HCPL-7100 Datasheet, PDF (14/14 Pages) Agilent(Hewlett-Packard) – High Speed CMOS Optocouplers

HCPL-7100/7101
Application Information
The HCPL-7100/7101 is
extremely easy to use. Because
the optocoupler uses high-speed
CMOS IC technology, the inputs
and output are fully compatible
with all +5 V TTL and CMOS
logic. TTL or CMOS logic can be
connected directly to the inputs
and output; no external interface
circuitry is required.
As shown in Figure 1, the only
external components required for
proper operation are two ceramic
bypass capacitors. Capacitor
values should be between 0.01 µF
and 0.1 µF. For each capacitor,
the total lead length between both
ends of the capacitor and the
power-supply pins should not
exceed 20 mm. Figure 2
illustrates the recommended
printed circuit board layout for
the HCPL-7100/7101.
Propagation Delay, Pulse-
Width Distortion, and
Propagation Delay Skew
Propagation delay is a figure of
merit which describes how
quickly a logic signal propagates
through a system. The propaga-
tion delay from low to high (t_PLH)
is the amount of time required for
an input signal to propagate to
the output, causing the output to
change from low to high.
Similarly, the propagation delay
from high to low (t_PHL) is the
amount of time required for the
input signal to propagate to the
output, causing the output to
change from high to low (see
Figure 7).
Pulse-width distortion (PWD)
results when t_PLH and t_PHL differ in
value. PWD is defined as the
difference between t_PLH and t_PHL
and often determines the maxi-
mum 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 exact figure depends on the
particular application (RS232,
RS422, T-1, etc.).
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 delays 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 propaga-
tion delays, either t_PLH or t_PHL, for
any given group of optocouplers
which are operating under the
same conditions (i.e., the same
supply voltage, output load, and
operating temperature). As illus-
trated 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 or t_PHL.
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 the data lines being
sent through optocouplers. The
figure shows data and clock
signals at the inputs and outputs
of the optocouplers. To obtain the
maximum data transmission rate,
both edges of the clock signal are
being used to clock the data; if
only one edge were used, the
clock signal would need to be
twice as fast.
Propagation delay skew repre-
sents 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
the 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-7101 optocoupler offers
the advantages 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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