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[FAQ] Can I estimate appropriate dampening resistor value for level-shifter outputs?

Michael Ikwuyum
Mastermind 32465 points

FAQ: Logic and Voltage Translation >  Auto-Bidirectional Level-Shifters > Fixed DIR / DIR CNTRL Level-Shifters >> Current FAQ

When in doubt, use a 0 ohm series resistor on the outputs and adjust accordingly, for your system needs.

This can also be simulated using available IBIS models.

To roughly estimate, consider the below (note, Vcc, VOH, IOH, VOL, IOL parameters specified in data sheets):

  • Output impedance of the level shifter while driving HIGH. Given in the data sheet or estimated by, ROH = (Vcc - VOH) / IOH.
  • Output impedance of the level shifter while driving LOW. Given in the data sheet or estimated by, ROL = VOL / IOL.
  • Impedance of transmission line connected to the output of the level-shifter, typically 50 ohms.

A good estimate for the series dampening resistor, R_damp = Impedance of transmission line - ROH (or ROL).

For more information, see [FAQ] What happens when I connect a logic device's output to a 50 ohm transmission line?.

In addition to impedance matching, consider the signal's rise/fall times (faster rise/fall typically requires lower resistance values), capacitive load (higher capacitive loads may require lower resistance values) and power dissipation (ensure expected current can be handled without overheating).

If inductance (in henries) and capacitance (in farads) of the transmission line or interface used is known: a general rule of thumb for R_damp (in ohms) = 2 * √(L/C).