HCPL-0708 データシートの表示（PDF） - HP => Agilent Technologies

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HCPL-0708

High Speed CMOS Optocoupler

HP => Agilent Technologies 

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 transmitted. Typically,

PWD on the order of 20 - 30% of

the minimum pulse width is

tolerable; the exact figure

depends on the particular

application.

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 volt-

age, output load, and operating

temperature). As illustrated in

Figure 9, 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 10

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.

VI

50%

VO

2.5 V,

CMOS

tPSK

VI

50%

VO

2.5 V,

CMOS

Figure 9. Propagation delay skew waveform.

DATA

INPUTS

CLOCK

DATA

OUTPUTS

CLOCK

tPSK

tPSK

Figure 10. Parallel data transmission example.

Propagation delay skew repre-

sents the uncertainty of where an

edge might be after being sent

through an optocoupler.

Figure 10 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-0708 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.

9

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