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DS90UB953-Q1: proper setup for EMI and Spread Spectrum

Andrew Barajas
Prodigy 20 points
Part Number: DS90UB953-Q1


Using this chip in one of our medical products. We are having issue with EMI despite following datasheet guides for layout. Are there any caveats that we may be over looking in terms of vulnerability with this part? (layout is pretty straight forward, don't really see issues, might post it later). Results in video loss during electrocautery surgery (high voltage spark gap). The other part of this question is there is suppose to be support for Spread Spectrum on this chip. Is that true and how do you properly set that up? The reason why I ask is we have found other products that use Maximum chip serializers with spread spectrum and PLL clock locking, they seem to be more invulnerable to these issues during our testing. I know its not really meant for that but the combination may allow for protection.

  • 0
    TI__Guru** 103215 points

    Hello Andrew,

    Yes 953 supports spread spectrum when paired with 954/960. To enable spread spectrum you must use the serializer in synchronous mode and apply a spread spectrum REFCLK to the deserializer side which will have the effect of spreading both the forward and back channels for the SERDES pair. Is this possible in your system?

    Best Regards,

    Casey 

  • 0 in reply to Casey McCrea
    Prodigy 20 points

    Hi Casey,

    That sounds about right. We don't have a multiphase oscillator implemented so I don't think we can implement this currently in the design but that was to be expected. We are already running in synchronous mode so we are good there. Do you have any references on how spread spectrum should be implemented for this chip in terms of circuit design? is there anything we have to do on the serializer side other than run at synchronous mode? Are there any additions that need to be done to take care of clock jitter or would that be taken care off if I just use something like this www.analog.com/.../6902f.pdf

  • 0 in reply to Andrew Barajas
    TI__Guru** 103215 points

    Hello Andrew,

    There is nothing needed to implement SSC on this link other than to apply an SSC REFCLK to the 954 side. The spread properties are shown in the 954 datasheet.

    One alternative option you could try is to use non-synchronous mode with internal AON clock and see if there is any improvement. To enable this mode, select AON clock mode in the 953 register 0x03 and set the 954 back channel rate to 10Mbps with 954 register 0x58. 

    The 953 AON clock has an inherent "pseudo-spread" effect just due to the fact that it is not as tightly frequency controlled as a dedicated external oscillator. We have seen some customers get significant improvement on their system level noise performance by trying that and it does not require any HW change

    Best Regards,

    Casey