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DS90LV048A: DS90LV048A

gary hawkins
Prodigy 30 points
Part Number: DS90LV048A
Other Parts Discussed in Thread: DS90LV047A, SN65MLVD047, SN65LVDS047

recently we have witnessed some lvds data transfer problems between boards on our multidrop backplane. The design dates to before mlvds so just uses a single driver with 6 receivers on the bus.

originally we only used national 047 and 048 parts but recently i have noticed some texas parts being used (mixed). after removing all the texas parts for national equivalents  we have as yet have not seen the problem

come back.

looking through our current stock i noticed one very odd thing about the texas parts, some devices have 650pf between diff input pins and other have 18-20pf, (18-20pf is similar to the national part).

how is this huge difference explained and how might this effect performance/behaviour 

  • 0
    TI__Genius 10250 points
    Hello Gary,

    A few questions:
    What is the problem you are noticing?
    What is the part number of the "Texas Parts"?
    How do you measure the capacitance? Is it consistent for all 4 receivers in the device?
    Do you notice any performance difference between the devices from the 2 groups?
    Can you please send me the marking on the packages that you have tested and indicate which has which capacitance value? Thanks.

    Regards,
    Yaser
  • 0 in reply to Yaser Ibrahim
    TI__Genius 10250 points
    Hi Gary,

    Do you still need support. I will close this thread for now, but if you still need support please feel free to respond and it will get open again.

    Regards,
    Yaser
  • 0 in reply to Yaser Ibrahim
    Prodigy 30 points

    Hello,

    Thank you for helping with this.

    I believe the problem we are occasionally seeing is due to loading on our system clock resulting in a very low differential impedance being seen by the driver & receivers.

    since the original design pre-dates mlvds we are at the limit of the driving capability of a single driver for six receivers on the clock. when all the various manufacturing tolerances

    are accounted for some combinations of boards result in a system on the limit. where we have had problems in these systems we have found that the driver has been a texas LVD048A in place of a national DS90LV047A.  also  we have found that some of the clock receivers  have 650pf between a pair of differential input pins. checking through new texas and national stock none of the national parts have this 650pf in fact they all tend to be around 15-18pf, for the texas parts we see many new having a mix of values these being either 650 or 15-18pf. 

    I have found a very old data sheet for the national part which gives and example of this type of configuration albeit limited to 5 receivers however for the texas part the driver is only ever used in point-point configurations.

    can the texas part really cope with driving six receivers over a large backplane with a loaded impedance of perhaps less than 60 ohms. 

  • 0 in reply to gary hawkins
    TI__Genius 10250 points
    Hello,

    60 ohms is very low and and will degrade the noise margins significantly (for any LVDS driver). As you said, it will put the system on the edge, especially if you add to that the fact that with multiple receivers and only one termination at the last receiver, there will likely be some reflections from the stubs at the intermediate receivers.

    Please consider using SN65MLVD047 instead which is Quad M-LVDS driver that is pin-to-pin compatible with DS90LV047A/SN65LVDS047. So that should be a more robust option. With the M-LVDS drivers (SN65MLVD047) you have the option to add a near-end 100 ohm termination which will help with the reflections.

    Regards,
    Yaser
  • 0 in reply to Yaser Ibrahim
    Prodigy 30 points
    Hello again,

    Thank you for your answer. The device you suggested is one i have here to try. We have held off from this because we thought it may have compatibility issues but i will try it to see if we can do something with.

    Thank you again for your help and suggestions.