REV. D

AD8001

–10–

THEORY OF OPERATION

A very simple analysis can put the operation of the AD8001, a

current feedback amplifier, in familiar terms. Being a current

feedback amplifier, the AD8001’s open-loop behavior is expressed

as transimpedance,

ance behaves just as the open-loop voltage gain of a voltage

feedback amplifier, that is, it has a large dc value and decreases

at roughly 6 dB/octave in frequency.

× g

conductance of the input stage. This results in a low open-loop

input impedance at the inverting input, a now familiar result.

Using this amplifier as a follower with gain, Figure 4, basic

analysis yields the following result.

V

V

G

TS

TS

G

R

R

G

R

R

Rg

O

IN

Z

ZIN

IN

M

=×

+×

+

=+

=

≈

()

()

/

1

1

1

2

150

Ω

R1

R2

Figure 4. Follower with Gain

Recognizing that G

× R

the first order that bandwidth for this amplifier is independent

of gain (G). This simple analysis in conjunction with Figure 5

can, in fact, predict the behavior of the AD8001 over a wide

range of conditions.

FREQUENCY – Hz

1M

10

100k

1M

1G

100M

10M

100

100k

10k

1k

Figure 5. Transimpedance vs. Frequency

Considering that additional poles contribute excess phase at

high frequencies, there is a minimum feedback resistance below

which peaking or oscillation may result. This fact is used to

parasitic capacitance at Pin 2 will also add phase in the feedback

Figure 6 illustrates this problem. Here the fine scale (0.1 dB/

div) flatness is plotted versus feedback resistance. These plots

were taken using an evaluation card which is available to cus-

tomers so that these results may readily be duplicated.

Achieving and maintaining gain flatness of better than 0.1 dB at

frequencies above 10 MHz requires careful consideration of

several issues.

0.1

0

–0.9

1M

10M

100M

–0.1

–0.2

–0.3

–0.4

–0.5

FREQUENCY – Hz

–0.6

–0.7

–0.8

G = +2

649

Figure 6. 0.1 dB Flatness vs. Frequency

Choice of Feedback and Gain Resistors

Because of the above-mentioned relationship between the band-

width and feedback resistor, the fine scale gain flatness will, to

some extent, vary with feedback resistance. It, therefore, is

recommended that once optimum resistor values have been

determined, 1% tolerance values should be used if it is desired to

maintain flatness over a wide range of production lots. In addition,

resistors of different construction have different associated parasitic

capacitance and inductance. Surface-mount resistors were used

for the bulk of the characterization for this data sheet. It is not

recommended that leaded components be used with the AD8001.