Termination in PCB DESIGN | Types of Termination | AC Termination
In this article we are going to discuss about what is Termination in PCB, types of termination and what is AC termination?
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
topological 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.
Types of Termination
1. Series Termination
2. Parallel Termination
Using Series termination resistor may help to lower possible crosstalk this is simple termination , Here termination resistor generally will find in circuit clock signal but some time find it other signal. It may be databases or address busses generally use in clock signal, For series termination Using a series resistor is simple as the resistor can be placed in-line with the trace and interconnect on the surface layer that is not require any vias, and the termination will be quite accurate. In this termination should be match impedance of series resistor with the signal source rather than the load. Series termination is also advantageous as it consumes a small amount of power.
Series Termination (At Driver)
· Avoids Signal Distortion
· It doesn’t eliminate the first reflection at the receiver
· It has no effect on the source current and DC voltage level
For a better termination use parallel termination by parallel resistor. In this technique connecting a parallel resistor between ground and load the load and ground which is equal to the line impedance. This is a better option in components without programmable output or input impedance.
Parallel Termination (At Receiver)
· Parallel resistor may be connected either to VCC or GND.
· It Eliminate all the reflections
· Increase the driver DC current
· Resistor to ground lowers the DC voltage level for high state
· Resistor to VCC raises the DC voltage level for low state.
What is AC termination?
AC termination adds a capacitive load to the driver and delay due to RC time constant, however, it consumes low power
AC termination is recommended most for clock
applications. An example of AC termination is when a 75Ω impedance is coupled
with a 100pF capacitor. To allow for leakage in input impedance of the
receiver, the resistor is selected to be larger than the trace impedance. To
allow for rapid transition of the clock edge, the capacitor value is selected
at 120pF. A higher capacitor value allows for heavier current levels to pass.
However, higher capacitive values increases power dissipation. Capacitor values
less than 50pF diminish the effectiveness of termination.
This method terminates the signal transition & therefore should be calculated based on the edge rate. This method’s disadvantage is that 2 components are needed for each line. Termination packs are sometimes used to reduce component count. Be aware that signal coupling can occur between signals in the termination pack. An RC circuit arranged in series to a voltage source, usually ground, makes a dynamic termination. The result of this is the cut off of glitches & overshoot.