Question
AD8134: I want to know the diode type of output stage in the fingure 33 of
the datasheet, please give some suggestion.
Answer
The diodes are only used in networks that use a daisy-chained driver
architecture, and are not required for other architectures. If you y do need
them, you also need to provide a return path in the Cat-5 cable for the diode
bias current. This is usually done with one wire in the fourth pair (the data
sheet discusses this as well). (We have had customers that have tried to use
this architecture, and have not provided the return current path -- it will not
work this way.) At the receiver, a common-mode tap needs to be provided at the
center-tap of the termination resistor. Typically, two 49.9 ohm resistors are
placed in series to provide the termination, and the common-mode center-tap is
taken between the two resistors. A series resistor is placed between the
center-tap and one wires of the fourth pair to set the diode bias current.
This wire is generally connected to ground at the receiver. The value of the
current-setting resistor is typically about 120 ohms. The diodes should be
Schottky types; HSMS-2825 diodes from Avago Technologies are good choices.
These are dual SMT diodes in an unconnected pair configuration.
This configuration can be difficult to implement, and is only required in
daisy-chained applications, so only use if your application needs it.
Attached is a picture showing how the system should be connected.