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Requesting information for DS125BR820

user4760726
Intellectual 1195 points
Other Parts Discussed in Thread: DS125BR820

Hi team, 

One of our customers wants to know Pre-channel loss and Post-channel loss of DS125BR820 for layout. Is it possible to provide the information? 

Just for reference: pre-channel loss means allowable loss budget between transmitter/Tx and re-driver/retimer and post-channel loss means allowable loss budget between re-driver/retimer and receiver/Rx. 

Please let me know if you have any questions. 

Thanks for your support. 

Leroy Song

  • 0
    TI__Mastermind 43795 points

    The DS125BR820 is not designed to compensate for the whole channel or even most of the channel.  It is designed to work with the SAS Tx/Rx during link training to optimize link performance.  The attenuation between the system Tx and the DS125BR820 input must be greater than the DS125BR820 CTLE boost setting.  If higher CTLE settings are required due to the total channel attenuation then the DS125BR820 must be placed far enough away from the SAS Tx to allow enough signal to attenuate to keep the DS125BR820 equalization linear. 

    For SAS-3 systems the Tx – Rx should be able to deal with 22-24 dB of attenuation in the channel.  If the channel attenuation is < 30 dB @ 6 GHz then the DS125BR820 can help to boost the signal to the SAS-3 Rx making the channel seem to have lower attenuation and allowing link training to complete.  For channels with attenuation > 35 dB @ 6 GHz the DS125BR820 will not be able to add enough gain to the system for reliable SAS-3 operation.

     

    Typically, adding the DS125BR820 into a SAS-3 system will increase the trace distance which can support 12G operation by 8-10" of FR4.

     

    For PCIe-3 the case is almost exactly the same in terms of channel attenuation and DS125BR820 device placement within the channel.

     

    Regards,

    Lee 

  • 0 in reply to Lee Sledjeski
    TI__Intellectual 1195 points

    Hi Lee,

    I have a follow-up question. The customer would like to know where or not the design can work. The block diagram is shown below (Note: The redriver is DS125BR820). The customer has calcuated the pre-channel loss (From PCH to redriver) is less than -14dB (includes driver compensation). The customer would like to transmit data using PCIe gen 3 and SAS 12G. Please advised. Thank you for your support.

     

    Leroy Song

  • 0 in reply to user4760726
    TI__Mastermind 29655 points
    Hi Leroy,

    A pre-channel loss of -14 dB should work well with the DS125BR820 in PCIe Gen 3 and SAS-3 applications, as this is more than the minimal amount of CTLE boost setting when DS125BR820 EQ = 0x00. When you say "less than -14 dB," how much less is it? Will there be at least -5 dB loss?

    Also, can the customer comment on how much loss is expected after the redriver and before the connector?

    Thanks,

    Michael