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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-
tion delay from low to high (t
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
from high to low (t
) is the
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amount of time required for the
input signal to propagate to the
output, causing the output to
change from high to low (see
Figure 5).
Pulse-width distortion (PWD)
results when t
and t
PLH
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value. PWD is defined as the
difference between t
and t
PLH
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 (RS232,
RS422, T-1, etc.).
Propagation delay skew, t
an important parameter to
consider in parallel data applica-
tions 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
delays is large enough, it will
1-312
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-
tion delays, either t
)
PLH
any given group of optocouplers
which are operating under the
same conditions (i.e., the same
drive current, supply voltage,
output load, and operating tem-
perature). As illustrated in
Figure 15, if the inputs of a group
of optocouplers are switched
either ON or OFF at the same
time, t
is the difference
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between the shortest propagation
delay, either t
longest propagation delay, either
t
or t
.
differ in
PLH
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As mentioned earlier, t
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determine the maximum parallel
data transmission rate. Figure 16
is the timing diagram 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 signals
are being used to clock the data;
if only one edge were used, the
clock signal would need to be
, is
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twice as fast.
Propagation delay skew repre-
sents the uncertainty of where an
edge might be after being sent
through an optocoupler.
Figure 16 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
or t
, for
parallel application is twice t
PLH
PHL
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-2400/30 optocouplers
offer the advantages of guaran-
teed specifications for propaga-
tion delays, pulse-width distortion,
and propagation delay skew over
or t
, and the
the recommended temperature,
PLH
PHL
input current, and power supply
ranges.
Application Circuit
can
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A recommended LED drive circuit
is shown in Figure 13. This circuit
utilizes several techniques to
minimize the total pulse-width
distortion at the output of the
optocoupler. By using two
inverting TTL gates connected in
series, the inherent pulse-width
distortion of each gate cancels the
distortion of the other gate. For
best results, the two series-
connected gates should be from
the same package.
The circuit in Figure 13 also uses
techniques known as prebias and
peaking to enhance the
performance of the optocoupler
LED. Prebias is a small forward
voltage applied to the LED when
the LED is off. This small prebias
voltage partially charges the
junction capacitance of the LED,
allowing the LED to turn on more
quickly. The speed of the LED is
further increased by applying
.
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