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SN74AVC16827: DOC, damping resistor and output drive capability

Wenwei Wang
Prodigy 50 points
Part Number: SN74AVC16827


It is great the SN74AVC16827 family of logic chips have Dynamic Output Control (DOC) so that no external series damping resistor is required. My questions are:

  1. How long a PCB trace can SN74AVC16827 family of chips support without using any external series damping resistor?
  2. Does TI provide any other low-cost logic chips that can provide strong output drive while reducing overshoot/undershoot so much that no external series damping resistor is required?

Please advise. Thank you so much.

Best Regards,

Wenwei Wang

  • 0
    TI__Genius 16735 points

    Hi Wenwei,

    1. Answering how much PCB trace the device can handle without external damping is a tough question to answer. There is an IBIS model available for the part, I recommend you use this to analyze what the output waveform of the device will look like in Hyperlynx. If your transmission line is well matched, then you can handle longer distances but this will still require you to place a series damping resistor because the output of the buffer is typically a low impedance output.

    Here are all the transmission line effects you will start to see as your line gets longer.

    2. These are both opposing requirements. If the device has a strong output drive, then it will have larger overshoot/undershoot. You cannot have both and so dynamic output control was developed to slightly reduce rise time and also reduce overshoot/undershoot. This typically causes the part to be able to work at lower frequencies.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 50 points

    Thank you so much for your quick response, Karan.

    A few more questions for you if you don't mind:

    1. If a logic output just goes to an input of another chip within a circuit board, when should we start to use a damping resistor?
    2. Any reason that there is no logic chips with 64mA or 100mA or 200mA output drive? Is it because nobody needs them? Or this is limited by overshoot/undershoot? Or limited by shoot-through (cross-conduction) current?

    Thanks,

    Wenwei Wang

  • 0 in reply to Wenwei Wang
    Guru 318050 points

    As mentioned in the FAQ, you begin to see transmission line effects at about 20 cm.

    When signal transmission becomes difficult, the solution is not to increase the drive strength, but to increase the voltage (RS-232) or to use differential signals (LVDS, RS-485/422). There are power logic devices with higher drive strength, but their purpose is not to transmit signals, but to switch loads (so they have open-drain outputs).

  • 0 in reply to Clemens Ladisch
    Prodigy 50 points

    Thanks for your guidance, Clemens.

    The signal transmission that I am concerned about is within a same circuit board, not in between circuit boards. So using differential drivers would be too much. Does TI provide any logic chips that increase the output rise/fall time while maintaining the same drive capability so that the signals can travel further away without using any damping resistors?

    Thanks,

    Wenwei

  • 0 in reply to Wenwei Wang
    TI__Genius 16735 points

    Hi,

    The device you have chosen has great output drive. If you have a trace that is longer than 20cm, then you will need to have a series resistor no matter how strong the output drive of the device is. This is because, the CMOS output is a low impedance output. With a long transmission line, you don't want any reflections so you will need to do matching. Without a series resistor, the signals will reflect off the high impedance inputs of your other chip. It is recommended you add line impedance to do matching. If you don't have a long transmission line, this device will work fine.

    Again as I have mentioned, quickest way to see signal degradation will be to create a hyperlynx simulation for your case.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 50 points

    Thank you so much for your expert opinions, Karan and Clemens.

    From your app note on 50 ohm transmission line, I learned that (please correct me if I got it wrong):

    • When transmission time t_t (or rise/fall time) is 1ns, the bandwidth frequency is 350MHz, the line length is limited to about 20cm without using a damping resistor
    • When transmission time t_t (or rise/fall time) is 2ns, the bandwidth frequency is 175MHz, the line length is limited to about 40cm without using a damping resistor
    • And so forth

    However, what confuses me is 

    1. Overshoots/undershoots and ringings caused by transmission line reflections when a line length is over 20 cm; versus
    2. Overshoots/undershoots and ringings caused by parasitic inductance (LC ringings) when line length is less than 20 cm.

    How does TI's logic chip handle LC ringings in addition to transmission line reflections?

    Thanks,

    Wenwei

  • 0 in reply to Wenwei Wang
    TI__Genius 16735 points

    Hi Wenwei,

    There is no special circuitry to handle transmission line reflection in this part. Some of our parts come with a output series resistor but the one in discussion does not. The best way to avoid transmission line effects is to add matching, or to use smaller traces.

    Thanks!

    -Karan