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SN75LVCP600S: SN75LVCP600S vs EVM - AC Coupling 10nF vs 100nF

Steven VALSESIA
Prodigy 30 points
Part Number: SN75LVCP600S

Hello,

We are experiencing some issue with this schematic :

It works well with SATA Gen2 3Gbps but a lot of errors in SATA Gen3 6Gbps.
This PCB is placed between a ComExpress board and a SATA SSD (cable + rack).

I now there is some capacitors missing on RX lanes, but on the datasheet 10nF is used for ac coupling, while on the evaluation board 100nF is used.
The SATA rev3.3 say 12nF max.

Can someone say why there is 100nF on the evaluation board?
Am I overthinking it? Does it just works better with 100nF? Or a mistake?

Thanks !

  • 0
    TI__Guru* 91440 points

    Hi Steven,

    Agreed. 10nF is more like it. 100nF has two side effects versus 10nF: it allows more low frequency content of the data sheet. Secondly it could be used to better match to trace impedance at Nyquist. Suggest is to go with 10nF and then have 100nF in the back of mind when checking for SATA Gen3 capability. 

    As it relates to SATA 3 bit error, suggestion is to start with minimum EQ and then increase - use no DE when sweeping EQ. Afterward you can enable DE and again start with minimum EQ and then increase.

    Regards, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 30 points

    Hi Nasser, thanks for the clarification.

    We learned that the seller of the ComExpress card put 220nF on both TX/RX sides, however we do not know if there is an ac coupling on SATA disks.

    If I understand correctly, we need to put 10nF to lower the capacity <12nF on all TX/RX "host"/"device" channels?

  • +1 in reply to Steven VALSESIA
    TI__Guru* 91440 points

    Greetings Steven,

    Typically this is done on the TX side. Most probably the TX driving RX of the device already had this AC coupling Caps as well. If it does not then it would be a good idea to put this on RX as well - mainly to isolate common mode voltages and reduce potential power sequencing side effects.

    Regards, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 30 points

    Thank you very much for all these information :)