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AM26C32: Output Damping Resistor

Kazuki Itoh
Genius 9175 points
Part Number: AM26C32
Other Parts Discussed in Thread: SN74LV244A

Hi team,

I'd like to know if damping resistor is needed for my system.

AM26C32 output is connected to SN74LV244A via L=142mm W=0.1mm trace.

I observe 6.604V peak overshoot and -1.366V peak undershoot at the SN74LV244A input. This violates SN74LV244A ratings.

When 33Ohm damping resistor is connected between AM26C32 and SN74LV244A, the overshoot and undershoot get much better.

Do you think 33Ohm resistor is neccessary?

Personally, this level of overshoot is unexpected to me. Is AM26C32 working correctly?

 Regards,

Itoh

  • 0
    TI__Guru 62910 points

    Hello Itoh-san,

    The AM26C32 output has a fast transition time (typically 4 ns), so even a relatively short PCB trace (in this case 142 mm, which may have a propagation delay near ~1 ns) could act as a transmission line, meaning that reflections could occur at interfaces between dissimilar impedances (such as the transition from the PCB trace to the SN74LV244A input) and result in visible over/under-shoots.  That is likely what is occurring here, since the AM26C32 output driver just uses a standard push-pull structure and couldn't generate voltages beyond VCC/GND itself.  For these cases a series damping resistance of ~33 Ohms is beneficial in reducing the output transition time and by helping to better match the output impedance of the device to the characteristic impedance of the PCB trace.

    Regards,
    Max Robertson