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THVD8000: Decoupling DC with common mode choke

Marc Engineer
Prodigy 10 points
Part Number: THVD8000


We would like to use THVD8000 for RS485 communication over DC power line.

Power line is loaded with up to 12 A. To reduce costs I'm thinking about replacing the two single decoupling inductors with one common mode choke as demonstrated here: https://e2e.ti.com/support/interface-group/interface/f/interface-forum/75687/data-over-dc-power-line-using-rs485-2-wire

Are there any considerations?

  • 0
    TI__Guru* 77271 points

    Hi Marc,

    Give me a day or two to verify with one of colleagues. 

    Sorry for the delay.

    -Bobby 

  • 0
    TI__Guru 53152 points

    Hi Marc,

    I am sorry for the delay here.

    No you can't use common mode chokes in place of the inductors - the link you refer to is not using common mode chokes - they are differential (the dots are on opposite sides of schematic symbol). They need to be differential because the signal they are blocking is differential not  common mode (they are used to block the A - B signal which is going to be primarily differential). Essentially you can use coupled inductors - but they must be differential.

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 10 points

    Thank you for clarifying. The figure shows differential mode chokes, but the text explicitly says "common-mode choke". So text must be wrong.

  • 0 in reply to Marc Engineer
    TI__Guru 53152 points

    Hi Marc,

    No problem - I am not sure why they put that in the text - I understand the confusion.

    Common mode chokes are used in THVD8000 applications (or power over bus - but generally THVD8000 is better than standard RS-485 over bus)  - but more for common mode noise mitigation on the shared bus - so if common mode chokes are used  it's not part of the coupling network but essentially just a filter on the line. 

    Please let me know if you have any other questions!

    Best,

    Parker Dodson