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Steve9's avatar
Steve9
Icon for Occasional Contributor rankOccasional Contributor
6 months ago
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Agilex 7 F/I Series True Differential Input Termination

Hi,  The Agilex F/I Series GPIO User Guide indicates that if using AC coupling for the true differential inputs you should add external voltage bias circuitry and has no examples of AC coupling with...
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  • FvM's avatar
    FvM
    6 months ago

    Hi Steve,

    reviewing documents and reference design, I agree that GPIO manual, datasheet and development kit schematic are inconsistent in several regards. Looking also at IBIS file, there seems to be a non-linear pull-up feature for TDS input standard which effectively pulls Vicm to about half supply with applied signal. Not sure if IBIS input characteristic is real or roughly simplified.

    So all-in-all, it seems like development kit implementation is a possible minimal effort termination option.

    Regards
    Frank

Steve9's avatar
Steve9
Icon for Occasional Contributor rankOccasional Contributor
6 months ago

Thanks. this is the dev kit I was referring to. It looks like the following differential pairs into the FPGA fabric are AC coupled with no external bias. There are bias resistors that are DNI'd on schematic/BOM so they are not installed. What do you think about those? If that was done on the dev kit it made me think AC coupling w/ no bias was acceptable. 

ToD_MASTER_CLK_125M_P/N

PTP_SAMPLE_CLK_250M_P/N

https://www.intel.com/content/www/us/en/content-details/843994/agilex-7-fpga-f-series-development-kit-production-1-2x-f-tile-revd-dk-dev-agf023fa-development-kit-board-schematic.html

FvM's avatar
FvM
Icon for Super Contributor rankSuper Contributor
6 months ago

Hi Steve,

reviewing documents and reference design, I agree that GPIO manual, datasheet and development kit schematic are inconsistent in several regards. Looking also at IBIS file, there seems to be a non-linear pull-up feature for TDS input standard which effectively pulls Vicm to about half supply with applied signal. Not sure if IBIS input characteristic is real or roughly simplified.

So all-in-all, it seems like development kit implementation is a possible minimal effort termination option.

Regards
Frank

  • Steve9's avatar
    Steve9
    Icon for Occasional Contributor rankOccasional Contributor
    6 months ago

    Thanks Frank. 

    Yes the documentation was not clear to me w/ regard to AC coupling/internal biasing. 

    I was able to get my hands on an Agilex -7 F series board and took some measurements of an AC coupled LVDS input. In this case the internal OCT was not enabled and there was an external 100 ohm termination resistor present on the Agilex side of the series AC coupling caps. 

    With no bias resistors at all on Agilex side of AC coupling caps: A single leg oscillated ~940mV to ~1.17V. So common mode of 1.055V and VID of 0.21V. 

    With a 10k pullup to 1.2V and a 10k pull down to GND on each leg (so should set a 0.6V bias for each leg: Swing of a single leg was from ~480mV to ~720mV. So Common mode of ~0.6V and swing of ~0.24V

    • AqidAyman_Altera's avatar
      AqidAyman_Altera
      Icon for Regular Contributor rankRegular Contributor
      6 months ago

      Hi Steve,

       

      May I know if you have additional question on this?

       

      • Steve9's avatar
        Steve9
        Icon for Occasional Contributor rankOccasional Contributor
        5 months ago

        I am all set here thanks! Sorry for the delay, I marked Frank's reply as the solution.