Pulse-width distortion (PWD) is
the difference 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 trans-
mitted. 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 con-
sider in parallel data applications
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 propa-
gation delays, either t
PLH
or t
PHL
,
for any given group of optocoup-
lers which are operating under
the same conditions (i.e., the same
drive current, supply voltage,
output load, and operating
temperature). As illustrated in
Figure 13, 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 14
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
DATA
50%
INPUTS
V
O
2.5 V,
CMOS
t
PSK
CLOCK
V
I
50%
DATA
OUTPUTS
CLOCK
t
PSK
V
O
2.5 V,
CMOS
t
PSK
Figure 13. Propagation delay skew waveform.
Figure 14. Parallel data transmission example.
Propagation delay skew repre-
sents the uncertainty of where an
edge might be after being sent
through an optocoupler. Figure 14
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.
The HCPL-772X/072X
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.
11
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