1-180

Figure 24. CMOS Gate Drive Circuit for HCPL-261A/-

261N Families.

Is Momentarily:

Is Momentarily:

positive (>0)

away from LED

away from LED

increased

decreased

negative (<0)

toward LED

toward LED

decreased

increased

Figure 23. TTL Open-Collector/Open Drain Gate Drive Circuit

for HCPL-261A/-261N Families.

820

Ω

1

3

2

4

VCC

74HC00

(OR ANY

OPEN-COLLECTOR/

OPEN-DRAIN

LOGIC GATE)

HCPL-261X

LED

750

Ω

1

3

2

4

VCC

74HC04

(OR ANY

TOTEM-POLE

OUTPUT LOGIC

GATE)

HCPL-261A/261N

1N4148

LED

Propagation Delay, Pulse-

Width Distortion and

Propagation Delay Skew

Propagation delay is a figure of

merit which describes how

quickly a logic signal propagates

through a system. The propaga-

is the amount of time required for

an input signal to propagate to

the output, causing the output to

change from low to high.

Similarly, the propagation delay

amount of time required for the

input signal to propagate to the

output, causing the output to

change from high to low (see

Figure 9).

Pulse-width distortion (PWD)

in value. PWD is defined as the

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 appli-

cation (RS232, RS422, T-1, etc.).

an important parameter to con-

sider in parallel data applications

where synchronization of signals

on parallel data lines is a con-

cern. If the parallel data is being

sent through a group of opto-

couplers, differences in propaga-

tion delays will cause the data to

arrive at the outputs of the opto-

couplers 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 propaga-

any given group of optocouplers

which are operating under the

same conditions (i.e., the same

drive current, supply voltage,

output load, and operating

temperature). As illustrated in

Figure 25, if the inputs of a group

of optocouplers are switched

either ON or OFF at the same

between the shortest propagation

longest propagation delay, either

determine the maximum parallel