Datasheet

Figure 11. Parallel data transmission example.
Propagation delay skew represents the uncertainty
of where an edge might be after being sent through
an optocoupler. 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 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 ACPL-x72L optocoupler offers the advantage of
guaranteed specifications for propagation delays, pulse-
width distortion, and propagation delay skew over the
recommended temperature and power supply ranges.
Figure 10. Propagation delay skew waveform
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
optocouplers. The figure shows data and clock signals at
the inputs and outputs of the optocouplers. In this case
the data is assumes to be clocked off of the rising edge of
the clock.
Propagation delay skew, t
PSK
, is an important parameter
to consider in parallel data applications where
synchronization of signals on parallel data lines is a
concern. If the parallel data is 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 optocouoplers
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 difference between
the shortest propagation delay, either t
PLH
or t
PHL
and the
longest propagation delay, either t
PLH
and t
PHL
.
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
For product information and a complete list of distributors, please go to our web site: www.avagotech.com
Avago, Avago Technologies, and the A logo are trademarks of Avago Technologies Limited in the United States and other countries.
Data subject to change. Copyright © 2006 Avago Technologies Limited. All rights reserved. Obsoletes AV01-0462EN
AV02-0324EN - June 3, 2008