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Altera_Forum's avatar
Altera_Forum
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16 years ago

Interfacing to S4/S3 LVDS

Hi All,

I would like to interface an ADC that has a Vocm of 820mV at 780Mbps to the S4 LVDS receiver. The S4 data sheet has the Vicm at:

0.05V < VICM < 1.8V for Dmax less than 700Mbps

and 1.05V < VICM < 1.55V for Dmax greater than 700Mbps

How best to connect the two devices?

For comparison, I looked at the Virtex6 LVDS and it has Vicm at:

0.3V < VICM < 2.2V with no separate rows based on Dmax.

Thanks,

DannyK

3 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    AC coupling is the most simple way, but is only permitted for DC balanced data, e.g. 8b/10b or manchester encoded. ADC data most likely aren't of this type. But I would prefer to know about the transmitter characteristics before making detail suggestions.

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    My personal feeling is that you won't have any issues if you just directly connect the two together (dc coupled).

    However, if you're concerned:

    My first suggestion would be to use an LVDS buffer from either Micrel or On semi:

    http://www.onsemi.com/pub/collateral/nb4n527s-d.pdf

    http://www.micrel.com/_pdf/hbw/sy58605u.pdf

    My second suggestion would be to use an AC coupling scheme with DC bias. This document might be helpful:

    http://focus.ti.com/lit/an/scaa059c/scaa059c.pdf

    Jake
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    What's the involved ADC type? The Vocm specification seems surprizing, because it doesn't meet the general LVDS specification. Strictly spoken, it shouldn't be designated LVDS. Depending on the transmitter and signal characteristic, there should be ways to perform a level shifting. At wost case, LVDS repeaters can be used.