8
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 7).
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 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
PSK
, is
an important parameter to
consider in parallel data appli-
cations 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 between the
minimum and maximum
propagation delays, either t
PLH
or
t
PHL
, for 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 15, if the
inputs of a group of optocouplers
are switched either ON or OFF at
the same time, t
PSK
is the
difference between the shortest
propagation delay, either t
PLH
or
t
PHL
, and the longest propagation
delay, either t
PLH
or t
PHL
.
As mentioned earlier, t
PSK
can
determine the maximum parallel
data transmission rate. Figure 11
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
represents 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
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
guaranteed specifications for
propagation delays, pulse-width
distortion and propagation delay
skew over the recommended
temperature, and input current,
and power supply ranges.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
