Data Sheet AD8468
Rev. A | Page 7 of 12
APPLICATIONS INFORMATION
POWER/GROUND LAYOUT AND BYPASSING
The AD8468 comparator is a high speed device. Despite
the low noise output stage, it is essential to use proper high
speed design techniques to achieve the specified performance.
Because comparators are uncompensated amplifiers, feedback
in any phase relationship is likely to cause oscillations or
undesired hysteresis. Of critical importance is the use of low
impedance supply planes, particularly the output supply plane
(V
CC
) and the ground plane (GND). Individual supply planes
are recommended as part of a multilayer board. Providing the
lowest inductance return path for switching currents ensures
the best possible performance in the target application.
It is also important to adequately bypass the input and output
supplies. A 0.1 μF bypass capacitor should be placed as close as
possible to the V
CC
supply pin. The capacitor should be connected
to the GND plane with redundant vias placed to provide a
physically short return path for output currents flowing back
from ground to the V
CC
pin. High frequency bypass capacitors
should be carefully selected for minimum inductance and ESR.
Parasitic layout inductance should also be strictly controlled to
maximize the effectiveness of the bypass at high frequencies.
TTL-/CMOS-COMPATIBLE OUTPUT STAGE
Specified propagation delay performance can be achieved only
by keeping the capacitive load at or below the specified minimums.
The output of the AD8468 is designed to directly drive one
Schottky TTL, three low power Schottky TTL loads, or the
equivalent. For large fanouts, buses, or transmission lines, use
an appropriate buffer to maintain the excellent speed and
stability of the comparator.
With the rated 15 pF load capacitance applied, more than half
of the total device propagation delay is output stage slew time.
Because of this, the total propagation delay decreases as V
CC
decreases, and instability in the power supply may appear as
excess delay dispersion.
Delay is measured to the 50% point for whatever supply is in
use; thus, the fastest times are observed with the V
CC
supply at
2.5 V, and larger values are observed when driving loads that
switch at other levels.
Overdrive and input slew rate dispersions are not significantly
affected by output loading and V
CC
variations.
The TTL-/CMOS-compatible output stage is shown in the
simplified schematic diagram (see Figure 9). Because of its
inherent symmetry and generally good behavior, this output
stage is readily adaptable for driving various filters and other
unusual loads.
OUTPUT
Q2
Q1
+IN
–IN
OUTPUT STAGE
V
LOGIC
GAIN STAGE
A2
A1
A
V
08853-009
Figure 9. Simplified Schematic Diagram of the
TTL-/CMOS-Compatible Output Stage
OPTIMIZING PERFORMANCE
As with any high speed comparator, proper design and layout
techniques are essential for obtaining the specified performance.
Stray capacitance, inductance, common power and ground
impedances, or other layout issues can severely limit performance
and can often cause oscillation. The source impedance should
be minimized as much as is practicable. High source impedance,
in combination with the parasitic input capacitance of the
comparator, causes an undesirable degradation in bandwidth at
the input, thus degrading the overall response. Higher impedances
encourage undesired coupling.
COMPARATOR PROPAGATION
DELAY DISPERSION
The AD8468 comparator is designed to reduce propagation
delay dispersion over a wide input overdrive range of 10 mV to
V
CC
– 1 V. Propagation delay dispersion is the variation in
propagation delay that results from a change in the degree of
overdrive or slew rate (how far or how fast the input signal
exceeds the switching threshold). See Figure 10 and Figure 11.
Propagation delay dispersion is a specification that becomes
important in high speed, time-critical applications, such as data
communication, automatic test and measurement, and instru-
mentation. It is also important in event-driven applications, such
as pulse spectroscopy, nuclear instrumentation, and medical
imaging.
The AD8468 overdrive dispersion is typically <12 ns as the
overdrive varies from 10 mV to 125 mV. This specification
applies to both positive and negative signals because the device
has very closely matched delays for both positive-going and
negative-going inputs and very low output skews. Remember to
add the actual device offset to the overdrive for repeatable
dispersion measurements.
