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SN74AHC1G125: When to use SN74AHC125 instead of a RS-422 or RS-485 differential line transceiver?

Lauren Mayfield
Expert 7180 points
Part Number: SN74AHC1G125

Hi all,

At what point would you use a digital buffer like SN74AHC1G125 for buffering digital signals to improve noise integrity over a long distance (like 1ft) vs something like a RS-422 or RS-485 differential line transceiver? 

My customer anticipates high ringing on the output of an ADC (operating at 24MHz) and would like to add circuitry to proactively improve signal noise integrity. 

I think a non-inverting buffer would be cheaper and easier to implement than an RS-422 or RS-485 transceiver at 1ft length distance, but I'm unsure if that still makes sense at 12 ft distance, 24 ft distance, etc.

What are potential problems and tradeoffs to consider?

Thanks!
Lauren

  • 0
    Guru 271300 points

    Ringing due to high-speed edges can be reduced or avoided with proper temination (even for single-ended signals; typically, you'd use source termination).

    Single-ended signals are sensitive to external noise. So, regardless of distance, for transmission in (possibly) noisy environments, you should use differential signals. (In general, differential transceivers are cheaper than shielded cables.)

    You can use LVDS or RS-422/485. LVDS is typically used for shorter distances and higher speeds; RS-4xx is somewhat more robust, but uses more power and is more expensive. ADC output signals are a typical application of LVDS; there even are some ADCs with integrated LVDS drivers.

  • 0 in reply to Clemens Ladisch
    TI__Expert 7180 points

    Hey Clemens,

    Is there an app note on how to achieve proper termination for the digital buffers? This is a topic I'm not familiar with.

    It seems like differential signals is the best way to go to really improve noise performance, but it seems to me that a digital buffer could work too (maybe not *as well*) and be significantly lower cost than LVDS or RS-485/422.

    Thanks,
    Lauren