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AN246

Transmission lines and terminations with Philips Advanced Logic families

Philips Electronics 

Philips Semiconductors

Transmission lines and terminations

with Philips Logic families

Application Note

AN246

TERMINATION CONSIDERATIONS AND

TECHNIQUES

As shown earlier, impedance mismatches between the source, line

and load can cause reflections. These reflection can cause signal

delays, such as the case of a stairstep type of response which

requires additional line delays to reach sufficient switching threshold

levels, mis-clocking from non-monotonic edges, or excess

voltage/current on inputs. A signal should be terminated if it won’t

settle on time, if it produces overshoot or undershoot that violates

the receivers input voltage or current ratings, or if it drives

edge-sensitive asynchronous inputs and has non-monotonic edges.

Several termination schemes can be used depending on drive

current capability, power dissipation requirements, and incident

wave switching requirements.

There are two basic approaches to line termination: source

termination and end termination. Both schemes will result in a

stable signal at the far end of the line after one line delay. Source

termination, however, results in a stable signal up to two line delays

for loads at intermediate points on the line and at the source. More

details of each scheme follows.

Source Terminations Methods

Figure 12 shows the configuration of a source terminated daisy

chain line.

Driver

RS

ZO’

ZO’

Receiver C

Receiver A

Receiver B

SH00117

Figure 12. Source termination configuration

The concept of this termination method is to try and match the

loaded line impedance with the sum of the driver output and series

resistor impedances. The series resistor value is equal to ZO’ minus

the driver impedance. The resistor should be located as close to the

driver as possible.

Since the sum of the driver impedance and series resistor equals

the line impedance, a half-height wave travels down the line from

the voltage divider effect. Assuming a reflection coefficient

approaching +1 at the end of the line, the reflection adds to the

half-height wave, and the voltage at the last receiver is at near full

amplitude. The wave amplitude at the first and intermediate

receivers, however, are half-height and require up to one additional

line delay for the reflected wave to reach the series terminator and

add to the initial wave. Figure 13 shows a SPICE simulation of the

reflections for this circuit and termination method.

Note that the last receiver is first to switch to the full signal

amplitude, while the first receiver is the last to reach full amplitude.

This means that any edge-sensitive asynchronous signals should be

located at the end of the line. Non-monotonic edges at the

beginning and intermediate points along the line could cause false

clocking of devices. Also, drivers at the beginning and at

intermediate points need to be able to tolerate roughly twice the

settling time.

As you can see, this termination method is not very good for lines

with daisy chain topologies. Source terminators work well, though,

for single receiver, point-to-point loads and star type of topologies.

They work well to dampen overshoot and undershoot.

Source terminators dissipate no quiescent power. The AC power

dissipation can be estimated by:

Eq. 12

ǒ Ǔ2

P

[

f2T

DV

2R

where f

where T

∆V

R

= pulse frequency

= one-way line delay

= VOH–VOL

= termination resistance

This approximation works if the pulse interval is greater than twice

the line delay. For shorter pulse intervals, you can assume a worst

case of DV/2 across the termination resistor at all times. With its

low power dissipation, series termination is recommended for low

voltage logic.

As mentioned previously, the sum of source impedance and the

series terminator should match the loaded line impedance. Since

output impedances are different in the logic low and high states,

there needs to be a compromise when choosing the termination

resistance. It’s probably better to slightly overdrive the line by

choosing a smaller resistor to ensure fast enough edge transitions to

a valid logic level. Typical values in applications range from 22 Ω to

33 Ω. Philips offers ABT, ALVC, ALVT, LVC, and LVT parts with

built-in series terminators that have equivalent output impedances of

30 Ω. These parts save board space by eliminating the need for a

terminating resistor. Part types are designated by a “2” prefix before

the part type number, e.g., 74ABT2245.

1998 Feb 05

9

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