Where do you place a termination resistor on a PCB?
In series termination, you place the resistor near the driver to increase the impedance at the source and prevent reflections on the driver end of the trace. A resistor value is selected so that the combined sum of the termination resistor and the driver output are equal to the impedance of the trace.
How is the value of a termination resistor chosen?
A resistor value is selected so that the combined sum of the termination resistor and the driver output are equal to the impedance of the trace. Series termination benefits from a lower power draw at the expense of reflection at the opposite end of the trace.
How are series terminations chosen in a PCB?
When we use series termination we place a resistor at the front end of the line, directly in series (hence the name) with the output of the driver. The value of the series resistor is chosen such that the sum of it plus the output impedance of the driver equals the characteristic impedance of the transmission line, Zo.
When to use series termination or series termination?
A closely related strategy is a Thevenin termination, where a pair of resistors, whose parallel combination is Zo, is used. When we use series termination we place a resistor at the front end of the line, directly in series (hence the name) with the output of the driver.
Why are parallel termination resistors used in VCC?
One resistor connects to Vcc while the other connects to ground, providing a pullup and pulldown pair that balances the driver’s high and low logic levels. This variation of the parallel termination can also perform well in distributed loads at the expense of a constant current leakage from Vcc to ground.
Why do you need a capacitor for parallel termination?
The addition of the capacitor mitigates the power dissipation problems of other parallel termination schemes, blocks low-frequency noise, and minimizes overshoot and undershoot. The added cost for this scheme is the added complexity of managing the RC time constant of the capacitor.