AD9617

–7–

REV. B

THEORY OF OPERATION

The AD9617 has been designed to combine the key attributes of

traditional “low frequency” precision amplifiers with exceptional

high frequency characteristics that are independent of closed-

loop gain. Previous “high frequency” closed-loop amplifiers have

low open loop gain relative to precision amplifiers. This results

in relatively poor dc nonlinearity and precision, as well as exces-

sive high frequency distortion due to open loop gain roll-off.

Operational amplifiers use two basic types of feedback correc-

tion, each with advantages and disadvantages. Voltage feedback

topologies exhibit an essentially constant gain bandwidth prod-

uct. This forces the closed-loop bandwidth to vary inversely with

closed-loop gain. Moreover, this type design typically slew rate

limits in a way that causes the large signal bandwidth to be

much lower than its small signal characteristics.

A newer approach is to use current feedback to realize better

dynamic performance. This architecture provides two key at-

tributes over voltage feedback configurations: (1) avoids slew

rate limiting and therefore large signal bandwidth can approach

small signal performance; and (2) low bandwidth variation ver-

sus gain settings, due to the inherently low open loop inverting

The AD9617 uses a new current feedback topology that over-

comes these limitations and combines the positive attributes of

both current feedback and voltage feedback designs. These

devices achieve excellent high frequency dynamics (slew, BW

and distortion) along with excellent low frequency linearity and

good dc precision.

DC GAIN CHARACTERISTICS

A simplified equivalent schematic is shown below. When operat-

ing the device in the inverting mode, the input signal error

Noninverting operation is similar, with the input signal applied

to the high impedance buffer (noninverting) input. As before, an

ance inverting input. The signal generated at the output is fed

back topology when exact equations are used.

+

–

Figure 13. Equivalent Circuit

The major difference lies in the front end architecture. A voltage

feedback amplifier has symmetrical high resistance (buffered)

inputs. A current feedback amplifier has a high noninverting

resistance (buffered) input and a low inverting (buffer output)

input resistance. The feedback mechanics can be easily devel-

oped using current feedback and transresistance open loop gain

T(s) to describe the I/O relationship. (See typical specification

chart.)

DC closed-loop gain for the AD9617 can be calculated using

the following equations:

G

=

≈

−R

1

+1/ LG

inverting

(1)

G

=

≈

1

+ R

1

+1/ LG

noninverting

(2)

where

1

LG

≈

()

Ts

S

()

(3)

Because the noninverting input buffer is not ideal, input resis-

approach the same value ( 7

Ω) for both at input frequencies

above 50 MHz. Below the open loop corner frequency, the

= 7 +

dc

(4)

G

× 600

proximated as:

= 7 +

dc

(5)

0.05% typically results for G = –1 and 0.15% for G = –40.

Moreover, the architecture linearizes the open loop gain over its

operating voltage range and temperature resulting in

≥16 bits of

linearity.

–2

0%

–1

0

12