Damping Resistor on Clock Output

Question: What is the value recommended for damping resistor on clock output of Cypress clock device?

 

Answer:

The value of a series damping resistor depends on several parameters:

 

Ø  Trace Length

Ø  Line Impedance

Ø  Output Impedance of Cypress clock device

Ø  Capacitive Load of the driven line

 

When these values are known, the value of the damping resistor can be found. Usually, one or more of these parameters are not known and a trial and error approach is taken to determine the correct value. If your trace length is not very long, you may not even need a damping resistor. If you feel you need a damping resistor to eliminate overshoot/undershoot and ringing, 22 ohms is a good place to start. Increasing this value to 33 would match a longer trace and even up to 47 ohms. Keep in mind that raising this value to high will cause the rise time at the load to increase. Of course, slower rise time will have better EMI.