AD8469 AD [Analog Devices], AD8469 Datasheet - Page 9

• Manufacturer Part Number: AD8469
• Description: Fast, Rail-to-Rail, Low Power, 2.5 V to 5.5 V
• Manufacturer: AD [Analog Devices]

Data Sheet
COMPARATOR HYSTERESIS
The addition of hysteresis to a comparator is often desirable in
noisy environments or when the differential input amplitudes
are relatively small or slow moving. The transfer function for a
comparator with hysteresis is shown in Figure 14.
As the input voltage approaches the threshold (0.0 V in Figure 14)
from below the threshold region in a positive direction, the com-
parator switches from low to high when the input crosses +V
The new switching threshold becomes −V
remains in the high state until the threshold, −V
from below the threshold region in a negative direction. In this
way, noise or feedback output signals centered on the 0.0 V input
cannot cause the comparator to switch states unless they exceed
the region bounded by ±V
INPUT VOLTAGE
INPUT VOLTAGE
Q/Q OUTPUT
Q/Q OUTPUT
Figure 12. Propagation Delay—Overdrive Dispersion
Figure 13. Propagation Delay—Slew Rate Dispersion
Figure 14. Comparator Hysteresis Transfer Function
–V
2
H
OUTPUT
0.0V
V
H
OL
/2.
10V/ns
V
OH
1V/ns
500mV OVERDRIVE
10mV OVERDRIVE
+V
2
DISPERSION
DISPERSION
H
H
INPUT
/2. The comparator
V
V
N
N
± V
± V
H
/2, is crossed
OS
OS
H
/2.
Rev. 0 | Page 9 of 12
The customary technique for introducing hysteresis into a
comparator uses positive feedback from the output back to the
input. One limitation of this approach is that the amount of
hysteresis varies with the output logic level, resulting in hysteresis
that is not symmetric about the threshold. The external feedback
network can also introduce significant parasitics that reduce high
speed performance and can even induce oscillation in some cases.
The
feature that significantly improves accuracy and stability. By
connecting an external pull-down resistor or current source
from the HYS pin to ground, the user can vary the amount of
hysteresis in a predictable, stable manner. Leaving the HYS pin
disconnected or driving it high removes the hysteresis. The
maximum hysteresis that can be applied using the HYS pin is
approximately 160 mV. Figure 15 illustrates the amount of
hysteresis applied as a function of the external resistor value.
The HYS pin appears as a 1.25 V bias voltage seen through a
series resistance of 7 kΩ ± 20% throughout the hysteresis control
range. The advantages of applying hysteresis in this manner are
improved accuracy, improved stability, reduced component
count, and maximum versatility. An external bypass capacitor is
not recommended on the HYS pin because it impairs the latch
function and often degrades the jitter performance of the device.
When the HYS pin is driven low, hysteresis may become large,
but in this device, the effect is not reliable or intended as a latch
function.
CROSSOVER BIAS POINT
Rail-to-rail inputs in both op amps and comparators have a dual
front-end design. Certain devices are active near the V
others are active near the V
in the common-mode range, a crossover occurs. At the crossover
point (normally V
and there are changes in measured offset voltages and currents.
The
points are at approximately 0.8 V and 1.6 V.
AD8469
AD8469
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
0
V
100
CC
comparator offers a programmable hysteresis
elaborates slightly on this scheme. The crossover
= 5.5V
200
Figure 15. Hysteresis vs. HYS Resistor
CC
300
/2), the direction of the bias current is reversed
V
CC
400
= 2.5V
HYS RESISTOR (kΩ)
500
EE
600
rail. At some predetermined point
700
800
900
1000
1100
1200
AD8469
CC
1300
rail, and