AD8564_15 datasheet(10/12 Pages) AD

10 / 12 page

AD8564

Rev. B | Page 10 of 12

USING HYSTERESIS

Hysteresis can easily be added to a comparator through the

addition of positive feedback. Adding hysteresis to a comparator

offers an advantage in noisy environments where it is not desirable

for the output to toggle between states when the input signal is

near the switching threshold. Figure 17 shows a method for

configuring the AD8564 with hysteresis.

COMPARATOR

SIGNAL

VREF

Figure 17. Configuring the AD8564 with Hysteresis

The input signal is connected directly to the inverting input of

the comparator. The output is fed back to the noninverting

input through R2 and R1. The ratio of R1 to R1 + R2 and the

output swing establishes the width of the hysteresis window,

with VREF setting the center of the window or the average

switching voltage. The output switches high when the input

voltage is greater than VHI and does not switch low again until

the input voltage is less than VLO, as given in Equation 2.

(

)

REF

−

(1)

⎟

⎠

⎞

⎜

⎝

⎛

−

REF

(2)

where V+ is the positive supply voltage.

The CF capacitor may also be added to introduce a pole into

the feedback network. This has the effect of increasing the

amount of hysteresis at high frequencies. This can be useful

when comparing a relatively slow signal in a high frequency

noise environment.

At frequencies greater than

, the hysteresis

window approaches VHI = V+ – 1 V and VLO = 0 V.

At frequencies less than fP, the threshold voltages remain as it is

in Equation 1.

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