ADCMP604/ADCMP605

Rev. A | Page 11 of 16

COMPARATOR PROPAGATION DELAY

DISPERSION

The ADCMP604/ADCMP605 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 (how far or how fast the input

signal is driven past the switching threshold).

Propagation delay dispersion is a specification that becomes

important in high speed, time-critical applications, such as data

communications, 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 17

and Figure 18).

The ADCMP604/ADCMP605 dispersion is typically <1.6 ns as

the overdrive varies from 10 mV to 125 mV. This specification

applies to both positive and negative signals because each of

the ADCMP604 and ADCMP605 has substantially equal delays

for positive-going and negative-going inputs and very low

output skews.

Q/Q OUTPUT

INPUT VOLTAGE

500mV OVERDRIVE

10mV OVERDRIVE

DISPERSION

Figure 17. Propagation Delay—Overdrive Dispersion

Q/Q OUTPUT

INPUT VOLTAGE

10V/ns

1V/ns

DISPERSION

Figure 18. 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. The transfer function for a

comparator with hysteresis is shown in Figure 19. As the input

voltage approaches the threshold (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 V input cannot

cause the comparator to switch states unless it exceeds the region

OUTPUT

INPUT

0V

2

2

Figure 19. 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.

The ADCMP605 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 hysteresis. The

maximum hysteresis that can be applied using this pin is

approximately 160 mV. Figure 20 illustrates the amount of

hysteresis applied as a function of external resistor value. Figure 11

illustrates hysteresis as a function of current.