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SN65LVDS100: How to select DC blocking capacitance based on Run Length?

John11513
Expert 5720 points
Part Number: SN65LVDS100

Hi Experts,

A question puzzled me when i read the 'LVDS Owner's Manual' ( a documents on our web site about some basics in LVDS). In this paper there is an equation to calculate the DC blocking capacitance based on Run Length of the bit stream (and 3% of signal droop): C=(7.8 x Run Length x Bit Period)/R. I cannot understand how does this equation reached? As my understanding if we consider C and R as a high pass filter, to keep signal droop less than 3% the constants in the equation should between 30~40, in stead of 7.8. I do not know where the factor 7.8 from?

Thank you!

John

  • 0
    TI__Guru* 93670 points

    Hi John,

    I did a back calculation and 40% is about 7.8dB. This could be where this number is coming from. We know that RC time constant should be greater than run length times bit period and then it is a matter of this multiplying factor to see what percentage of either one or zero level we need to get to. If we go with 30% then we have to multiply by 10.5 approximately.

    Regards,, Nasser