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Altera_Forum's avatar
Altera_Forum
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14 years ago

parallel lvds outputs

I need to drive an IR led driver at high speed. the driver has multiple channels (6) an all are paralleled in a single enable input. The driver's enable input is lvds but the input capacitance per pin could be as high as 760pF.

Pulses of 25nsec at 50ns (50% at 20MHz) are injected to the LED driver and I'm wondering if the large input capacitance will be an issue for a single FPGA output channel (cyclone III). Can be connected in parallel multiple FPGA's lvds output channels and avoid any current issue?

Regards

OAM

4 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    Yikes! 760pF is huge.

    Quartus will take that capacitance into account for timing analysis if you set it in the board trace model parameters.

    A quick experiment says that 760 pF is adding 33 ns of delay for a Cyclone III.

    I think you should be able to use multiple outputs to increase the drive capacity.

    You should set timing constraints on those outputs and set their load capacitance.

    If you're using N outputs to drive a single input, then set the load capacitance as 760p/N for each output.
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    LVDS outputs are dfferential current sources with 100 ohm termination at the receiver according to the standard. Capacitive load won't be a problem for the driver but of course limits the link speed.

    I never heard about a "LVDS" receiver with multiple 100 pF input capacitance and can't see a reasonable purpose for it. Are you sure about the info?

    But definitely not a question related to Altera FPGA anyhow.
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    Hello,

    Yes, the delay is a lot.. and it will be an issue at 20Mhz. I have read the IC driver spec and they specify 760pF per input. A recommended configuration for the IC driver is to use all channels in parallel I can use a TTL instead LVDS still evaluating this so, that is why I was thinking on use paralleled outputs.

    Thanks for the info
  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    The specifications don't seem to go well together, either a strange device or some misunderstanding. If it's a commercial available type, can you tell manufacturer and component name?