ADCMP609(RevPrA) 데이터 시트보기 (PDF) - Analog Devices

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ADCMP609
(Rev.:RevPrA)

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

Analog Devices 

ADCMP608/ADCMP609

COMPARATOR PROPAGATION

DELAY DISPERSION

The ADCMP608 and ADCMP609 comparator is 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 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

(see Figure 14 and Figure 15).

ADCMP608 and ADCMP609 dispersion is typically <5 ns as

the overdrive varies from 5 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 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.

500mV OVERDRIVE

INPUT VOLTAGE

10mV OVERDRIVE

VN ± VOS

DISPERSION

Q/Q OUTPUT

Figure 14. Propagation Delay—Overdrive Dispersion

INPUT VOLTAGE

1V/ns

10V/ns

VN ± VOS

DISPERSION

Q/Q OUTPUT

Figure 15. Propagation Delay—Slew Rate Dispersion

Preliminary Technical Data

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 16. 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 a low to a high when the input crosses

+VH/2. The new switching threshold becomes −VH/2. The

comparator remains in the high state until the threshold −VH/2

is crossed from 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 ±VH/2.

OUTPUT

VOH

VOL

0

–VH

2

INPUT

+VH

2

Figure 16. Comparator Hysteresis Transfer Function

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 can even

induce oscillation in some cases.

The ADCMP609 comparator offers a programmable hysteresis

feature that significantly improves 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 and stable manner. Leaving the LE/HYS pin

disconnected or driving it high removes the hysteresis. The

maximum hysteresis that can be applied using this pin is

approximately 160 mV. Figure 17 illustrates the amount of

hysteresis applied as a function of external resistor value.

Figure TBD illustrates hysteresis as a function of current.

Rev. PrA | Page 10 of 16

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