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DS15EA101: Selection of AC coupling capacitors reference data sheet SNLS235H Figure 3

Robert Curtis
Intellectual 430 points
Part Number: DS15EA101


Hello,

In figure 3 of the DS15EA101 data sheet, 1uf  capacitors are used to AC couple the differential input signal to the DS15EA101.

How was the 1uF value determined?

Could a much smaller value (475pF) be used.

Thank you for consideration

Rob Curtis

  • 0
    TI__Guru* 93570 points

    Hi Rob,

    Choice of AC coupling Cap is a function of the data pattern and operating data rate. Internal to the deice there is about 1.23K ohm Resistance. 1.23K with 1uF cap gives us about 1.23mS. Meaning as long as total number of zeros or ones at a required data rate is less than 1.23mS - considering or give and take different tolerances - then device would not get affected by the DC wander caused by data pattern and data rate. To check if 475pF is ok, you should check your data pattern maximum number of zeros or ones at your operating data pattern and make sure is way less than 475e-12 * 1230 or about 0.6nS. Suggestion is if you can stay with 1uF cap or something close to accommodate different data patterns/rate. 

    Regards,, Nasser