HCPL-0710 Datasheet, PDF (8/16 Pages) Agilent(Hewlett-Packard) – 40 ns Prop. Delay, SO-8 Optocoupler

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 13.
INPUT
V_I
OUTPUT
V_O
10%
t_PLH
90%
Figure 13.
t_PHL
50%
90%
10%
5 V CMOS
0V
V_OH
2.5 V CMOS
V_OL
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. The PWD
specification for the HCPL-0710
is 8 ns (10%) maximum across
recommended operating condi-
tions. 10% maximum is dictated
by the most stringent of the three
fieldbus standards, PROFIBUS.
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 volt-
age, output load, and operating
temperature). As illustrated in
Figure 14, 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 15
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.