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SN65LVDS96: LVDSVCC/PLLVCC filtering requirements and unused LVDS input recomendations

DaveIsNotHere
Prodigy 10 points
Part Number: SN65LVDS96
Other Parts Discussed in Thread: DS90CF366

I'm looking at an old design that we are reusing. I see FB filtering for both LVDS and PLL power pins. I think this may have been copied from a TI demo board schematic. Interestingly the datasheet gives zero description, requirements, recommendations or info for these pins. I've checked for other documentation with no luck. Is there any requirement for the filters on these pins?

In this same design I see that we are not using some of the LVDS inputs. These are left unconnected (floating). The outputs for the channels are tied to ground. Is this correct? Again the datasheet offers nothing.

In general I have to say that I am taken back a bit by the datasheet for these parts, I've come to expect much, much better from TI.

  • 0
    TI__Mastermind 35130 points

    Hi Dave,

    Although it's not stated as a requirement in the datasheet, in general it's good practice to have good power supply filtering for the LVDS and PLL power pins. Three parallel-connected decoupling capacitors (multi-layered ceramic type in surface mount form factor) between each VCC (VCC, PLL VCC, LVDS VCC) and the ground plane(s) are recommended. The recommended values are 0.1 μF, 0.01 μF, and 0.001 μF, and the preferred capacitor size is 0402.

    In regards to unused pins, you can reference the equivalent input/output schematic diagrams in the datasheet:

    Looking at these, it's fine to leave unused LVDS inputs open since the internal pullups would just pull both lanes to Vcc. However, since this would put the output in an indeterminate state, it would be better to leave the outputs for these channels open instead of tied to ground. 

    Are you looking to use this device in a new design? I agree the datasheet is less than satisfactory, but please note that the last time it was updated was in 2006. I would recommend using the DS90CF366 instead as it is (relatively) newer and the datasheet is more up to date with the information you requested.

    Regards,

    I.K.