Pulse-width distortion (PWD) results when t
and t
PHL
Bypassing and PC Board Layout
PLH
differ in value. PWD is defined as the difference between
and t and often PWD is defined as the difference
The ACPL-061L/ACPL-C61L/ACNW261L optocouplers are
extremely easy to use. ACPL-061L/ACPL-C61L/ACNW261L
provide CMOS logic output due to the high-speed CMOS
IC technology used.
t
PLH
PHL
PLH
between t
and t . This parameter determines the
PHL
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.
Typically, PWD in 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.).
The external components required for proper operation
are the input limiting resistors and the output bypass
capacitor. Capacitor values should be 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.
Propagation delay skew, t , is an important parameter
PSK
to consider in parallel data applications where synchroni-
zation of signals on parallel data lines is a concern. If the
parallel data is being sent through a group of optocou-
plers, 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, 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
propagation delay from low to high (t ) is the amount
PLH
of time required for an input signal to propagate to the
output, causing the output to change from low to high.
Propagation delay skew is defined as the difference
between the minimum and maximum propagation delays,
Similarly, the propagation delay from high to low (t
)
PHL
either t
or t , for any given group of optocouplers
PLH
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 9).
which are operating under the same conditions (i.e., the
same supply voltage, output load, and operating temper-
ature). As illustrated in Figure 10, if the inputs of a group of
optocouplers are switched either ON or OFF at the same
RT = R1 + R2, R1/R2 ≈ 1.5
time,
is the difference between the shortest propaga-
tPSK
tion delay, either t
or t , and the longest propagation
VDD
PLH
PHL
1
2
3
4
8
7
6
5
R1
R2
IF
50%
VI
VI
VE
C = 0.1 µF
GND1
VO
2.5 V,
CMOS
VO
VI
tPSK
GND2
ACPL-061L / ACPL-C61L / ACNW261L
50%
3.3V / 5V
V
DD
8
7
1
2
I
F
Anode
C = 0.1µF
2.5 V,
CMOS
VE
VO
B
A
6
5
Figure 10. Propagation delay skew waveform
3
4
VO
Output
Monitoring
node
Cathode
DATA
INPUTS
Shield
+
GND
CLOCK
−
V
CM
Pulse Gen
VCM (PEAK)
VCM
0 V
DATA
SWITCH AT A: IF = 0 mA
VDD
CMH
VO
OUTPUTS
tPSK
V (min.)
O
CLOCK
SWITCH AT B: I F= 2 mA (ACPL-061L) / 5 mA (ACPL-C61L/ACNW261L)
V (max.)
O
VO
tPSK
GND
CM L
Figure 11. Parallel data transmission example
Figure 9. Recommended printed circuit board layout
14
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
