In this tutorial we are going to learn about WHAT IS TERMINATION IN PCB DESIGN ?
A component (typically a resistor/diode) added at the end of the line to establish a relationship of the load /source impedance with line impedance. The termination of interconnect typologies is required when increasing clock frequencies & faster rise & fall times begin to impact the electrical performance parameters on the PCB. For high speed board design, there are 5 common termination techniques described below which may be a lied to the design to contain the impact on signal integrity & losses. Some commonly known termination techniques are series termination, parallel termination & Thevenin termination. Series/Source termination: For most TTL devices, the driver output impedance is less than the Transmission line characteristic’s impedance. When such a TTL driver is driving a small no.of devices, series termination is recommended. The advantage of series termination is that its simple,consumes low power, & uses only one resistor. The disadvantage of series termination is that it increases the rise & fall times of the signal. Generally secondary reflection off the driver end, series termination helps with signal quality.
The concept of this termination method is to try & match the loaded line impedance with the sum of the driver output & series resistor impedance. The series resistor value is equal to Impedance ’ minus the driver impedance. The resistor always place close to the driver as possible. These are most useful in high speed applications where most of the loads are at the far end of the line. Series termination is not very good for lines with daisy chain typologies. Source terminators work well, though, for single receiver, point-to-point loads & star type of typologies. They work well to dampen overshoot & undershoot. Source terminators dissipate no quiescent power. In this circuit configuration, a resistor is put in series between a driver & a receiver. This is a common application for point to point routing to match the impedance of the driver with the impedance of the circuit trace, where Zs + Rs =Zo. For best EMI characteristics, the series resistor should be 75–100 W, not 22–33 W (this needs to be balanced against the timing requirements of the signal).