HCPL-7721 datasheet(11/18 Pages) HP

Part #	HCPL-7721

Description	40 ns Propagation Delay CMOS Optocoupler

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Figure 13. Propagation delay skew waveform.

Figure 14. Parallel data transmission example.

Propagation delay skew repre-

sents the uncertainty of where an

edge might be after being sent

through an optocoupler. Figure 14

shows that there will be

uncertainty in both the data and

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 application is twice tPSK.

A cautious design should use a

slightly longer pulse width to

ensure that any additional

uncertainty in the rest of the

circuit does not cause a problem.

The HCPL-772X/072X

optocouplers offer the advantage

of guaranteed specifications for

propagation delays, pulse-width

distortion, and propagation delay

skew over the recommended

temperature and power supply

ranges.

Pulse-width distortion (PWD) is

the difference between tPHL and

tPLH and often determines the

maximum data rate capability of a

transmission system. PWD can be

expressed in percent by dividing

the PWD (in ns) by the minimum

pulse width (in ns) being trans-

mitted. Typically, PWD on the

order of 20 - 30% of the minimum

pulse width is tolerable.

Propagation delay skew, tPSK, is

an important parameter to con-

sider in parallel data applications

where synchronization of signals

on parallel data lines is a concern.

If the parallel data is being sent

through a group of optocouplers,

differences in propagation delays

will cause the data to arrive at the

outputs of the optocouplers at

different times. If this difference

in propagation delay is large

enough it will determine the

maximum rate at which parallel

data can be sent through the

optocouplers.

Propagation delay skew is defined

as the difference between the

minimum and maximum propa-

gation delays, either tPLH or tPHL,

for any given group of optocoup-

lers which are operating under

the same conditions (i.e., the same

drive current, supply voltage,

output load, and operating

temperature). As illustrated in

Figure 13, if the inputs of a group

of optocouplers are switched

either ON or OFF at the same

time, tPSK is the difference

between the shortest propagation

delay, either tPLH or tPHL, and the

longest propagation delay, either

tPLH or tPHL.

As mentioned earlier, tPSK can

determine the maximum parallel

data transmission rate. Figure 14

is the timing diagram of a typical

parallel data application with

both the clock and data lines

being sent through the

optocouplers. The figure shows

data and clock signals at the

inputs and outputs of the

optocouplers. In this case the data

is assumed to be clocked off of the

rising edge of the clock.

50%

tPSK

VI

VO

VI

VO

2.5 V,

CMOS

2.5 V,

CMOS

DATA

INPUTS

CLOCK

DATA

OUTPUTS

CLOCK

tPSK

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HCPL-7721 Datasheet(PDF) 11 Page - Agilent(Hewlett-Packard)

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