ADCMP551_0410 Datasheet, PDF (12/16 Pages) Analog Devices – Single-Supply, High Speed PECL/LVPECL Comparators

ADCMP551/ADCMP552/ADCMP553
Propagation delay dispersion is an important specification in
critical timing applications such as ATE, bench instruments,
and nuclear instrumentation. Overdrive dispersion is defined as
the variation in propagation delay as the input overdrive
conditions are changed (Figure 18). For the ADCMP55x,
overdrive dispersion is typically 125 ps as the overdrive is
changed from 20 mV to 1 V. This specification applies for both
positive and negative overdrive since the ADCMP55x has equal
delays for positive- and negative-going inputs.
1.5V OVERDRIVE
INPUT VOLTAGE
20mV OVERDRIVE
V_REF ± V_OS
Q OUTPUT
DISPERSION
Figure 18. 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
comparator 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 19. If the input voltage
approaches the threshold from the negative direction, the
comparator switches from a 0 to a 1 when the input crosses
+VH/2. The new switching threshold becomes −VH/2. The
comparator remains in a 1 state until the −VH/2 threshold 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 ±VH/2.
Positive feedback from the output to the input is often used to
produce hysteresis in a comparator (Figure 23). 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 ADCMP552, hysteresis is generated through the
programmable hysteresis pin. A resistor from the HYS pin to
VCCI 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 20.
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 16.
–V_H
+V_H
2
2
0V
INPUT
1
0
OUTPUT
Figure 19. Comparator Hysteresis Transfer Function
120
100
80
60
40
20
0
100
10
1
R_HYS (kΩ)
Figure 20. Comparator Hysteresis Transfer Function
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.
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