ADCMP604/ADCMP605
APPLICATION INFORMATION
POWER/GROUND LAYOUT AND BYPASSING
USING/DISABLING THE LATCH FEATURE
The ADCMP604/ADCMP605 comparators are very high speed
devices. 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 (VCCO) 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.
The latch input is designed for maximum versatility. It can
safely be left floating or it can be driven low by any standard
TTL/CMOS device as a high speed latch. In addition, the pin
can be operated as a hysteresis control pin with a bias voltage of
1.25 V nominal and an input resistance of approximately
7000 Ω. This allows the comparator hysteresis to be easily
controlled by either a resistor or an inexpensive CMOS DAC.
Driving this pin high or floating the pin disables all hysteresis.
Hysteresis control and latch mode can be used together if an
open drain, an open collector, or a three-state driver is connected in
parallel to the hysteresis control resistor or current source.
Due to the programmable hysteresis feature, the logic threshold
It is also important to adequately bypass the input and output
supplies. Multiple high quality 0.01 μF bypass capacitors should
be placed as close as possible to each of the VCCI and VCCO supply
pins and should be connected to the GND plane with redundant
vias. At least one of these should be placed to provide a physically
short return path for output currents flowing back from ground
to the VCC 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.
of the latch pin is approximately 1.1 V regardless of VCC
.
OPTIMIZING PERFORMANCE
As with any high speed comparator, proper design and layout
techniques are essential for obtaining the specified performance.
Stray capacitance, inductance, inductive power and ground
impedances, or other layout issues can severely limit performance
and often cause oscillation. Large discontinuities along input
and output transmission lines can also limit the specified pulse-
width dispersion performance. 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. Thermal noise
from large resistances can easily cause extra jitter with slowly
slewing input signals. Higher impedances encourage undesired
coupling.
If the package allows, and the input and output supplies have
been connected separately (VCCI ≠ VCCO), be sure to bypass each
of these supplies separately to the GND plane. Do not connect a
bypass capacitor between these supplies. It is recommended
that the GND plane separate the VCCI and VCCO planes when the
circuit board layout is designed to minimize coupling between
the two supplies to take advantage of the additional bypass
capacitance from each respective supply to the ground plane.
This enhances the performance when split input/output supplies
are used. If the input and output supplies are connected
together for single-supply operation (VCCI = VCCO), then
coupling between the two supplies is unavoidable; however,
careful board placement can help keep output return currents
away from the inputs.
COMPARATOR PROPAGATION DELAY
DISPERSION
The ADCMP604/ADCMP605 comparators are designed to
reduce propagation delay dispersion over a wide input overdrive
range of 5 mV to VCCI − 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 is driven past the switching threshold).
LVDS-COMPATIBLE OUTPUT STAGE
Specified propagation delay dispersion performance is only
achieved by keeping parasitic capacitive loads at or below the
specified minimums. The outputs of the ADCMP604 and
ADCMP605 are designed to directly drive any standard LVDS-
compatible input.
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. Dispersion is defined as the variation in propagation
delay as the input overdrive conditions are changed (Figure 17
and Figure 18).
Rev. 0 | Page 10 of 16
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