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DS90UB913/914A AC coupling capacitor

Randy Luck
Prodigy 240 points

Hello-

The datasheet for the DS90UB913/914 shows AC coupling capacitors at both ends of the coax or twin-ax cable.  (Actually this is true for all FPD-LIII devices).

Why is a capacitor used at both ends?

Would putting it at one end only work as well?

If one cap would work, should the value be halved to 0.047uF?

Thanks,

-Randy

  • 0
    TI__Genius 13475 points

    The input to an FPD deserializer, and the output of the serializer are differential structures.  When operating with a coax or other single ended media, the unused half of the input/output is terminated to provide a balanced load to the differential pair, the termination on the inactive side should match, as closely as possible the impedance of the active side.

    The load seen by the output consists of the AC coupling cap, the transmission line (coax cable), another AC coupling cap at the receive end, and finally the resistive termination within the chip.   To mimic this, we typically terminate with an AC coupled termination resistor, with the value of the capacitor being 1/2 the value of the capacitor on the other side - this to simulate the series combination of the AC coupling caps at each end of the cable.

     

  • 0 in reply to Mark Sauerwald88474
    Prodigy 240 points

    Mark-

    To be clear, you are saying the following:

    The data sheet shows the configuration using coax that looks like this:

    But this configuration would also be OK to use:

    Thanks,

    -Randy

  • 0 in reply to Randy Luck
    TI__Expert 5210 points

    Hello Randy,


    We do not recommend adding the capacitors in series nor changing their values. This is because the recommended application circuit values and configuration have been simulated to produce the topology you see in your first diagram above (AC coupling cap on both sides).


    With that said, one of the main reasons why we have AC coupling caps on both sides for serializer and deserializer is for automotive qualification, AEC-Q100. By putting an AC coupling cap on both sides, this prevents any DC current from entering either chip and shorting them. A short is most common during production before the interconnect link (i.e. STP, coax) has been connected between the Ser/Des. This is why the AC coupling caps are put on both sides instead of just one (to protect BOTH chips).

    -Sean