ADCMP565

Rev. 0 | Page 10 of 16

as the variation in propagation delay as the input overdrive

conditions are changed (Figure 4). For the ADCMP565,

overdrive dispersion is typically 50 ps as the overdrive is

changed from 100 mV to 1 V. This specification applies for

both positive and negative overdrive since the ADCMP565 has

equal delays for positive and negative going inputs.

The 50 ps propagation delay dispersion of the ADCMP565

offers considerable improvement of the 100 ps dispersion of

other similar series comparators.

Q OUTPUT

INPUT VOLTAGE

1.5V OVERDRIVE

20mV OVERDRIVE

DISPERSION

02820-0-004

Figure 4. 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 com-

parator 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 5. If the input voltage

approaches the threshold from the negative direction, the

comparator will switch 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 will not cause the comparator to

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

produce hysteresis in a comparator (Figure 9). 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.

Another method to implement hysteresis is generated by

introducing a differential voltage between the LATCH ENABLE

and LATCH ENABLE inputs (Figure 10). Hysteresis generated

in this manner is independent of output swing and is symmetri-

cal around zero. The variation of hysteresis with input voltage is

shown in Figure 6.

OUTPUT

INPUT

0

1

0V

2

2

02820-0-005

Figure 5. Comparator Hysteresis Transfer Function

0

20

60

∆ LATCH = LE – LEB (mV)

–20

20

02820-0-006

–15

–10

–5

0

5

10

15

40

50

30

10

Figure 6. Comparator Hysteresis Transfer Function

Using Latch Enable Input

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 5 V/µs

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

than 5 V/µs are used, then hysteresis should be added to reduce

the oscillation.