity by impeding high frequency signals. We typically recommend it because of this. What kind of application is this?

    Regards,

  • 0 in reply to Eric Hackett
    TI__Genius 10865 points

    Hi Eric,

    Thanks for your reply.

    May I ask 

    • Whether customer need to measure device alone without bus or measure the device on the bus? could you please give some guidance how to measure?
    • Adding more cap is an option of this test. Here where did you mean to add cap.

  • 0 in reply to Amelie Zheng
    TI__Guru 60022 points

    Amelie,

    I was curious how this was measured to see if the bus capacitance/inductance also played a factor. Measuring this on the bus that it will be communicating on will be more applicable since that's when the signal integrity will matter. Where on the schematic are these waveforms being measured?

    And more capacitance can be added where C1007 and C1002 are placed on the bus in the schematic.

    Both of these questions/suggestions were meant to be tests to see if the ringing was caused by introduction to the CAN bus and/or the resonance of the capacitane and inductance on the bus pins.

    Regards,