AD5663ARMZ datasheet(19/24 Pages) AD

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AD5663

Rev. 0 | Page 19 of 24

APPLICATIONS

CHOOSING A REFERENCE FOR THE AD5663

To achieve the optimum performance from the AD5663,

thought should be given to the choice of a precision voltage

reference. The AD5663 has only one reference input, VREF.

The voltage on the reference input is used to supply the positive

input to the DAC. Therefore, any error in the reference is

reflected in the DAC.

When choosing a voltage reference for high accuracy applica-

tions, the sources of error are initial accuracy, ppm drift, long-

term drift, and output voltage noise. Initial accuracy on the

output voltage of the DAC leads to a full-scale error in the

DAC. To minimize these errors, a reference with high initial

accuracy is preferred. Also, choosing a reference with an output

trim adjustment, such as the ADR423, allows a system designer

to trim system errors out by setting a reference voltage to a

voltage other than the nominal. The trim adjustment can also

be used at temperature to trim out any error.

Long-term drift is a measurement of how much the reference

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

cation ensures that the overall solution remains relatively stable

during its entire lifetime.

The temperature coefficient of a reference’s output voltage

affects INL, DNL, and TUE. A reference with a tight temperature

coefficient specification should be chosen to reduce temperature

dependence of the DAC output voltage in ambient conditions.

In high accuracy applications, which have a relatively low noise

budget, reference output voltage noise needs to be considered.

It is important to choose a reference with as low an output noise

voltage as practical for the system noise resolution required.

Precision voltage references, such as the ADR425, produce low

output noise in the 0.1 Hz to 10 Hz range. Examples of recom-

mended precision references for use as supplies to the AD5663

are shown in the Table 15.

USING A REFERENCE AS A POWER SUPPLY FOR

THE AD5663

Because the supply current required by the AD5663 is extremely

low, an alternative option is to use a voltage reference to supply

the required voltage to the part (see Figure 36). This is especially

useful if the power supply is quite noisy, or if the system supply

voltages are at some value other than 5 V or 3 V (for example,

15 V). The voltage reference outputs a steady supply voltage for

the AD5663; see Table 15 for a suitable reference. If the low drop-

out REF195 is used, it must supply 250 μA of current to the

AD5663, with no load on the output of the DAC. When the

DAC output is loaded, the REF195 also needs to supply the

current to the load. The total current required (with a 5 kΩ

load on the DAC output) is

250 μA + (5 V/5 kΩ) = 1.25 mA

The load regulation of the REF195 is typically 2 ppm/mA,

which results in a 2.5 ppm (12.5 μV) error for the 1.25 mA

current drawn from it. This corresponds to a 0.164 LSB error.

AD5663

THREE-WIRE

SERIAL

INTERFACE

SYNC

SCLK

DIN

15V

500µA

VOUT = 0V TO 5V

VREF

VDD

REF195

Figure 36. REF195 as Power Supply to the AD5663

Table 15. Partial List of Precision References for Use with the AD5663

Part No.

Initial Accuracy (mV Max)

Temperature Drift (ppm

C Max)

0.1 Hz to 10 Hz Noise (μV p-p Typ)

VOUT (V)

ADR425

±2

3.4

ADR395

±6

REF195

±2

AD780

±2

2.5/3

ADR423

±2

3.4

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