Propagation Delay,
Pulse-Width Distortion and Propagation Delay Skew
Propagation delay is a gure of merit which describes how
quickly a logic signal propagates through a system. The
propagation delay from low to high (tPLH) 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 (tPHL)
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 tPLH and tPHL
dier in value. PWD is dened as the dierence between
tPLH and tPHL 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 gure depends on the particular
application (RS232, RS422, T-1, etc.).
Propagation delay skew, tPSK, is an important parameter to
consider in parallel data applications where synchroniza-
tion of signals on parallel data lines is a concern. If the
parallel data is being sent through a group of optocouplers,
dierences in propagation delays will cause the data to
arrive at the outputs of the optocouplers at dierent times.
If this dierence 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 dened as the dierence be-
tween the minimum and maximum propagation delays,
either tPLH or tPHL, 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, tPSK is the dierence between
the shortest propagation delay, either tPLH or tPHL, and the
longest propagation delay, either tPLH or tPHL.
As mentioned earlier, tPSK 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 gure 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 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 tPSK. 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 tPSK specied optocouplers oer the advantages of
guaranteed specications for propagation delays, pulse-
width distortion and propagation delay skew over the
recommended temperature, and input current, and power
supply ranges.