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AD53519

Dual Ultrafast Voltage Comparator

Analog Devices 

PRELIMINARY TECHNICAL DATA

AD53519

PROPAGATION DELAY DISPERSION

1.5 V OVERDRIVE

INPUT VOLTAGE

20 mV OVERDRI VE

VREF ± VOS

Q OUTPUT

DISPE RSION

COMPARATOR HYSTERESIS

Figure 4

The customary technique for introducing hysteresis into a

comparator uses positive feedback. The major problems with

this approach are 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 LATCH ENABLE

and /LATCH ENABLE as shown in Figure X.X. Hysteresis

generated in this manner is independent of output swing and is

symmetrical around zero. The variation of hysteresis with input

voltage is shown in Figure 5.

COMPARATOR HYSTERESIS TRANSFER

FUNCTION USING LATCH ENABLE INPUT

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 4 below. 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 +VH/2. The “new” switching threshold now becomes –

VH/2. The comparator will remain in a “1” state until the

threshold –VH/2 is crossed coming from the positive direction.

In this manner, noise centered around 0 V input will not cause

the comparator to switch states unless it exceeds the region

bounded by ±VH/2.

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

produce hysteresis in a comparator.

COMPARATOR HYSTERESIS TRANSFER FUNCTION

-VH

+VH

2

2

0V

INPUT

“1"

0

-30 -20 -10

0

10 20 30

DIFFERENTIAL LATCH VOLTAGE - mV

Figure 6

THERMAL CONSIDERATIONS

The AD53519 PLCC package option has a theta JA (junction to

ambient thermal resistance) of 89.4 °C /W in still air.

Upgrading the SPT9689 and AD96687

The AD53519 dual comparator is pin-for-pin compatible with

the SPT9689 and AD96687 and offers many improvements over

these devices. The most notable difference is in propagation

delay. The SPT9689 and AD96687 can be easily replaced with

the higher performance AD53519, but there are differences and

it is useful to check that these ensure proper operation.

“0"

OUTPUT

The major differences between the SPT9689 and AD53519

include Propagation Delay, Latch to Output Delay, Bandwidth,

Rise Time, Fall Time, Input Offset Voltage (SPT9689B) and

Offset Voltage Tempco (SPT9689B).

Figure 5

REV. Pr J July 22, 2002

-8-

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