18
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
PLH
) 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
PHL
) is
the amount of time required for
the input signal to propagate to
the output causing the output to
change from high to low (see
Figure 16).
Pulse-width distortion (PWD)
results when t
PLH
and t
PHL
differ
in value. PWD is defined as the
difference between t
PLH
and t
PHL
and often determines the maxi-
mum 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 transmit-
ted. Typically, PWD on the order of
20-30% of the minimum pulse
width is tolerable; the exact figure
depends on the particular applica-
tion (RS232, RS422, T-l, etc.).
Propagation delay skew,t
PSK
, is
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 determine the
maximum rate at which parallel
data can be sent through the
optocouplers.
Propagation delay skew is
defined as the difference be-
tween the minimum and maxi-
mum propagation delays,either
t
PLH
or t
PHL
, for any given group
of optocouplers which are
operating under the same condi-
tions (i.e., the same drive cur-
rent, supply voltage, output load,
and operating temperature). As
illustrated in Figure 23, if the
inputs of a group of optocouplers
are switched either ON or OFF at
the same time, t
PSK
is the differ-
ence between the shortest
propagation delay,either t
PLH
or
tPHL, and the longest propaga-
tion delay,either t
PLH
or t
PHL
.
As mentioned earlier,t
PSK
can
determine the maximum parallel
data transmission rate. Figure 24
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
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 24 shows that there will
be uncertainty in both the data
and the clock lines. It is impor-
tant that these two areas of
uncertainty not overlap, other-
wise 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 consider-
ations, the absolute minimum
pulse width that can be sent
through optocouplers in a
parallel application is twice t
PSK
.
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 t
PSK
specified optocouplers
offer the advantages of guaran-
teed specifications for propaga-
tion delays, pulsewidth distor-
tion and propagation delay skew
over the recommended tempera-
ture, input current, and power
supply ranges.
Figure 23. Propagation delay skew – t
PSK
.
Figure 24. Parallel data transmission example.
