ADCMP563/ADCMP564

Rev. A | Page 12 of 16

Propagation delay dispersion is important in critical timing

applications such as ATE, bench instruments, and nuclear

instrumentation. Overdrive dispersion is defined as the varia-

tion in propagation delay as the input overdrive conditions are

changed (Figure 19). For the ADCMP563/ADCMP564, over-

drive 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 ADCMP563 and

the ADCMP564 have equal delays for positive and negative

going inputs.

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

Q OUTPUT

INPUT VOLTAGE

1.5V OVERDRIVE

20mV OVERDRIVE

DISPERSION

Figure 20. Comparator Hysteresis Transfer Function

160

140

120

100

80

60

40

20

0

50

0

10

20

30

40

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 compar-

ator 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

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.

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 ADCMP564, 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.