AD8611ARMZ-R2 datasheet(12/20 Pages) AD

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AD8611/AD8612

Data Sheet

Rev. B | Page 12 of 20

The capacitor CF is optional and can be added to introduce a

pole into the feedback network. This has the effect of increasing

the amount of hysteresis at high frequencies, which is useful

when comparing relatively slow signals in high frequency noise

environments. At frequencies greater than fP, the hysteresis

window approaches VHI = V+ − 1.5 V and VLO = 0 V. For

frequencies less than fP, the threshold voltages remain as in

Equation 1.

CLOCK TIMING RECOVERY

Comparators are often used in digital systems to recover clock

timing signals. High speed square waves transmitted over any

distance, even tens of centimeters, can become distorted due to

stray capacitance and inductance. Poor layout or improper

termination can also cause reflections on the transmission line,

further distorting the signal waveform. A high speed comparator

can recover the distorted waveform while maintaining a

minimum of delay.

Figure 25 shows VOUT vs. VIN as the AD8611 recovers a 65 MHz,

100 mV peak-to-peak distorted clock signal into a 4 V peak-to-

peak square wave. The lower trace is the input to the AD8611,

and the upper trace is the QA or QB output from the comparator.

The AD8611 is powered from a 5 V single supply.

TIME (10ns/DIV)

2V/DIV

VOUT

VIN

20mV/DIV

Figure 25. Using the AD8611 to Recover a Noisy Clock Signal

A 5 V, HIGH SPEED WINDOW COMPARATOR

A window comparator circuit detects when a signal is between

two fixed voltages. The AD8612 can create a high speed window

comparator, as shown in Figure 26. In this example, the

reference window voltages are set as:

The output of the A1 comparator goes high when the input

signal exceeds VHI, and the output of A2 goes high only when

VIN drops below VLO. When the input voltage is between VHI

and VLO, both comparator outputs are low, turning off both Q1

and Q2, thus driving VOUT to a high state. If the input signal

goes outside of the reference voltage window, VOUT goes low.

To ensure a minimum of switching delay, the use of high speed

transistors is recommended for Q1 and Q2. Using the AD8612

with 2N3960 transistors provides a total propagation delay from

VIN to VOUT of less than 10 ns.

Table 8. Window Comparator Output States

VOUT

Input Voltage

≈ 200 mV

VIN < VLO

+5 V

VLO < VIN < VHI

≈ 200 mV

VIN > VHI

VHI

VIN

AD8612

Ω

VLO

AD8612

Ω

VOUT

Ω

500

Ω

500

Ω

NOTES

1. Q1, Q2 = 2N3960.

2. PINS 2 AND 13 ARE NO CONNECTS.

Figure 26. A High Speed Window Comparator

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