Pulse-width distortion (PWD) is
the difference between t and
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 delay is large enough
it will determine the maximum
rate at which parallel data can be
sent through the optocouplers.
illustrated in Figure 4, if the
inputs of a group of optocouplers
are switched either ON or OFF at
PHL
t
and often determines the
PHL
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 same time, t
is the
PSK
difference between the shortest
propagation delay, either t
or
PLH
t
, and the longest propagation
PHL
delay, either t
or t
.
PLH
PHL
As mentioned earlier, t
can
PSK
Propagation delay skew is defined
as the difference between the
minimum and maximum
determine the maximum parallel
data transmission rate. Figure 5
is the timing diagram of a typical
parallel data application with
both the clock and data lines
being sent through the
optocouplers. The figure shows
data and clock signals at the
inputs and outputs of the
Propagation delay skew, t , is
propagation delays, either t
or
PSK
PLH
an important parameter to
t
, for any given group of
PHL
consider in parallel data applica-
tions where synchronization of
signals on parallel data lines is a
concern. If the parallel data is
being sent through a group of
optocouplers which are operating
under the same conditions (i.e.,
the same drive current, supply
voltage, output load, and
operating temperature). As
optocouplers. In this case the
data is assumed to be clocked off
of the rising edge of the clock.
V
I
50%
Propagation delay skew
represents the uncertainty of
where an edge might be after
being sent through an
optocoupler. Figure 5 shows that
there will be uncertainty in both
the data and clock lines. It is
important that these two areas of
uncertainty not overlap,
2.5 V,
CMOS
V
O
t
PSK
V
50%
I
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
2.5 V,
CMOS
V
O
Figure 4. Timing diagram to illustrate propagation delay skew, tpsk.
considerations, the absolute
minimum pulse width that can be
sent through optocouplers in a
DATA
parallel application is twice t
.
PSK
A cautious design should use a
slightly longer pulse width to
ensure that any additional
INPUTS
CLOCK
uncertainty in the rest of the
circuit does not cause a problem.
DATA
The HCPL-7723/0723
optocouplers offer the advantage
of guaranteed specifications for
propagation delays, pulse-width
distortion, and propagation delay
skew over the recommended
temperature and power supply
ranges.
OUTPUTS
t
PSK
CLOCK
t
PSK
Figure 5. Parallel data transmission example.
11
HP [ HEWLETT-PACKARD ]
