ADA4817-1/ADA4817-2

Rev. 0 | Page 13 of 24

THEORY OF OPERATION

The ADA4817-1/ADA4817-2 are voltage feedback operational

amplifiers that combine new architecture for FET input opera-

tional amplifiers with the eXtra Fast Complementary Bipolar

(XFCB) process from Analog Devices resulting in an outstanding

combination of speed and low noise. The innovative high speed

FET input stage handles common-mode signals from the nega-

tive supply to within 2.3 V of the positive rail. This stage is

combined with an H-bridge to attain a 870 V/μs slew rate and

low distortion, in addition to 4 nV/√Hz input voltage noise.

The amplifier features a high speed output stage capable of driving

heavy loads sourcing and sinking up to 70 mA of linear current.

Supply current and offset current are laser trimmed for optimum

performance. These specifications make the ADA4817-1/

ADA4817-2 a great choice for high speed instrumentation

and high resolution data acquisition systems. Its low noise,

picoamp input current, precision offset, and high speed make

them superb preamps for fast photodiode applications.

CLOSED-LOOP FREQUENCY RESPONSE

The ADA4817-1/ADA4817-2 are classic voltage feedback

amplifiers with an open-loop frequency response that can be

approximated as the integrator response shown in Figure 40.

Basic closed-loop frequency response for inverting and nonin-

verting configurations can be derived from the schematics shown

in Figure 38 and Figure 39.

A

Figure 38. Noninverting Configuration

A

Figure 39. Inverting Configuration

NONINVERTING CLOSED-LOOP FREQUENCY

RESPONSE

Solving for the transfer function,

(

)

()

G

CROSSOVER

G

F

F

G

CROSSOVER

I

O

R

f

S

R

R

R

R

f

V

V

×

×

π

+

+

+

×

π

=

2

2

(4)

gain equals 0 dB.

At dc

G

G

F

I

O

R

R

R

V

V

+

=

(5)

Closed-loop −3 dB frequency

G

F

G

CROSSOVER

3dB

R

R

R

f

f

+

×

=

−

(6)

INVERTING CLOSED-LOOP FREQUENCY RESPONSE

Solving for the transfer function,

()

G

CROSSOVER

G

F

F

CROSSOVER

I

O

R

f

S

R

R

R

f

V

V

×

×

π

+

+

×

×

π

−

=

2

2

(7)

At dc

G

F

I

O

R

R

V

V

−

=

(8)

Solve for closed-loop −3 dB frequency by,

G

F

G

CROSSOVER

dB

R

R

R

f

f

+

×

=

−3

(9)

FREQUENCY (MHz)

80

60

0.1

1000

1

100

10

40

20

0

A = (2π ×

Figure 40. Open-Loop Gain vs. Frequency and Basic Connections

The closed-loop bandwidth is inversely proportional to the noise

accurate for noise gains above 2. The actual bandwidth of circuits

with noise gains at or below 2 is higher than those predicted

with this model due to the influence of other poles in the

frequency response of the real op amp.

Figure 41 shows a voltage feedback amplifier’s dc errors. For

both inverting and noninverting configurations,

()

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

+

×

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

×

=

−

+

G

F

G

OS

F

b

G

F

G

S

b

OUT

R

R

R

V

R

I

R

R

R

R

I

error

V

(10)

A

Figure 41. Voltage Feedback Amplifier’s DC Errors