ADCMP551/ADCMP552/ADCMP553

Data Sheet

Rev. A | Page 12 of 16

Figure 18. 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 19. 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 23). 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 ADCMP552, hysteresis is generated through the

programmable hysteresis pin. A resistor from the HYS pin to

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 20.

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 16.

Figure 19. Comparator Hysteresis Transfer Function

Figure 20. Comparator Hysteresis Transfer Function

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.

Q OUTPUT

INPUT VOLTAGE

1.5V OVERDRIVE

20mV OVERDRIVE

DISPERSION

OUTPUT

INPUT

0

1

0V

2

2

120

100

80

60

40

20

0

100

10

1