CAN termination vs. choke

Hi All - Anybody expert on CAN termination ? I'm looking at a board with the standard CAN terminator (120 ohm split into two

60 ohm with a 6.4pf to ground in the middle). The board also has a choke on the CAN input (CHOKE DOUBLE .051MH 500MA SMD EPCOS B82790S0513N201).

All this looks OK except: The terminator is placed behind the choke. Doesn't that reduce the effectiveness of the termination (especially the cap which helps to damp any HF noise) ?

I searched about but didn't find any references... Any pointers ?

Thanks in advance, Best Regards, Dave

Reply to
Dave Nadler
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At a guess, the choke is there to suppress EMI from the board from leaking out along the CAN lines, rather than as a component of the CANbus termination as such.

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Rich Webb     Norfolk, VA
Reply to
Rich Webb

Yes, of course. The terminator is on the transceiver side, then the choke, then connection to CAN lines.

If the terminator would be on the line side, the common mode choke would not work.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

Sorry, I am missing something here.

The choke on the CAN bus side, before the terminator, means the impedance seen on the CAN bus does not match the required 120 ohm (except at DC), no ?

Can you explain your point Vladimir ?

Thanks, Best Regards, Dave

Reply to
Dave Nadler

then connection to CAN lines.

work.

It is a COMMON MODE choke.

Sure. I'l send you a bill.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

It's a common mode choke, meaning that it's bifilar wound (or should be), to allow differential currents to pass without being molested, but to block common-mode currents.

At 51uH it has an impedance of 100 ohms at 300kHz or so -- if the CAN bus is operating at significantly slower than that, the inductance wouldn't matter even if it weren't a common-mode choke.

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www.wescottdesign.com
Reply to
Tim Wescott

Thanks Tim. This choke is actually sector-wound, not bifilar, so has a very high Lstray (either

2700 nH if you believe TI measurements, or
Reply to
Dave Nadler

Looks like it was placed here out of concern for the ~3v pp common-mode noise on the CAN bus... Still digging...

Reply to
Dave Nadler

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