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Data Sheet

AD5761/AD5721

Rev. C | Page 29 of 31

APPLICATIONS INFORMATION

TYPICAL OPERATING CIRCUIT

Figure 66 shows the typical operating circuit for the AD5761/

AD5721. The only external components needed for these precision

16-/12-bit DACs are decoupling capacitors, supply voltage, and an

external reference. The integration of a reference buffer in the

AD5761/AD5721 results in overall savings in both cost and board

space.

In Figure 66, VDD is connected to 15 V and VSS is connected to

−15 V, but VDD and VSS can operate with supplies from 4.75 V to

30 V and from −16.5 V to 0 V, respectively.

Precision Voltage Reference Selection

An external reference is required for the AD5761/AD5721. Take

care in the selection of the same because any error in the voltage

reference is reflected in the output of the device.

There are four possible sources of error to consider when choosing

a voltage reference for high accuracy applications: initial accuracy,

temperature coefficient of the output voltage, long-term drift,

and output voltage noise.

Initial accuracy error on the output voltage of an external

reference may lead to a full-scale error in the DAC. Therefore,

to minimize these errors, a reference with low initial accuracy

error specification is preferred. Choosing a reference with an

output trim adjustment, such as the ADR421, allows a system

designer to trim system errors out by setting the reference

voltage to a voltage other than the nominal. The trim adjustment

can also be used at ambient temperature to trim out any error.

Long-term drift is a measure of how much the reference output

voltage drifts over time. A reference with a tight long-term drift

specification ensures that the overall solution remains relatively

stable over its entire lifetime.

The temperature coefficient of a reference output voltage affects

gain error and TUE. Choose a reference with a tight temperature

coefficient specification to reduce the dependence of the DAC

output voltage on ambient conditions.

In high accuracy applications, which have a relatively low noise

budget, reference output voltage noise must be considered. It is

important to choose a reference with as low an output noise

voltage as practical for the system resolution that is required.

Precision voltage references, such as the ADR4525, produce low

output noise in the 0.1 Hz to 10 Hz region. However, as the circuit

bandwidth increases, filtering the output of the reference may

be required to minimize the output noise.

VOUT

–15V

10µF

100nF

VREFIN

+15V

SDI

DGND

SDO

DVCC

DNC

NOTES

1. DNC = DO NOT CONNECT. DO NOT CONNECT TO THIS PIN.

SCLK

AD5761/

AD5721

SYNC

CLEAR

RESET

LDAC

SDI

SDO

SCLK

SYNC

LDAC

ALERT

AGND

VOUT

VSS

VREFIN

VDD

CLEAR

RESET

ALERT

100nF

10µF

+5V

Figure 66. Typical Operating Circuit

POWER SUPPLY CONSIDERATIONS

The AD5761/AD5721 must be powered by the following three

supplies to provide any of the eight output voltage ranges available

on the DAC: VDD = +21 V, VSS = −11 V, and DVCC = +5 V.

For applications requiring optimal high power efficiency and

low noise performance, it is recommended that the ADP5070

switching regulator be used to convert the 5 V input rail into

two intermediate rails (+23 V and −13 V). These intermediate

rails are then postregulated by very low noise, low dropout (LDO)

regulators (ADP7142 and ADP7182). Figure 67 shows the

recommended method.

ADP7182

LDO

ADP5070

DC-TO-DC

SWITCHING

REGULATOR

–11V: VSS

ADP7142

LDO

+5V: DVCC

ADP7142

LDO

SWITCHING

REGULATOR

+21V: VDD

5V INPUT

+23V

–13V

5V INPUT

ADP5070

DC-TO-DC

Figure 67. Postregulation by ADP7142 and ADP7182

EVALUATION BOARD

An evaluation board is available for the AD5761R to aid designers

in evaluating the high performance of the device with minimum

effort. This evaluation board can be used to evaluate the AD5761/

AD5721. The AD5761R evaluation kit includes a populated and

tested AD5761R printed circuit board (PCB). The evaluation

board interfaces to the USB port of a PC. Software is available

with the evaluation board to allow the user to easily program

the AD5761R. The EVAL-AD5761RSDZ user guide gives full

details on the operation of the evaluation board.

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