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Sayma AMC/RTM issue tracker
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sayma: sysref coupling + termination #588

Closed hartytp closed 5 years ago

hartytp commented 6 years ago

Sysref should be DC coupled and 50Ohm terminated at the DAC. Currently, it's AC coupled and untermianted (AFAICT, the DAC sysref inputs are high-Z).

hartytp commented 6 years ago

NB need to be careful with the logic level conversion here since the optimal common-mode voltage for the SYSREF inputs is 600mV. So, need a resistor network to match that to the LVPECL/CML/whatever driver.

NB in my tests, I'm using AC-coupled LVPECL since it would be very tough to add such a network to Sayma (too fine pitch everywhere, no convenient access to ground, etc).

gkasprow commented 6 years ago

It's easier to remove terminating resistors and switch to LVDS mode. At the moment the termination resistors are soldered directly to the DAC pins. AFAIR I did it not only for clock but also for sysref inputs.

marmeladapk commented 5 years ago

@gkasprow @hartytp What is the conclusion here? In schematics SYSREF is AC coupled, has 100R differential termination and 200R to ground at source.

Source: obraz DAC: obraz

hartytp commented 5 years ago

Closing in favour of https://github.com/sinara-hw/Sayma_RTM/issues/13

gkasprow commented 5 years ago

200R is pulldown for open emitter outputs. 100R is differential termination. Such a solution consumes far less power than Thevenin termination.