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CDCLVD1208 unused N-inputs

Jim Carrillo
Genius 9147 points
Other Parts Discussed in Thread: CDCLVD1208

How should the unused N-input be treated on the CDCLVD1208 - pull-up or pull-down to ground?

I've seen conflicting guidance for both.

Thanks,

Jim

 

  • 0
    TI__Guru* 80843 points

    Hi Jim,

    The CDCLVD1208 has two universal inputs, two pins on each input. If one of the inputs is unused, the datasheet recommends pulling both pins (P and N) to ground using a 1k resistor.

    If an input is being driven by a single-ended CMOS signal, then only the P side will toggle and the N side will be connected to mid-rail (VIH-VIL)/2.

    Best regards,

    Matt

  • 0
    Prodigy 200 points

    Actually lets verify how to correctly address both the p and n unused inputs.

    The CDCLVD1208 spec sheet clearly states “For unused input, it is recommended to ground both input pins (INP, INN) using 1 kΩ resistors.” 

    An e-mail with Clancy Soehren stated  “The factory recommends to pull the unused differential input of the CDCLVD1208 to a status level by having a pull-dn on the P-input and a pull-up on the N-input.”  Is the data sheet wrong?

  • 0 in reply to Alfred Key
    TI__Guru* 80843 points

    Hi Alfred,

    There is more than one way to address unused inputs for the CDCLVD12xx devices. The datasheet method will work okay. Clancy's suggestion will work as well as I have seen this used on other devices. Her suggestion to pull the inputs in opposite directions is helpful in the case where no input hysterisis is present to prevent noise from causing the input to toggle.

    The CDCLVD12xx input buffers do have hysterisis built in so even floating inputs should not be a problem. The 1k pull down resistors are recommended just to be conservative. Additionally, only one input can be selected at a time using the IN_SEL pin, so activity on the unused input will not propogate to the output. This would not be the case for a dual buffer like the CDCLVD21xx.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 120 points

    Alfred Key said:

    The factory recommends to pull the unused differential input of the CDCLVD1208 to a status level by having a pull-dn on the P-input and a pull-up on the N-input.

    Matt Sunna said:

    The CDCLVD12xx input buffers do have hysterisis built in so even floating inputs should not be a problem. The 1k pull down resistors are recommended just to be conservative.

    We have an LVDS clock coming from a connector going to the CDCLVD1208, so there may be a signal or not.

    • How weak can the pullups be? 100 kOhm would be useful. 1 kOhm pulldowns (as recommended by the datasheet) disturb the signal.
    • Is it allowed to leave unused inputs open without disturbing the other channel?