AD8564

–5–

REV. A

3.000

2.500

0.000

24

12

68

10

2.000

1.500

1.000

0.500

rent/Comparator vs. Supply Voltage

TEMPERATURE – C

2.000

1.500

0.000

–75 –50

150

–25

0

25

75

100 125

50

1.000

0.500

Comparator vs. Temperature

APPLICATIONS

OPTIMIZING HIGH SPEED PERFORMANCE

As with any high speed comparator or amplifier, proper design

and layout techniques should be used to ensure optimal perfor-

mance from the AD8564. The performance limits of high speed

circuitry can easily be a result of stray capacitance, improper

ground impedance or other layout issues.

Minimizing resistance from source to the input is an important

consideration in maximizing the high speed operation of the

AD8564. Source resistance in combination with equivalent

input capacitance could cause a lagged response at the input,

thus delaying the output. The input capacitance of the AD8564

in combination with stray capacitance from an input pin to

ground could result in several picofarads of equivalent capaci-

tance. A combination of 3 k

Ω source resistance and 5 pF of

input capacitance yields a time constant of 15 ns, which is

slower than the 5 ns capability of the AD8564. Source imped-

ances should be less than 1 k

Ω for the best performance.

It is also important to provide bypass capacitors for the power

supply in a high speed application. A 1

µF electrolytic bypass

capacitor should be placed within 0.5 inches of each power

supply pin to ground. These capacitors will reduce any potential

voltage ripples from the power supply. In addition, a 10 nF

ceramic capacitor should be placed as close as possible from the

power supply pins to ground. These capacitors act as a charge

reservoir for the device during high frequency switching.

A ground plane is recommended for proper high speed perfor-

mance. This can be created by using a continuous conductive

plane over the surface of the circuit board, only allowing breaks

in the plane for necessary current paths. The ground plane

provides a low inductance ground, eliminating any potential

differences at different ground points throughout the circuit

board caused from “ground bounce.” A proper ground plane

also minimizes the effects of stray capacitance on the circuit

board.

OUTPUT LOADING CONSIDERATIONS

The AD8564 output can deliver up to 40 mA of output current

without any significant increase in propagation delay. The out-

put of the device should not be connected to more than twenty

(20) TTL input logic gates, or drive a load resistance less than

100

Ω.

To ensure the best performance from the AD8564 it is impor-

tant to minimize capacitive loading of the output of the device.

Capacitive loads greater than 50 pF will cause ringing on the

output waveform and will reduce the operating bandwidth of

the comparator. Propagation delay will also increase with

capacitive loads above 100 pF.

TEMPERATURE – C

0.000

–1.000

–5.000

–75 –50

150

–25

0

25

75

100 125

50

–2.000

–3.000

–4.000

rent/Comparator vs. Temperature

TEMPERATURE – C

5.000

4.000

0

–75 –50

150

–25

0

25

75

100 125

50

3.000

2.000

1.000

rent/Comparator vs. Temperature