ADCMP551/ADCMP552/ADCMP553

Preliminary Technical Data

Rev. PrB | Page 12 of 14

Propagation delay dispersion is an important specification 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 condi-

tions are changed (Figure 17). For the ADCMP55x, overdrive

dispersion is typically 100 ps as the overdrive is changed from

100 mV to 1 V. This specification applies for both positive and

negative overdrive since the ADCMP55x has equal delays for

positive- and negative-going inputs.

Q OUTPUT

INPUT VOLTAGE

1.5V OVERDRIVE

20mV OVERDRIVE

DISPERSION

Figure 17. 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 18. 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 22). 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 19.

Another method to implement hysteresis is generated by

ABLE

introducing a differential voltage between the LATCH EN

and LATCH ENABLE. inputs (Figure 23).

OUTPUT

INPUT

0

1

0V

2

2

Figure 18. Comparator Hysteresis Transfer Function

TBD

–000

–000

–000

–000

–000

–000

–000

–000

–000

–000

ALL CAPS (Initial caps)

Figure 19. Comparator Hysteresis Transfer Function

MINIMUM INPUT SLEW RATE REQUIREMENT

ust

he

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

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

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

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.