HCPL-7723 Datasheet, PDF (11/12 Pages) Agilent(Hewlett-Packard) – 50 MBd 2 ns PWD High Speed CMOS Optocoupler

Pulse-width distortion (PWD) is
the difference between t_PHL 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.
Propagation delay skew, t_PSK, is
an important parameter to
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, 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.
Propagation delay skew is defined
as the difference between 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
operating temperature). As
illustrated in Figure 4, if the
inputs 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 propagation
delay, either t_PLH or t_PHL.
As mentioned earlier, t_PSK can
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
optocouplers. In this case the
data is assumed to be clocked off
of the rising edge of the clock.
V_I
50%
V_O
2.5 V,
CMOS
t_PSK
V_I
50%
V_O
2.5 V,
CMOS
Figure 4. Timing diagram to illustrate propagation delay skew, tpsk.
DATA
INPUTS
CLOCK
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,
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 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.
DATA
OUTPUTS
CLOCK
t_PSK
t_PSK
Figure 5. Parallel data transmission example.
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.
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