ad53519 Analog Devices, Inc., ad53519 Datasheet - Page 8

• Manufacturer Part Number: ad53519
• Description: Dual Ultrafast Voltage Comparator
• Manufacturer: Analog Devices, Inc.

AD53519
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 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 +V
V
threshold –V
In this manner, noise centered around 0 V input will not cause
the comparator to switch states unless it exceeds the region
bounded by rV
Positive feedback from the output to the input is often used to
produce hysteresis in a comparator.
COMPARATOR HYSTERESIS TRANSFER FUNCTION
Figure 5
REV. Pr J July 22, 2002
Q OUTPUT
H
/2. The comparator will remain in a “1” state until the
INPUT VOLTAGE
“0"
H
/2. The “new” switching threshold now becomes –
H
/2 is crossed coming from the positive direction.
H
/2.
-V H
2
20 mV OVERDRI VE
0 V
1.5 V OVERDRIVE
PRELIMINARY TECHNICAL DATA
OUTPUT
DISPERSION
V REF ± V OS
+V H
2
Figure 4
INPUT
“1"
- 8 -
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.
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).
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
-30
0
DIFFERENTIAL LATCH VOLTAGE - mV
-20
-10
0
10
20
30