Datasheet AD8469 (Analog Devices) - 9

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Data Sheet AD8469 500mV OVERDRIVE
The customary technique for introducing hysteresis into a comparator uses positive feedback from the output back to the
INPUT VOLTAGE
input. One limitation of this approach is that the amount of hysteresis varies with the output logic level, resulting in hysteresis
10mV OVERDRIVE
that is not symmetric about the threshold. The external feedback
VN ± VOS
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
DISPERSION
012 feature that significantly improves accuracy and stability. By
Q/Q OUTPUT
10490- connecting an external pull-down resistor or current source Figure 12. Propagation Delay—Overdrive Dispersion from the HYS pin to ground, the user can vary the amount of hysteresis in a predictable, stable manner. Leaving the HYS pin
INPUT VOLTAGE
disconnected or driving it high removes the hysteresis. The
1V/ns
maximum hysteresis that can be applied using the HYS pin is approximately 160 mV. Figure 15 illustrates the amount of
VN ± VOS 10V/ns
hysteresis applied as a function of the external resistor value.
160 150 140 130 DISPERSION 120
013
) 110 Q/Q OUTPUT V
10490-
100 (m
Figure 13. Propagation Delay—Slew Rate Dispersion
IS 90 S E 80 COMPARATOR HYSTERESIS R E 70 T S 60
The addition of hysteresis to a comparator is often desirable in
Y H 50 VCC = 2.5V
noisy environments or when the differential input amplitudes
40
are relatively small or slow moving. The transfer function for a
30 20 VCC = 5.5V
comparator with hysteresis is shown in Figure 14.
10 0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
019
OUTPUT HYS RESISTOR (kΩ)
10490- Figure 15. Hysteresis vs. HYS Resistor
VOH
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
VOL
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.
0.0V INPUT
014
–V +V
When the HYS pin is driven low, hysteresis may become large,
H H 2 2
10490- but in this device, the effect is not reliable or intended as a latch Figure 14. Comparator Hysteresis Transfer Function function. As the input voltage approaches the threshold (0.0 V in Figure 14)
CROSSOVER BIAS POINT
from below the threshold region in a positive direction, the com- parator switches from low to high when the input crosses +V Rail-to-rail inputs in both op amps and comparators have a dual H/2. The new switching threshold becomes −V front-end design. Certain devices are active near the VCC rail, and H/2. The comparator remains in the high state until the threshold, −V others are active near the V H/2, is crossed EE rail. At some predetermined point from below the threshold region in a negative direction. In this in the common-mode range, a crossover occurs. At the crossover way, noise or feedback output signals centered on the 0.0 V input point (normal y VCC/2), the direction of the bias current is reversed cannot cause the comparator to switch states unless they exceed and there are changes in measured offset voltages and currents. the region bounded by ±VH/2. The AD8469 elaborates slightly on this scheme. The crossover points are at approximately 0.8 V and 1.6 V. Rev. 0 | Page 9 of 12 Document Outline Features Applications General Description Functional Block Diagram Table of Contents Revision History Specifications Electrical Characteristics Absolute Maximum Ratings Thermal Resistance ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Applications Information Power/Ground Layout and Bypassing TTL-/CMOS-Compatible Output Stage Optimizing Performance Comparator Propagation Delay Dispersion Comparator Hysteresis Crossover Bias Point Minimum Input Slew Rate Requirement Typical Applications Circuits Outline Dimensions Ordering Guide Automotive Products