ADCMP606/ADCMP607

Rev. 0 | Page 11 of 16

COMPARATOR PROPAGATION DELAY

DISPERSION

The ADCMP606/ADCMP607 comparators are designed to

reduce propagation delay dispersion over a wide input overdrive

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 (Figure 15

and Figure 16).

The device dispersion is typically 2.3 ns as the overdrive varies

from 10 mV to 125 mV. 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.

Q/Q OUTPUT

INPUT VOLTAGE

500mV OVERDRIVE

10mV OVERDRIVE

DISPERSION

Figure 15. Propagation Delay—Overdrive Dispersion

Q/Q OUTPUT

INPUT VOLTAGE

10V/ns

1V/ns

DISPERSION

Figure 16. Propagation Delay—Slew Rate Dispersion

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

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

OUTPUT

INPUT

0

2

2

Figure 17. 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 induce

oscillation in some cases.

This 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 this pin high removes hysteresis. The

maximum hysteresis that can be applied using this pin is

approximately 160 mV. Figure 18 illustrates typical hysteresis

applied as a function of the external resistor value, and Figure 7

illustrates typical hysteresis as a function of the current.

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