ADCMP608/ADCMP609

Preliminary Technical Data

Rev. PrA | Page 10 of 16

COMPARATOR PROPAGATION

DELAY 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

The ADCMP608 and ADCMP609 comparator is 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 exceeds the switching threshold).

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

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

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

OUTPUT

INPUT

0

2

2

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.

Q/Q OUTPUT

INPUT VOLTAGE

500mV OVERDRIVE

10mV OVERDRIVE

DISPERSION

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.

Figure 14. Propagation Delay—Overdrive Dispersion

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.

Q/Q OUTPUT

INPUT VOLTAGE

10V/ns

1V/ns

DISPERSION

Figure 17 illustrates the amount of

hysteresis applied as a function of external resistor value.

Figure TBD illustrates hysteresis as a function of current.

Figure 15. Propagation Delay—Slew Rate Dispersion