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AV02-0643EN - February 26, 2013
Pulse-width distortion (PWD) is the dierence between
t
PHL
and t
PLH
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.
Propagation delay skew, t
PSK
, is an important parameter
to consider in parallel data applica tions where synchro-
nization of signals on parallel data lines is a concern. If
the parallel data is being sent through a group of opto-
couplers, dierences in propagation delays will cause
the data to arrive at the outputs of the optocouplers at
dierent times. If this dierence in propagation delay
is large enough it will determine the maximum rate at
which parallel data can be sent through the optocou-
plers.
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 opto-
couplers which are operating under the same conditions
(i.e., the same drive current, supply voltage, output load,
and operating temperature). As illustrated in Figure 10,
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
.
Figure 10. Timing diagram to illustrate propagation delay skew, tpsk.
50%
50%
t
PSK
V
I
V
O
V
I
V
O
2.5 V,
CMOS
2.5 V,
CMOS
DATA
INPUTS
CLOCK
DATA
OUTPUTS
CLOCK
t
PSK
t
PSK
Figure 11. Parallel data transmission example.
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 data lines being sent through the opto-
couplers. The gure shows data and clock signals at the
inputs and outputs of the optocouplers. In this case the
data is assumed to be clocked o of the rising edge of
the clock.
Propagation delay skew represents the uncertainty of
where an edge might be after being sent through an op-
tocoupler. Figure 11 shows that there will be uncertainty
in both the data and 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 consid-
erations, 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 HCPL-7723/0723 optocouplers oer the advantage of
guaranteed specications for propagation delays, pulse-
width distortion, and propagation delay skew over the
recommended temperature and power supply ranges.
