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AD712AH

Dual Precision, Low Cost,High Speed, BiFET Op Amp

Analog Devices 

AD712

D/A CONVERTER APPLICATIONS

The AD712 is an excellent output amplifier for CMOS DACs.

It can be used to perform both 2 quadrant and 4 quadrant op-

eration. The output impedance of a DAC using an inverted

R-2R ladder approaches R for codes containing many 1s, 3R for

codes containing a single 1, and for codes containing all zero,

the output impedance is infinite.

For example, the output resistance of the AD7545 will modu-

late between 11 kΩ and 33 kΩ. Therefore, with the DAC’s

internal feedback resistance of 11 kΩ, the noise gain will vary

from 2 to 4/3. This changing noise gain modulates the effect of

the input offset voltage of the amplifier, resulting in nonlinear

DAC amplifier performance.

The AD712K with guaranteed 700 µV offset voltage minimizes

this effect to achieve 12-bit performance.

Figures 31 and 32 show the AD712 and AD7545 (12-bit

CMOS DAC) configured for unipolar binary (2-quadrant multi-

plication) or bipolar (4-quadrant multiplication) operation.

Capacitor C1 provides phase compensation to reduce overshoot

and ringing.

GAIN

ADJUST

VIN

R1A*

*REFER TO

TABLE I

VDD

R2A*

VDD

RFB

OUT1

VREF AD7545

AGND

DGND

C1A

33pF

ANALOG

COMMON

+15V

0.1␮F

1/2

AD712

DB11–DB0

GAIN

ADJUST

VIN

R1B*

*REFER TO

TABLE I

VDD

R2B*

VDD

RFB

OUT1

VREF AD7545

AGND

DGND

DB11–DB0

C1B

33pF

ANALOG

COMMON

1/2

AD712

0.1␮F

–15V

Figure 31. Unipolar Binary Operation

VOUTA

VOUTB

R1 and R2 calibrate the zero offset and gain error of the DAC.

Specific values for these resistors depend upon the grade of

AD7545 and are shown below.

Table I. Recommended Trim Resistor Values vs. Grades

of the AD7545 for VDD = +5 V

Trim

Resistor

JN/AQ/SD KN/BQ/TD LN/UD GLN/GUD

R1

500 Ω

200 Ω

100 Ω 20 Ω

R2

150 Ω

68 Ω

33 Ω

6.8 Ω

GAIN

ADJUST

VIN

R1*

VDD

R2*

VDD

RFB

OUT1

VREF AD7545

AGND

DB11–DB0 DGND

12

C1

33pF

+15V

0.1␮F

R4

20k⍀ 1%

R5

20k⍀ 1%

1/2

AD712

R3

10k⍀ 1%

1/2

AD712

0.1␮F

DATA INPUT

*FOR VALUES OF

R1 AND R2 SEE TABLE I

ANALOG

COMMON

–15V

Figure 32. Bipolar Operation

VOUT

–10–

REV. B

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