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Original question:

LMX2594: OSCin differential drive datasheet suggestion

LMX2594: OSCinP and OSCinM termination with 50 Ohm to ground or 100 Ohm in parallel

Franz Castro
Prodigy 50 points
Part Number: LMX2594

Hello TI Staff,

On the frontpage of the data sheet is a simplified schamatic which shows two resistor at OSCinP and OSCinM to ground. But if I check on page 61 (Figure51. TypicalApplicationSchematic), there is a 100 Ohm resistor connected between both inputs.

I mentioned on my PCB that the temperature of the LMX2594 highly depends on those resistors. I got a much lower temperature with the 100 Ohms in parallel.

Since I couldn't find any explanation for those 2 options I'm wondering if you could give me a reference to where I have to search for information.

Greetings Franz


  • 0
    TI__Guru* 93970 points

    Hi Franz,

    Two 50 Ω shunts or a 100 Ω between pins depends on the input signal format.

    If the input is a differential signal, a 100 Ω between OSCIN pins is the best configuration.

    On the other hand, if the input is single-ended, then two 50 Ω shunt seems to work better.

    In any configuration, make sure the OSCIN pins are AC-couple to the resistors because the pins have internal biasing.

  • 0 in reply to Noel Fung
    Prodigy 50 points

    Hi Noel,

    Thank you for your support and fast reply.
    On my pcb I didn't realize that I had a DC Offset on this path.
    So the heat came from the 50 Ohm shunts in parallel to the LVPECL Driver Emitter Resistors.
    Thus the resistance was pretty low and I had high current to ground. Since it is differential I'm using the 100 Ohms between OSCin Pins. This reduces the heat by 25° now.

    Greetings Franz!