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AV02-0235EN - March 25, 2014

Propagation Delay, Pulse-Width Distortion and Propagation Delay Skew

operating temperature). As illustrated in Figure 23, if the

inputs of a group of optocouplers are switched either ON

or OFF at the same time, t

shortest propagation delay,either t

longest propagation delay,either t

As mentioned earlier,t

parallel data transmission rate. Figure 24 is the timing

diagram of a typical parallel data application with both

the clock and the data lines being sent through opto-

couplers. The figure shows data and clock signals at the

inputs and outputs of the optocouplers. To obtain the

maximum data transmission rate, both edges of the

clock signal are being used to clock the data;if only one

edge were used, the clock signal would need to be twice

as fast.

Propagation delay skew repre- sents the uncertainty of

where an edge might be after being sent through an op-

tocoupler. Figure 24 shows that there will be uncertainty

in both the data and the clock lines. It is important that

these two areas of uncertainty not overlap, otherwise the

clock signal might arrive before all of the data outputs

have settled,or some of the data outputs may start to

change before the clock signal has arrived. From these

considerations, the absolute minimum pulse width that

can be sent through optocouplers in a parallel applica-

tion is twice t

longer pulse width to ensure that any additional uncer-

tainty in the rest of the circuit does not cause a problem.

The t

guaranteed specifications for propagation delays, pulse-

width distortion and propagation delay skew over the

recommended temperature, input current, and power

supply ranges.

Figure 23. Propagation delay skew – t

Figure 24. Parallel data transmission example.

Propagation delay is a figure of merit which de-

scribes how quickly a logic signal propagates through

a system. The propagation delay from low to high

(t

propagate to the output, causing the output to change

from low to high. Similarly, the propagation delay from

high to low (t

input signal to propagate to the output causing the

output to change from high to low (see Figure 16).

Pulse-width distortion (PWD) results when t

in value. PWD is defined as the difference between t

t

of a transmission system. PWD can be expressed in percent

by dividing the PWD (in ns) by the minimum pulse width

(in ns) being transmitted. Typically, PWD on the order of

20-30% of the minimum pulse width is tolerable; the exact

figure depends on the particular application (RS232, RS422,

T-l, etc.).

Propagation delay skew,t

to consider in parallel data applica- tions where synchro-

nization of signals on parallel data lines is a concern. If

the parallel data is being sent through a group of op-

tocouplers, differences in propagation delays will cause

the data to arrive at the outputs of the optocouplers at

different times. If this difference in propagation delays

is large enough, it will determine the maximum rate at

which parallel data can be sent through the optocou-

plers.

Propagation delay skew is defined as the difference

between the minimum and maximum propagation

delays,either t

plers which are operating under the same conditions (i.e.,

the same drive current, supply voltage, output load, and

50%

50%

1.5 V

1.5 V

DATA

INPUTS

CLOCK

DATA

OUTPUTS

CLOCK