Data Sheet AD8469
Rev. 0 | Page 9 of 12
Q/Q OUTPUT
INPUT VOLTAGE
500mV OVERDRIVE
10mV OVERDRIVE
DISPERSION
V
N
± V
OS
10490-012
Figure 12. Propagation Delay—Overdrive Dispersion
Q/Q OUTPUT
INPUT VOLTAGE
10V/ns
1V/ns
DISPERSION
V
N
± V
OS
10490-013
Figure 13. Propagation Delay—Slew Rate Dispersion
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.
OUTPUT
INPUT
0.0V
V
OL
V
OH
+V
H
2
–V
H
2
10490-014
Figure 14. Comparator Hysteresis Transfer Function
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
H
/2.
The new switching threshold becomes −V
H
/2. The comparator
remains in the high state until the threshold, −V
H
/2, is crossed
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
H
/2.
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 AD8469 comparator offers a programmable hysteresis
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.
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
HYSTERESIS (mV)
13001200110010009008007006005004003002001000
HYS RESISTOR (kΩ)
10490-019
V
CC
= 5.5V
V
CC
= 2.5V
Figure 15. Hysteresis vs. HYS Resistor
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
CC
rail, and
others are active near the V
EE
rail. At some predetermined point
in the common-mode range, a crossover occurs. At the crossover
point (normally V
CC
/2), the direction of the bias current is reversed
and there are changes in measured offset voltages and currents.
The AD8469 elaborates slightly on this scheme. The crossover
points are at approximately 0.8 V and 1.6 V.
