With the increasing speed of data transmission (PC, TV, ...), engineersare naturally confrontedwith effects

that were of less significance with slower circuits. Among these are the effects described in transmission

line theory : line reflections provide signal distortions and the overshoots or undershoots produced on the

signal edges can finally cause the malfunction of the whole system.

1. Reflection at a non terminated line

The figure below shows the circuit of a transmission system in which a memory device (input impedance of

100 k

Ω) is connectedat the end of a line with line impedance Zo.

APPLICATION NOTE : BUS TERMINATION

10

Ω

100 k

Ω

If this line is not properly terminated, a certain amount of the energy is reflected back, inducing a reflection

phenomena that can distort the signal. This can result in improper operation of the system.

The simulation shown on the above figure illustrates the signal distortion produced by line reflection at the

end of the line which is not well terminated.

Even if the signal at the start of the line has the correct form (upper curve), considerable distortion arises at

the end of the line (lower curve). On the positive edge, the overshoot can exceed the maximum operating

voltage of the used circuit technology which will then be destroyed. Also, the following negativeundershoot

may reach a level low enough to change the value of the logic state. If it affects of an address line, a wrong

memory cell will be addressed, and in the case of a data line, the data can be corrupted.

This phenomena also occurs on the falling edge.

To avoid these negative effects from leading to problems in a system, a suitable termination is

required.

RCD16-47B

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