AD8469WBRMZ-RL Просмотр технического описания (PDF) - Analog Devices

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AD8469WBRMZ-RL

Fast, Rail-to-Rail, Low Power, 2.5 V to 5.5 V, Single-Supply TTL/CMOS Comparator

Analog Devices 

Data Sheet

INPUT VOLTAGE

500mV OVERDRIVE

10mV OVERDRIVE

VN ± VOS

DISPERSION

Q/Q OUTPUT

Figure 12. Propagation Delay—Overdrive Dispersion

INPUT VOLTAGE

1V/ns

10V/ns

VN ± VOS

Q/Q OUTPUT

DISPERSION

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

VOH

VOL

0.0V

–VH

2

INPUT

+VH

2

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 +VH/2.

The new switching threshold becomes −VH/2. The comparator

remains in the high state until the threshold, −VH/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 ±VH/2.

AD8469

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

VCC = 2.5V

40

30

20 VCC = 5.5V

10

0

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

HYS RESISTOR (kΩ)

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 VCC rail, and

others are active near the VEE rail. At some predetermined point

in the common-mode range, a crossover occurs. At the crossover

point (normally VCC/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.

Rev. 0 | Page 9 of 12

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