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Original question:

DS90UB921-Q1: power over coax_921&940

DS90UB921-Q1: DS90UB921 with DS90UB940 combo

David Winstanley
Prodigy 70 points
Part Number: DS90UB921-Q1

The DS90UB921 datasheet 8.3.2 Page 44 states that the link capacitors should be the same at both the serialiser and the desierialiser and 0.33uF and 0.15uF are recommended elsewhere in the datasheet

but the DS90UB940 datasheet 9.2.1 states that there is a DC-balanced decoding scheme - recommend 33nF and 15nF which are an 10 times smaller than this.

What are the requirements for the DC-balancing scheme to work? Should we use 33nF and 15nF at both ends for the UB921/UB940 combo?

  • 0
    TI__Expert 8135 points
    Hello-

    The 921 back-channel requires larger ac-coupling capacitance regardless of the companion deserializer. For the UB921/UB940 combo, the 330nF/150nF capacitors are recommended.

    Regards,
    Davor
  • 0 in reply to Davor Glisic
    Prodigy 70 points

    Thanks for that Davor,

    Just to clarify - would i design in 330nF/150nF at both ends of the link?

    We have 330nF/150nF at the serialiser end and 33nF/15nF(as per datasheet) at the deserialiser end at present, if this is wrong what effect may it have from a dc balance/signal integrity point of view?

    Regards,

    Dave

  • 0 in reply to David Winstanley
    TI__Expert 8135 points

    Hello Dave-

    Yes, 330nF/150nF capacitors should be used at both ends of the link.

    The 33nF/15nF recommendation in the 940 datasheet is valid only when interfacing the device to the 94x serializers. I will file a documentation error so that the necessary information is included in the next datasheet revision.

    With the 33nF/15nF at the deserializer side, the back-channel signal may be attenuated more than acceptable, so you may see back-channel errors (I2C, GPIO communication) especially if using a long cable.

    Regards,
    Davor

  • 0 in reply to Davor Glisic
    Prodigy 70 points
    Hi Davor,

    If we are not using the back channel, is it safe to stay with 330nF/150nF at the ds90ub921 end and 33nF/15nF at the ds90ub940 end without causing any problems? Is the back channel used in the background by the 'system' for anything?

    Regards,

    Dave.
  • 0 in reply to David Winstanley
    TI__Expert 8135 points
    Hello Dave,

    Errors on the back-channel could potentially cause issues on the forward-channel as the serializer is constantly getting the deserializer status via the back-channel (e.g. is it locked?). If the serializer gets the unlocked status from the deserializer, it will interrupt the forward channel transmission and attempt to reinitialize the link.

    Regards,
    Davor
  • 0 in reply to Davor Glisic
    Prodigy 70 points

    Hi Davor,

    Thanks for this information I think this is what we are experiencing. This level of detail of how the system works seems to be missing from datasheets and application/evaluation information at present. Perhaps TI need to add a bit more 'under the bonnet' info on FPD-LINK operation.

    Regards,

    Dave.