ad8465 Analog Devices, Inc., ad8465 Datasheet - Page 11

• Manufacturer Part Number: ad8465
• Description: Rail-to-rail, Very Fast, 2.5 V To 5.5 V, Single-supply Lvds Comparator
• Manufacturer: Analog Devices, Inc.

COMPARATOR PROPAGATION DELAY
DISPERSION
The AD8465 comparator is designed to reduce propagation
delay dispersion over a wide input overdrive range of 5 mV
to V
propagation delay that results from a change in the degree of
overdrive or slew rate (how far or how fast the input signal is
driven past the switching threshold).
Propagation delay dispersion is a specification that becomes
important in high speed, time-critical applications, such as data
communications, automatic test and measurement, and instru-
mentation. It is also important in event-driven applications, such
as pulse spectroscopy, nuclear instrumentation, and medical
imaging. Dispersion is defined as the variation in propagation
delay as the input overdrive conditions are changed (see Figure 16
and Figure 17).
The AD8465 dispersion is typically <1.6 ns as the overdrive
varies from 10 mV to 125 mV. This specification applies to
both positive and negative signals because the AD8465 has
substantially equal delays for positive-going and negative-
going inputs and very low output skews.
CCI
− 1 V. Propagation delay dispersion is the variation in
INPUT VOLTAGE
INPUT VOLTAGE
Q/Q OUTPUT
Q/Q OUTPUT
Figure 16. Propagation Delay—Overdrive Dispersion
Figure 17. Propagation Delay—Slew Rate Dispersion
10V/ns
1V/ns
500mV OVERDRIVE
10mV OVERDRIVE
DISPERSION
DISPERSION
V
V
N
N
± V
± V
OS
OS
Rev. 0 | Page 11 of 16
COMPARATOR HYSTERESIS
The addition of hysteresis to a comparator is often desirable in
a noisy environment, or when the differential input amplitudes
are relatively small or slow moving. The transfer function for a
comparator with hysteresis is shown in Figure 18. As the input
voltage approaches the threshold (0 V, in this example) from
below the threshold region in a positive direction, the comparator
switches from low to high when the input crosses +V
new switching threshold becomes −V
in the high state until the −V
the threshold region in a negative direction. In this manner, noise
or feedback output signals centered on 0 V input cannot cause
the comparator to switch states unless it exceeds the region
bounded by ±V
The customary technique for introducing hysteresis into a
comparator uses positive feedback from the output back to
the input. One limitation of this approach is that the amount
of hysteresis varies with the output logic levels, resulting in
hysteresis that is not symmetric about the threshold. The
external feedback network can also introduce significant
parasitics that reduce high-speed performance and induce
oscillation in some cases.
Figure 18. Comparator Hysteresis Transfer Function
H
/2.
–V
2
H
OUTPUT
V
OL
0V
H
/2 threshold is crossed from below
V
OH
H
/2. The comparator remains
+V
2
H
INPUT
AD8465
H
/2. The