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SN75LVDS83B: EMC How to reduce the external radiation energy of LVDS line

Yuxi Liu(Tony)
Expert 3360 points
Part Number: SN75LVDS83B


Hello 

I am Tony Liu. My customer encountered serious EMC problems when using the SN75LVDS83BDGGR chip. Because their LVDS signal lines were too long, they radiated energy outwards.

Do we have HSSC devices that can help reduce emissions from LVDS process traces? In addition, other domestic manufacturers in China have recommended the SSDCI1108AF chip to improve this problem. I don't know much about the function of this chip. Does TI have similar chips or solutions?

  • 0
    Guru 318050 points

    The SN75LVDS83B datasheet says that it supports spread spectrum clocking. So it would be compatible if the device connected to its CLKIN pin uses a spread-spectrum clock. (I do not know which device is connected to the Dxx/CKLIN pins, or how that device generates its clock.)

    Please note that differential signals should not generate much EMI if routed correctly. Are you using twisted pair cables? Also, inserting common-mode chokes might help.

  • 0 in reply to Clemens Ladisch
    TI__Guru 66880 points

    Hi Yuxi, 

    Emissions are largely system and implementation specific, ie. grounding techniques, trace and routing, reference planes, impedance control, and so on. 

    As Clemens pointed out, this device does support SSC clock input, which can spread the frequency and lower the overall peaks being observed. Otherwise, customer might need to further optimize their routing/layout with either shorted routing/cable, burying on internal layers, etc.

    Regards ,

    Logan

  • 0 in reply to Clemens Ladisch
    TI__Expert 3360 points

    Hi Clemens,

    Yes customer use twisted pair cables, 10cm.

    How to select a suitable common-mode chokes ?

    In terms of SSDCI1108AF , Do we have some IC which is same function?

    Datasheet:

    https://datasheet.lcsc.com/lcsc/1912111437_Shenzhen-Top-Flight-Tech-SSDCI1108AF_C347524.pdf

  • 0 in reply to Yuxi Liu(Tony)
    Guru 318050 points

    10 cm is not very long. Are there long traces? What is the total length?

    In general, you want a common-mode choke with a low-enough differential-mode impedance (< 10 Ω) and a high common-mode impedance (e.g., 1 kΩ) at the signal/noise frequencies. For signal transmissions, the maximum current rating is not a problem. There are many chokes with a size of 2.0mm×1.2mm or 3.2mm×1.6mm from many manufacturers, e.g., ACE1V2012-102-R or ACM-31-102M-T.

    The SSDCI1108AF is just a clock generator. I do not know if the graphics device that is connected to the Dxx/CKLIN pins is capable of using such a clock generator; please specify what device this is.

    Also see High-Speed Layout Guidelines for Reducing EMI for LVDS SerDes Designs.