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cdclvp1204 phase wrong

Barry V
Intellectual 305 points
Other Parts Discussed in Thread: CDCLVP1204, DAC5682Z

I'm using  a CDCLVP1204 clock driver at 360MHz, and when I look at the output phase between OUTPx and OUTNx, I see very little phase shift (maybe 10 degrees).  As I change my clock frequency, the phase between the output changes.  At 270MHz, they are beautifully 180 degrees out of phase, at 110MHz, they are back to being close to in phase.  

The actual application for this has the outputs pulled to ground through 150 ohm resistors, and at the receiver end (a DAC5682z), they are capacitively (.01uF) coupled to a 100 ohm differential terminator.  I tried disconnecting the terminating end (lifted the two caps), but this didn't seem to have much of an effect.

What could be causing this performance? This device is supposed to be capable of operating up to 2GHz.

Thanks,

Barry

  • 0
    TI__Mastermind 32575 points

    Could you please share the scope plots you took showing what you described?  Did you use two identical high-speed active probes (>1 GHz BW) with short, low-inductance ground leads?  Are you able to measure the differential output with a high-speed diff probe?  What is the transmission line connecting the clock driver and receiver?

    Regards,
    Alan

  • 0 in reply to Alan O
    Intellectual 305 points

    I used two 500MHz probes and short ground leads(about 1/4")  The PCB traces are about 4" long from driver to receiver; they calculate to be about 78 ohms.  I will capture the plots and post them shortly.