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 (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 6).
Pulse-width distortion (PWD) results when t
PLH
and t
PHL
dier in value. PWD is dened as the dierence between
t
PLH
and t
PHL
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, t
PSK
, is an important parameter
to consider in parallel data applications where synchroni-
zation of signals on parallel data lines is a concern. If the
parallel data is being sent through a group of optocou-
plers, 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
between the minimum and maximum propagation
delays, either t
PLH
or t
PHL
, for any given group of optocou-
plers which are operating under the same conditions (i.e.,
the same drive current, supply voltage, output load, and
Figure 14. Illustration of Propagation Delay Skew – t
PSK
Figure 15. Parallel Data Transmission Example
50%
1.5 V
I
F
V
O
50%I
F
V
O
t
PSK
1.5 V
DATA
t
PSK
INPUTS
CLOCK
DATA
OUTPUTS
CLOCK
t
PSK
Propagation Delay, Pulse-Width Distortion and Propagation Delay Skew
operating temperature). As illustrated in Figure 14, if the
inputs of a group of optocouplers are switched either ON
or OFF at the same time, t
PSK
is the dierence 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 15 is the timing
diagram of a typical parallel data application with both
the clock and the data lines being sent through optocou-
plers. 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 15 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 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 problem.
The t
PSK
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.
For product information and a complete list of distributors, please go to our web site: www.avagotech.com
Avago, Avago Technologies, the A logo and R
2
Coupler™ are trademarks of Avago Technologies in the United States and other countries.
Data subject to change. Copyright © 2005-2013 Avago Technologies. All rights reserved.
AV02-0950EN - September 30, 2013
