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DP83848C: What Class of Output Drivers Does DP83848 Have?

James Olson
Prodigy 170 points
Part Number: DP83848C

When selecting an RJ45 jack with built-in magnetics to use with DP83848 I note that there are two general classes of these devices.

One class is known in the literature as "current driving outputs", the other "voltage driving outputs".  These descriptions refer to the mode that the output transistors on the IC (=DP83848 for me) drive the output transformer ahead of the RJ45 jack.

The voltage driving class of RJ45 has built-in 75 ohm resistors in series with the output transformer's center tap.  The current driving class of RJ45 brings the transformer CT directly out with no 75 ohm resistor in series.

The type of magnetics shown in the DP83848 data sheet is the variant called current driving outputs.  No 75 ohm resistors are shown in the schematic of the DP83848 data sheet.

However, in the RJ45 marketplace the current driving type of jack magnetics is quite rare.  The more common type of magnetics today is the voltage mode with 75 ohm resistors in the center taps.

Can voltage mode RJ45 magnetics be used successfully with DP83848?

How can I find schematics that show successful interconnections of common RJ45 jacks to DP83848?

  • 0
    Prodigy 170 points

    When the DP83848 data sheet listed compatible RJ45 devices, it mentioned part numbers associated with Pulse Engineering, Inc. magnetics-only potted transformer modules.  Pulse Engr was long ago absorbed into larger Asian conglomerates and today magnetics integrated with the jack is now the purchasing norm.

  • 0 in reply to James Olson
    Prodigy 170 points

    You can ignore these posts because I have resolved the matter by further study.  The 75 ohm resistor I mention is not in the CT pin of the transformer side facing the DP83848.  It's on the opposite side.

    Upon further reflection it seems that the two classes of RJ45 magnetics I describe (current vs voltage) are likely associated with whether the driving IC has open drain output or push pull output.  The variant with the CT pin tied to +3V is for open drain ICs.  The variant with the CT tied to a capacitor are for ICs with push pull out.

    Thanks again for this forum.

    Jim Olson

    Indianapolis, IN US