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Altera_Forum's avatar
Altera_Forum
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15 years ago

Cyclone IV receiving 5V PECL

I want to drive differential input pairs with 5V PECL. Can I use the diff input with a 3.3V VCCIO or is this prevented? If I can, what is the max common mode and max input voltage? I'm considering DC level shifting (and attenuating) rather than AC level shift.

I'm considering a 121 ohm diff resistor with a 162 and 121 ohm resistor divider to ground at each input. This should work with a 2.5V VCCIO - question is whether a 3.3V VCCIO is allowed and if it enables reducing the shift and attenuation. The alternative to the five resistors is two bias resistors, two blocking caps, two 50 ohm term resistors and a common mode termination voltage.

The DC coupling would also support peaking compensation if this proved beneficial - 5pf caps or so across the 162 ohm resistors.

1 Reply

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    15 years ago

    The differential inputs aren't specified for 3.3V VCCIO, thus I won't use it.

    The LVDS common mode input range has different specifications depending on the speed, e.g. with Cyclone IV maximum VCM is 1.55 V for hghest data rate. Considering the high differential level of PECL, an impedance matched resistive divider will do, like 120/80 ohm.