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DS90UB953-Q1: AC coupling capacitor values

Jon Bilbao
Intellectual 1415 points
Part Number: DS90UB953-Q1

Hello,

Why is it recommended to have 100nF and 47nF on the outputs DOUT+ and DOUT- respectively for the AC coupling capacitors for coaxial asynchronous mode?

Why can we not place same capacitor values on both (+ and -)?

We think this would make the output driver stage loaded non-symmetrically which is usually never good idea for EMC.

Is there something special in output driver stage that needs non-symetrical impedance termination?

Regards,

Jon

  • 0
    TI__Guru* 90860 points

    Hello Jon,

    Once you are using single ended (Coax cable) you need to use double the value of the terminated output. this is because you are using two capacitors in series, one on the Serializer output and one at the Deserializer input. The addition of these two capacitors will be half the value, which is here ~47nF. At the end you have symmetrical load.

    100nF in Series with 100nF = 50nF hence the 47nF standard value. And the terminated output has 47nF

     

  • 0 in reply to Hamzeh Jaradat
    TI__Intellectual 1415 points

    Hi Hamzeh, 

    This would be of course OK in case the serializer and deserilizers are placed very close to each other. But does it also work when you have 2 connectors and couple of meter of coax cable in between? Due to attenuation the cap on the far side has probably no or minimal effect.  Am i wrong?

    Regards,

    Jon