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 sys-
tem. 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 sig-
nal 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. Typi-
cally, PWD on the order of 20-30% of the minimum pulse
width is tolerable; the exact figure depends on the par-
ticular appli cation (RS232, RS422, T-1, etc.).
Propagation delay skew, t
PSK
, is an important param-
eter to consider in parallel data appli cations where
synchroniza tion of signals on parallel data lines is a con-
cern. If the parallel data is being sent through a group
of optocouplers, differ ences in propagation delays will
cause the data to arrive at the outputs of the optocou-
plers 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 op-
tocouplers.
Propagation delay skew is defined as the difference be-
tween 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 op-
erating tempera ture). As illustrated in Figure 15, if the in-
puts 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 propaga tion 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 opto-
couplers. 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 op-
tocoupler. 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 out-
puts 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 applica-
tion is twice t
PSK
. A cautious design should use a slightly
longer pulse width to ensure that any additional uncer-
tainty in the rest of the circuit does not cause a prob-
lem.
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.
