AD8047/AD8048

REV. 0

–13–

circuit when driving large capacitive loads will be dominated by

R

F

R

SERIES

R

L

1k

Ω

C

L

AD8047

Figure 54. Driving Capacitive Loads

5ns

500mV

Figure 55. AD8047 Large Signal Transient Response;

R

F

R

SERIES

R

L

1k

Ω

C

L

AD8048

R

IN

Figure 56. Driving Capacitive Loads

5ns

500mV

Figure 57. AD8048 Large Signal Transient Response;

APPLICATIONS

The AD8047 and AD8048 are voltage feedback amplifiers well

suited for such applications as photodetectors, active filters, and

log amplifiers. The devices’ wide bandwidth (260 MHz), phase

margin (65

°), low noise current (1.0 pA/√Hz), and slew rate

(1000 V/

µs) give higher performance capabilities to these appli-

cations over previous voltage feedback designs.

With a settling time of 30 ns to 0.01% and 13 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 su-

perb linearity at relatively high signal frequencies, the AD8047

and AD8048 are ideal drivers for ADCs up to 12 bits.

Operation as a Video Line Driver

The AD8047 and AD8048 have been designed to offer out-

standing performance as video line drivers. The important

specifications of differential gain (0.01%) and differential phase

(0.02

°) meet the most exacting HDTV demands for driving

video loads.

75

Ω

CABLE

200

Ω

200

Ω

75

Ω

CABLE

75

Ω

75

Ω

V

OUT

+V

S

–V

S

75

Ω

V

IN

0.1

µF

10

µF

AD8047/

AD8048

3

2

7

0.1

µF

10

µF

4

6

Figure 58. Video Line Driver

Active Filters

The wide bandwidth and low distortion of the AD8047 and

AD8048 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 AD8047 and AD8048

in a voltage feedback configuration. Many active filter configu-

rations 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

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

shift of the amplifier are to be avoided.

Figure 59 is an example of a 20 MHz low pass multiple feed-

back active filter using an AD8048.

1

R4

154

Ω

C1

50pF

C2

100pF

R1

154

Ω

AD8048

R3

78.7

Ω

+5V

0.1

µF

3

2

100

Ω

6

10

µF

5

0.1

µF

–5V

10

µF

4

7

Figure 59. Active Filter Circuit

Choose:

α = Damping Ratio = 1/Q = 2