REV. B

–10–

AD8011

This analysis assumes perfect current sources and infinite tran-

These assumptions result in actual vs. model open loop voltage

gain and associated input referred error terms being less accurate

for low gain (G) noninverting operation at the frequencies below

the open loop pole of the AD8011. This is primarily a result of

ing operation, the actual vs. model dc error terms are relatively

much less.

AC TRANSFER CHARACTERISTICS

The ac small signal transfer derivations below are based on a

simplified single-pole model. Though inaccurate at frequencies

approaching the closed-loop BW (CLBW) of the AD8011 at low

noninverting external gains, they still provide a fair approximation

and a intuitive understanding of its primary ac small signal

characteristics.

For inverting operation and high noninverting gains these trans-

fer equations provide a good approximation to the actual ac

performance of the device.

nonexpanded noninverting gain relationship:

V

G

G

A

+

R

F

T

+ 1

with

R1

× g

S

τ1

1E+03

1E+04

1E+05

1E+06

1E+07

1E+08

1E+09

80

70

60

50

40

20

10

FREQUENCY – Hz

30

–90

–100

–110

–120

–160

PHASE

GAIN

0

–10

–20

–30

–170

–180

–190

–200

–130

–140

–150

Figure 30. Open-Loop Voltage Gain and Phase

(error current times the open loop inverting input resistance)

that results (see Figure 29), a more exact low frequency closed

loop transfer functions can be described as:

A

G

1

+

G

× R

I

T

O

+

R

F

T

O

=

G

1

+

G

A

O

+

R

F

T

O

for noninverting (G is positive)

A

G

1

+

1 – G

A

O

+

R

F

T

O

for inverting (G is negative)

IE

where G is the ideal gain as previously described. With

expression (positive G) clearly relates to the classical voltage

the open-loop dc voltage and transresistance gains of the ampli-

fier respectively. These key transfer variables can be described as:

A

Rg mf

A

g

R

RA

R

g

mc

R

g

mf

OO

I

mc

T

=

××

×

=

×

=

−×

×

12

1

1

12

2

11

2

|

|

( –

)

|

|

;

and

therefore

× R1 product has a design value that

results in a negative dc open loop gain of typically –2500 V/V (see

Figure 30).

Though atypical of conventional CF or VF amps, this negative

open-loop voltage gain results in an input referred error term

Ω, results in a error of –3 mV using the A

tion above.