ADCMP608(Rev0) 데이터 시트보기 (PDF) - Analog Devices

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ADCMP608
(Rev.:Rev0)

Rail-to-Rail, Fast, Low Power 2.5 V to 5.5 V, Single-Supply TTL/CMOS Comparator

Analog Devices 

ADCMP608

APPLICATION INFORMATION

POWER/GROUND LAYOUT AND BYPASSING

The ADCMP608 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 (VCC) 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 VCC 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 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.

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 ADCMP608 is designed to directly drive one

Schottky TTL, or three low power Schottky TTL loads, or the

equivalent. For large fan outs, 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 VCC

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 VCC 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 VCC 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.

VLOGIC

A1

Q1

+IN

AV

–IN

OUTPUT

A2

Q2

GAIN STAGE

OUTPUT STAGE

Figure 9. Simplified Schematic Diagram of

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 ADCMP608 comparator is designed to reduce propagation

delay dispersion over a wide input overdrive range of 10 mV to

VCC – 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).

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 10

and Figure 11).

ADCMP608 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.

Rev. 0 | Page 7 of 12

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