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Data Sheet

AD9631/AD9632

Rev. D | Page 17 of 20

APPLICATIONS INFORMATION

The AD9631/AD9632 are voltage feedback amplifiers well

suited for applications such as photodetectors, active filters,

and log amplifiers. The wide bandwidth (320 MHz), phase

margin (65°), low current noise (2.0 pA/√Hz), and slew rate

(1300 V/µs) of the devices give higher performance capabilities

to these applications over previous voltage feedback designs.

With a settling time of 16 ns to 0.01% and 11 ns to 0.1%, the

devices are an excellent choice for DAC I/V conversion. The

same characteristics along with low harmonic distortion make

them a good choice for ADC buffering/amplification. With

superb linearity at relatively high signal frequencies, the

AD9631/AD9632 are ideal drivers for ADCs up to 12 bits.

OPERATION AS A VIDEO LINE DRIVER

The AD9631/AD9632 have been designed to offer outstanding

performance as video line drivers. The important specifications

of differential gain (0.02%) and differential phase (0.02°) meet

the most exacting HDTV demands for driving video loads.

Figure 64. Video Line Driver

ACTIVE FILTERS

The wide bandwidth and low distortion of the AD9631/

AD9632 are ideal for the realization of higher bandwidth active

filters. These characteristics, while being more common in

many current feedback op amps, are offered in the AD9631/

AD9632 in a voltage feedback configuration. Many active

filter configurations are not realizable with current feedback

amplifiers.

A multiple feedback active filter requires a voltage feedback

amplifier and is more demanding of op amp performance than

other active filter configurations, such as the Sallen-Key. In

general, the amplifier should have a bandwidth that is at least

10 times the bandwidth of the filter if problems due to phase

shift of the amplifier are to be avoided.

Figure 65 is an example of a 20 MHz low-pass multiple feedback

active filter using an AD9632.

Figure 65. Active Filter Circuit

Choose

FO = cutoff frequency = 20 MHz

α = damping ratio = 1/Q = 2

H = absolute value of circuit gain =

4 =

−

Then

)

(

)

(

)

R4 =

+VS

0.1µF

10µF

–VS

VOUT

VIN

AD9631/

AD9632

75Ω

274Ω

75Ω

CABLE

75Ω

CABLE

0.1µF

10µF

0.1µF

10µF

VOUT

VIN

+5V

–5V

AD9632

100Ω

50pF

78.7Ω

154Ω

100pF

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