ADCMP561/ADCMP562

Rev. A | Page 12 of 16

Propagation delay dispersion is a specification that is important

in critical timing applications such as ATE, bench instruments,

and nuclear instrumentation. Overdrive dispersion is defined

as the variation in propagation delay as the input overdrive

conditions are changed (Figure 19). For the ADCMP561 and

ADCMP562, overdrive dispersion is typically 75 ps as the

overdrive is changed from 100 mV to 1.5 V. This specification

applies for both positive and negative overdrive because the

ADCMP561/ADCMP562 have equal delays for positive and

negative going inputs.

Q OUTPUT

INPUT VOLTAGE

1.5V OVERDRIVE

20mV OVERDRIVE

DISPERSION

Figure 19. Propagation Delay Dispersion

COMPARATOR HYSTERESIS

The addition of hysteresis to a comparator is often useful in a

noisy environment, or where it is not desirable for the

comparator to toggle between states when the input signal is at

the switching threshold. The transfer function for a comparator

with hysteresis is shown in Figure 20. If the input voltage

approaches the threshold from the negative direction, the

comparator switches from a 0 to a 1 when the input crosses

crossed, coming from the positive direction. In this manner,

noise centered on 0 V input does not cause the comparator to

Positive feedback from the output to the input is often used to

produce hysteresis in a comparator (Figure 24). The major

problem with this approach is that the amount of hysteresis

varies with the output logic levels, resulting in a hysteresis that

is not symmetrical around zero.

In the ADCMP562, hysteresis is generated through the

programmable hysteresis pin. A resistor from the HYS pin to

GND creates a current into the part that is used to generate

hysteresis. Hysteresis generated in this manner is independent

of output swing and is symmetrical around the trip point. The

hysteresis versus resistance curve is shown in Figure 21.

A current source can also be used with the HYS pin. The

relationship between the current applied to the HYS pin and the

resulting hysteresis is shown in Figure 17.

OUTPUT

INPUT

0

1

0V

2

2

Figure 20. Comparator Hysteresis Transfer Function

160

140

120

100

80

60

40

20

0

0

10

20

30

40

50

MINIMUM INPUT SLEW RATE REQUIREMENT

As for all high speed comparators, a minimum slew rate must

be met to ensure that the device does not oscillate when the

input crosses the threshold. This oscillation is due in part to the

high input bandwidth of the comparator and the parasitics of

the package. Analog Devices recommends a slew rate of 1 V/µs

or faster to ensure a clean output transition. If slew rates less

than 1 V/µs are used, hysteresis should be added to reduce the

oscillation.