Data Sheet ADCMP609
Rev. C | Page 9 of 12
Q/Q OUTPUT
INPUT VOLTAGE
500mV OVERDRIVE
10mV OVERDRIVE
DISPERSION
V
N
± V
OS
06918-012
Figure 12. Propagation Delay—Overdrive Dispersion
Q/Q OUTPUT
INPUT VOLTAGE
10V/ns
1V/ns
DISPERSION
V
N
± V
OS
06918-013
Figure 13. Propagation Delay—Slew Rate Dispersion
COMPARATOR HYSTERESIS
The addition of hysteresis to a comparator is often desirable in a
noisy environment, 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 comparator
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 manner, noise or
feedback output signals centered on 0.0 V input cannot cause
the comparator to switch states unless it exceeds the region
bounded by ±V
H
/2.
OUTPUT
INPUT
0.0V
V
OL
V
OH
+V
H
2
–V
H
2
6918-014
Figure 14. Comparator Hysteresis Transfer Function
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 levels, 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 ADCMP609 comparator offers a programmable hysteresis
feature that significantly improves accuracy and stability.
Connecting an external pull-down resistor or a current source
from the HYS pin to ground varies 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 this pin is approximately
160 mV. Figure 15 illustrates the amount of hysteresis applied as
a function of the external resistor value.
06918-006
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
HYSTERESIS (
V)
13001200110010009008007006005004003002001000
HYS RESISTOR (kΩ)
V
CC
= 5.5
V
CC
= 2.5
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.
With the pin 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 of this type, 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 predeter-
mined point in the common-mode range, a crossover occurs. At
this point, normally V
CC
/2, the direction of the bias current reverses
and there are changes in measured offset voltages and currents.
The ADCMP609 slightly elaborates on this scheme. The
crossover points are at approximately 0.8 V and 1.6 V.
