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Original question:

DP83869HM: Pair of PHYs falling out of sync

DS15BA101: LVDS buffer for DP83869HM

Michael9q
Intellectual 565 points
Part Number: DS15BA101
Other Parts Discussed in Thread: DP83869HM, , SN65LVDS100

Hello There,

we use DP83869HM as PHY for 100M media converter. In order to connect Optical receiver we need to meet PHY input specification of 0.3-2.0 V for differential signals (8.5 datasheet).

Our Receiver has differential output of 0.19 - 0.32 V which not fully in spec of PHY. 

We need simple reliable device allowing interface between receiver and PHY, with minimum form factor, low price, minimum additional components.

We chose your DS15BA101 as buffer. RX (0.19-0.39V) --> DS15BA101 (0.4V) --> DP83869HM (0.3-2.0V)

We would like to ask:

  • Is this fully compatible?
  • Is there another buffer more suitable for this application?
  • Should it be always RX --> DS15 --> DP83639 AC coupled at all interfaces for proper functionality

Usually in most other device datasheets (SN65LVDS100) is differential voltage level 250-450 mV - Does this mean, it is not compatible with the PHY? or this voltage level is double?

Initial topic was started already in PHY forum: https://e2e.ti.com/support/interface-group/interface/f/interface-forum/1342757/dp83869hm-pair-of-phys-falling-out-of-sync/5123129#5123129

Thank you for help here.

Best 

Michael 

  • +1
    TI__Guru* 93670 points

    Hi Michael,

    Based on your description, DS15BA101 should meet this requirement. 

    It is preferred to AC couple since this will isolate common mode voltages. As long as your RX common mode voltage is within the range of the DS15BA101 you can DC couple as well. But AC coupling provides more flexibility.

    I think DS15BA101 should be optimum and it seems to meet your DS83869 input swing requirement as well.

    Regards, Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 565 points

    Hello Nasser,

    thanks for confirmation.

    For future product could you please help me understand the LVDS OUTPUT DC SPECIFICATIONS (OUT) in DS90LV001TLD datasheet and other LVDS devices:

    • Does it mean the guaranteed differential voltage is at least 250mV in any batch / any operational condition OR it is per pin so double?

    I am trying to understand what is then a standard of LVDS level. It should be +/- 200mV but the PHY DP83869HM has minimum of 300mV which is above minimum of LVDS. 

    Thanks

    Michael

  • +1 in reply to Michael9q
    TI__Guru* 93670 points

    Hi Michael,

    It is 250mV differential or 125mV peak to peak per pin.

    Regards, nasser

  • 0
    Intellectual 565 points

    Hi Nasser,

    maybe it would make also sense to use some of the SN65LVP, which are smaller and have:

    Vout differential: 0.6-1.0V

    Vin differential: 0.08-1.0V

    Is there any good practice which of the buffer should be used? 

    thank you 

    Michael 

  • +1 in reply to Michael9q
    TI__Guru* 93670 points

    Hi Michael,

    SN65LVP seems to work fine as well. Using DS15BA, you can set the output swing. It is up to you.

    Regards, Nasser 

  • 0 in reply to Nasser Mohammadi
    Intellectual 565 points

    Thanks Nasser for support here. 

    today we tested with this additional component and it solved the issue. 

    One big warning for this component DS15BA101: it is crucial to keep the path to the RVO resistor as close as possible to VCC. 20 mm vs 3 mm wire made this device functional from not working state!

    Thank you,

    Michael 

  • 0 in reply to Michael9q
    TI__Guru* 93670 points

    Hi Michael,

    Thanks for the update and glad you were able to find this out.

    Regards, Nasser