8

Propagation delay skew is defined as the difference be-

tween the minimum and maximum propagation delays,

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

the same supply voltage, output load, and operating

temperature). As illustrated in Figure 8, if the inputs

of a group of optocouplers are switched either ON or

parallel data transmission rate. Figure 8 is the timing dia-

gram of a typical parallel data application with both the

clock and the data lines being sent through optocouplers.

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 optocoupler. Figure 7 shows that there will be

uncertainty in both the data and the clock lines. It is im-

portant 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

slightlylongerpulsewidthtoensurethatanyadditionalun-

certaintyintherestofthecircuitdoesnotcauseaproblem.

guaranteed specifications for propagation delays, pulse-

width distortion and propagation delay skew over the

recommended temperature, and power supply ranges.

Figure 7. Propagation delay skew waveform.

Figure 8. Parallel data transmission example.

50%

50%

tPSK

IF

VO

IF

VO

2.5 V,

CMOS

2.5 V,

CMOS

DATA

INPUTS

CLOCK

DATA

OUTPUTS

CLOCK

tPSK

tPSK

8

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