AD8465

Rev. 0 | Page 11 of 16

COMPARATOR PROPAGATION DELAY

DISPERSION

The AD8465 comparator is designed to reduce propagation

delay dispersion over a wide input overdrive range of 5 mV

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 16

and Figure 17).

The AD8465 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 the AD8465 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 16. Propagation Delay—Overdrive Dispersion

Q/Q OUTPUT

INPUT VOLTAGE

10V/ns

1V/ns

DISPERSION

Figure 17. 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 18. As the input

voltage approaches the threshold (0 V, in this example) from

below the threshold region in a positive direction, the comparator

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 18. 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.