AD9621

REV. 0

–4–

phase margin (55

°), low noise current (3.6 pA/

rate (1200 V/

µs) give higher performance capabilities to these

applications over previous voltage feedback designs.

With a settling time of 11 ns to 0.01% and 7 ns to 0.1%, the de-

vice is an excellent choice for DAC I/V conversion. The same

characteristics, along with low harmonic distortion, make it a

good choice for ADC buffering/amplification. With its superb

linearity at relatively high signal frequencies, it is an ideal driver

for ADCs up to 14 bits.

Layout Considerations

As with all wide bandwidth components, printed circuit layout

is critical to obtain best dynamic performance with the AD9621.

The ground plane in the area of the amplifier and its associated

components should cover as much of the component side of the

board as possible (or first interior layer of a multi layer surface

mount board).

The ground plane should be removed in the area of the inputs

supply trace should be decoupled close to the package with a

0.1

µF ceramic capacitor, plus a 6.8 µF tantalum nearby.

All lead lengths for input, output, and feedback resistor should

be kept as short as possible. All gain setting resistors should be

chosen for low values of parasitic capacitance and inductance,

i.e., microwave resistors and/or carbon resistors.

Microstrip techniques should be used for lead lengths in excess

of one inch. Sockets should be avoided if at all possible because

of their high series inductance. If sockets are necessary, indi-

vidual pin sockets such as AMP p/n 6-330808-3 should be used.

These contribute far less stray reactance than molded socket

assemblies.

An evaluation board is available from Analog Devices for a

nominal charge.

Pulse Response

Unlike a traditional voltage feedback amplifier in which slew

speed is dictated by its front end dc quiescent current and gain

bandwidth product, the AD9621 provides “on demand” trans-

conductance current that increases proportionally to the input

“step” signal amplitude. This results in slew speeds (1200 V/

µs)

comparable to wideband current feedback designs. This, com-

bined with relatively low input noise current (3.6 pA/

the AD9621 the best attributes of both voltage and current feed-

back amplifiers.

Under certain conditions, it can be used to further enhance set-

tling time performance.

µF) connects to the internal high im-

pedance nodes of the amplifier. Using this capacitor will reduce

the large signal (4 V) step output settling time by 3 to 5 ns for

0.05% or greater accuracy. For settling accuracy less than

0.05% or for smaller step sizes, its effect will be less apparent.

Under heavy slew conditions, this capacitor forces the internal

signal (initial step) amplitude to be controlled by the “on”

(slewed) transistor, preventing its complement from completely

turning off. This allows for faster settling time of these internal

nodes and also the output.

In the frequency domain, total (high frequency) distortion will

APPLICATIONS

The AD9621 is a voltage feedback amplifier and is well suited

for such applications as photo-detector preamp, active filters,

and log amplifiers. The device’s wide bandwidth (350 MHz),

R

F

C

F

C

I

V

OUT

Figure 1. Transimpedance

Configuration

2

3

4

7

6

0.1

µF

0.1

µF

1

8

6.8

µF

R

G

V

IN

6.8

µF

+V

S

–V

S

C

B (OPTIONAL)

C

F

V

OUT

R

F

500

Ω

A

R

F

R

G

Figure 3. Noninverting Gain Connection

Diagram

2

3

4

7

6

0.1

µF

0.1

µF

1

8

6.8

µF

R

F

R

G

R

G

V

IN

6.8

µF

+V

S

–V

S

C

B (OPTIONAL)

C

F

V

OUT

R

F

500

Ω

A

–R

F

R

G

Figure 2. Inverting Gain Connection

Diagram