ADCMP563/ADCMP564

Rev. C | 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 21). 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.

Q OUTPUT

INPUT VOLTAGE

1.5V OVERDRIVE

20mV OVERDRIVE

DISPERSION

Figure 21. 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 22. If the input voltage

approaches the threshold from the negative direction, the

coming from the positive direction. In this manner, noise

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

The customary technique for introducing hysteresis into a

comparator uses positive feedback from the output back to the

input. A limitation of this approach is that the amount of

hysteresis varies with the output logic levels, resulting in

hysteresis that can be load dependent and is not symmetrical

about the threshold. The external feedback network can also

introduce significant parasitics, which reduce high speed

performance and can induce oscillation in some cases.

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 vs. resistance curve is shown in Figure 23.

A current may be sourced into the HYS pin. The pin is biased

approximately 1 V below AGND and has a 3 kΩ series

resistance. The relationship between the current applied to the

HYS pin and the resulting hysteresis is shown in Figure 19.

OUTPUT

INPUT

0

1

0V

2

2

Figure 22. Comparator Hysteresis Transfer Function

160

140

120

100

80

60

40

20

0

50

0

10

20

30

40

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 as 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. ADI 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 can be added to prevent the oscillation.