ADCMP606 AD [Analog Devices], ADCMP606 Datasheet - Page 10

• Manufacturer Part Number: ADCMP606
• Description: Rail-to-Rail, Very Fast, 2.5 V to 5.5 V, Single-Supply CML Comparators
• Manufacturer: AD [Analog Devices]

ADCMP606/ADCMP607
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.
COMPARATOR PROPAGATION
DELAY DISPERSION
The ADCMP606/ADCMP607 comparators are designed to
reduce propagation delay dispersion over a wide input overdrive
range of 5 mV to V
variation in propagation delay that results from a change in the
degree of overdrive or slew rate (that is, how far or how fast the
input signal exceeds the switching threshold).
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 (
and Figure 16).
ADCMP606/ADCMP607 dispersion is typically <TBD ps as the
overdrive varies from 10 mV to 500 mV and the input slew rate
varies from 2 V/ns to 10 V/ns. This specification applies to both
positive and negative signals because each device has very closely
matched delays for positive-going and negative-going inputs, as
well as very low output skews.
INPUT VOLTAGE
Q/Q OUTPUT
Figure 15. Propagation Delay—Overdrive Dispersion
CCI
– 1 V. Propagation delay dispersion is the
500mV OVERDRIVE
10mV OVERDRIVE
DISPERSION
V
N
± V
OS
Figure 15
Rev. PrA | Page 10 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. Figure 17 shows the transfer
function for a comparator with hysteresis. As the input voltage
approaches the threshold (0.0 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 new threshold, −V
below the threshold region in a negative direction. In this manner,
noise or feedback output signals centered on 0.0 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.
The ADCMP607 comparator offers a programmable hysteresis
feature that can significantly improve accuracy and stability.
Connecting an external pull-down resistor or a current source
from the LE/HYS pin to GND, varies the amount of hysteresis
in a predictable, stable manner. Leaving the LE/HYS pin
disconnected or driving it high removes the hysteresis. The
INPUT VOLTAGE
Q/Q OUTPUT
Figure 16. Propagation Delay—Slew Rate Dispersion
Figure 17. Comparator Hysteresis Transfer Function
H
/2.
–V
2
Preliminary Technical Data
H
OUTPUT
V
OL
0
10V/ns
V
OH
1V/ns
H
/2. The comparator remains
+V
2
DISPERSION
H
INPUT
H
/2, is crossed from
V
N
± V
OS
H
/2, and the