Measuring output impedance of high speed differential driver (LVDS)

[cirthix]

Hi,
I have an ASIC which drives a 100-ohm differential pair, to be routed and terminated properly.  The datasheet for this chip is pretty poor.  I am supposed to use a series resistor on each output pin close to the driver, but I do not know the optimal value.  33ohm works well, 75ohm works but not that well.  10ohm similarly works but not that well. 

I know that the purpose of the series resistors is to terminate reflections and provide a uniform 100-ohm transmission line.  The value of the series resistor should be (100-driver_impedance)/2, right? (of course 75 breaks this and does not work well)

I'd like to determine the optimal value, but my scope is not fast enough to directly look at the waveforms (chip rise/fall times are ~130/170 picoseconds). 

I suppose I could just keep using the 33ohm value because it works and makes sense, but I'd like it to be properly done.

Thanks

[CM800]

I'm not to familiar with high speed design, but could you just put the output into resistors, measure the current and use V=IR to get the output impedence, or at least the resistance, from there do the calculation?

[Miyuki]

1st rule: Don't touch it when it works. It will always be worse.

[Gyro]

The only thing I can think of is measuring the output voltage swing with different value load resistors close to the chip - that ought to give you a practical indication of internal resistance.