Figure 9. Propagation delay skew waveform.
Figure 10. Parallel data transmission example.
Propagation delay skew represents the uncertainty of
where an edge might be after being sent through an op-
tocoupler. Figure 10 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 consider-
ations, the absolute minimum pulse width that can be sent
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. Typical-
ly, PWD on the order of 20 - 30% of the minimum pulse
width is tolerable; the exact gure depends on the par-
ticular application.
Propagation delay skew, t
PSK
, is an important parameter
to consider in parallel data applications 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 dier-
ent 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 optocouplers.
50%
50%
t
PSK
I
F
V
O
I
F
V
O
2.5 V,
CMOS
2.5 V,
CMOS
DATA
INPUTS
CLOCK
DATA
OUTPUTS
CLOCK
t
PSK
t
PSK
Propagation delay skew is dened as the dierence be-
tween 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 op-
erating temperature). As illustrated in Figure 9, if the in-
puts 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 10 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.
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-0708 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.
