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ADCMP563/ADCMP564
Propagation delay dispersion is important in critical timing
applications such as ATE, bench instruments, and nuclear
instrumentation. Overdrive dispersion is defined as the varia-
tion in propagation delay as the input overdrive conditions are
changed (Figure 19). For the ADCMP563/ADCMP564, over-
drive dispersion is typically 75 ps as the overdrive is changed
from 100 mV to 1.5 V. This specification applies for both
positive and negative overdrive because the ADCMP563 and
the ADCMP564 have equal delays for positive and negative
going inputs.
Rev. A | Page 12 of 16
A current source can also be used with the HYS pin. The
relationship between the current applied to the HYS pin and the
resulting hysteresis is shown in Figure 17.
OUTPUT
INPUT
0
1
0V
–V
H
2
+V
H
2
0
Q OUTPUT
INPUT VOLTAGE
1.5V OVERDRIVE
20mV OVERDRIVE
DISPERSION
V
REF
± V
OS
0
Figure 20. Comparator Hysteresis Transfer Function
160
140
120
100
80
60
40
20
0
50
0
10
20
30
40
0
R
HYS
(k
)
P
Figure 19. Propagation Delay Dispersion
COMPARATOR HYSTERESIS
The addition of hysteresis to a comparator is often useful in a
noisy environment, or where it is not desirable for the compar-
ator to toggle between states when the input signal is at the
switching threshold. The transfer function for a comparator
with hysteresis is shown in Figure 20. If the input voltage
approaches the threshold from the negative direction, the
comparator switches from a 0 to a 1 when the input crosses
+V
H
/2. The new switching threshold becomes V
H
/2. The
comparator remains in a 1 state until the threshold V
H
/2 is
crossed coming from the positive direction. In this manner,
noise centered on 0 V input does not cause the comparator to
switch states unless it exceeds the region bounded by ±V
H
/2.
Figure 21. Comparator Hysteresis vs. R
HYS
MINIMUM INPUT SLEW RATE REQUIREMENT
As for all high speed comparators, a minimum slew rate must
be met to ensure that the device does not oscillate when the
input crosses the threshold. This oscillation is due in part to the
high input bandwidth of the comparator and the parasitics of
the package. Analog Devices recommends a slew rate of 1 V/μs
or faster to ensure a clean output transition. If slew rates less
than 1 V/μs are used, hysteresis should be added to reduce the
oscillation.
Positive feedback from the output to the input is often used to
produce hysteresis in a comparator (Figure 24). The major
problem with this approach is that the amount of hysteresis
varies with the output logic levels, resulting in a hysteresis that
is not symmetrical around zero.
In the ADCMP564, hysteresis is generated through the
programmable hysteresis pin. A resistor from the HYS pin to
GND creates a current into the part that is used to generate
hysteresis. Hysteresis generated in this manner is independent
of output swing and is symmetrical around the trip point. The
hysteresis versus resistance curve is shown in Figure 21.