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DS25CP114: Best IC for CML SERDES Repeater & Additional Information

John Frye
Prodigy 10 points
Part Number: DS25CP114
Other Parts Discussed in Thread: DS125BR401, DS80PCI800, DS80PCI402, , DS64BR111

Hello,

I'm looking at designing a simple SERDES repeater – something that takes in 3.125 gbps CML differential pairs (100 ohm differential impedance), and buffers/repeats the signal such that we can drive additional transmission line length. My need is protocol agnostic.

I’ve found the following parts that seem applicable  for this need - 

  • DS125BR401
  • DS80PCI402 / DS80PCI800
  • DS25CP114

 I’m sure there are others. My desired characteristics are -

 Configurable pre/de-emphasis suitable for 3.125 gbps (i.e. – I need to understand the duration of the emphasis for some of these ICs that have configurable rate, which is not obvious from some of the datasheets)

  • Configurable RX equalization (with a disable setting)
  • High-sensitivity on the receiver input – will work with < 200mVpp differential on the input (not obvious from the datasheets)
  • Output swing of at least 1V at max setting

Questions:

  • Are any of the three parts I’ve identified suitable for this purpose?
  • Can you provide some additional information on the characteristics (particularly minimum input differential voltage, pre-emphasis duration) for these that I am not able to find in the datasheet?
  • Are there any other parts we should consider?
  • 0
    TI__Mastermind 20192 points

    Hi,

    John Frye said:
    Can you provide some additional information on the characteristics (particularly minimum input differential voltage, pre-emphasis duration) for these that I am not able to find in the datasheet?

    In general, I expect our redrivers for PCIe and Ethernet to have a maximum differential output swing of at least 1 Vpp.  These devices do not perform data recovery, and as a result, there is not a clear recommended minimum differential input level on many of these devices.  As long as you meet the signal detect threshold (or manually enable signal detect), these devices will transmit whatever they receive on the input plus some equalization and de-emphasis.

    I can check in regarding the de-emphasis duration, but I don't have any immediate resources on this.

    John Frye said:
    Are any of the three parts I’ve identified suitable for this purpose?

    The DS25CP114 is not suitable since the maximum VOD is less than 1 Vpp.  The other devices you mentioned should be suitable based on the criteria you mentioned.

    John Frye said:
    Are there any other parts we should consider?

    You might also consider the DS64BR111.

    Thanks,
    Drew

  • 0 in reply to Drew Miller1
    Prodigy 10 points

    Hi Drew,

    Thanks very much for the quick and comprehensive response. 

    I am very interested in the emphasis duration for the DS64BR111, DS125BR401, and DS80PCI402, since it's important to emphasize close to a bit period down at 3.125 gbps, so I hope you can find some information on that. 

    With regard to your statement:

    "As long as you meet the signal detect threshold (or manually enable signal detect), these devices will transmit whatever they receive on the input plus some equalization and de-emphasis."

    When you say that they will transmit "whatever they receive plus...", just to confirm - these devices will gain/buffer a lossy signal up to the selected VOD & emphasis settings, correct? 

    In addition - you're saying that meeting the signal detect threshold (i.e. 160 mV for the DS64BR111) guarantees no errors by the receiver in terms of deciding whether each symbol is a '0' or '1'? 

    Also - how does the signal detect differ from the "Vtx" specified at the receiver inputs, e.g. for the DS64BR111? On another thread, the minimum value of 190mV is mentioned as an acceptable Vdiff for the receiver.

    Thanks,
    John

     

  • 0 in reply to John Frye
    TI__Mastermind 20192 points

    Hi John,

    I'm checking in with another team member in order to get more details about the de-emphasis.  There are a couple of high level comments I can make on the de-emphasis at the moment.

    • Because these redrivers do not have any CDR system, they do not track the data rate of the signal.  Because of this, the de-emphasis duration is independent of data rate in these devices.
    • The de-emphasis in these devices is RC based (as opposed to a delay based de-emphasis)
    John Frye said:
    When you say that they will transmit "whatever they receive plus...", just to confirm - these devices will gain/buffer a lossy signal up to the selected VOD & emphasis settings, correct? 

    Yes this is correct, these devices apply gain/equalization on the input signal.  Please note that equalization is not a broadband gain and instead consists of applying gain to high frequency content in the input signals.

    John Frye said:

    In addition - you're saying that meeting the signal detect threshold (i.e. 160 mV for the DS64BR111) guarantees no errors by the receiver in terms of deciding whether each symbol is a '0' or '1'? 

    Also - how does the signal detect differ from the "Vtx" specified at the receiver inputs, e.g. for the DS64BR111? On another thread, the minimum value of 190mV is mentioned as an acceptable Vdiff for the receiver.

    The signal detect threshold is not based on any metric like BER.  This threshold is just the voltage level that is required in order to have the device automatically "turn on" the internal high speed circuits.  You will likely want a transmitter VOD that is will above the signal detect threshold in order to achieve good BER.

    The Vtx metric specified in the electrical characteristic section of the DS64BR111 datasheet refers to the transmit voltage of whatever device transmits signal to the DS64BR111.  I believe the upper and lower bounds of this are governed by two separate limitations.  I believe the minimum value is likely related to the signal detect threshold, and the upper value is likely governed by the absolute maximum ratings of the device.  I see there is another forum post inquiring about this metric (linked below).  If possible, I would recommend following the guideline provided by the other post of having a transmit VOD between 600-1000mVpp.

    https://e2e.ti.com/support/interface-group/interface/f/interface-forum/910785/ds64br111-repeater-configuration-and-datasheet-details?tisearch=e2e-sitesearch&keymatch=ds64br111%20Vtx#

    Thanks,

    Drew

  • 0 in reply to Drew Miller1
    TI__Mastermind 20192 points

    Hi John,

    Just wanted to follow up with you to let you know that unfortunately I was not able to locate any documentation containing more details on the de-emphasis of the parts we discussed.

    Thanks,

    Drew