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DS90UB926-Q1 - CMLOUT schematics

Ryuji
Guru 20090 points

Hello,

Could you please let me know the recommended schematics on the CMLOUT pins of DS90UB926?
Please let me know the recommended schematics for differential probe and passive probe.

In the datasheet typical schematics, there is the 100ohm termination only.
However, there are the 100ohm termination and Capacitor for AC coupling in the DS90UB926Q1EVM user's guide.
Also, there are caps only in the reference design.
CML OUT.pdf

The following is my understanding.
For Differential probe : 100ohm termination and probing the pad
For Passive probe(50ohm termination) : ac coupling and mount the  SMA connector

Best Regards,
Ryuji Asaka

  • 0
    Guru 20090 points
    AC coupling is recommended.
    Usually CMLOUT is biased.
    For 2.5 V CML, the common mode level is 2.5 V - Vod.
    If Vod = 100 mV, +/- 50 mV swings around 2.4 V.
    If you connect its output to an input terminated with 50 Ω to GND, 115 mW will be dissipated.
    Some oscilloscopes cause thermal damage.
    Also, observing the 100 mV signal offset by 2.4 V will damage the dynamic range.

    AC coupling is unnecessary if using a high impedance differential probe and the differential probe's common mode is within the common mode level range of the output. In this case, it is necessary to separately prepare a far-end termination with a differential of 100 Ω.