AD5624/AD5664

Data Sheet

Rev. A | Page 20 of 23

APPLICATIONS INFORMATION

CHOOSING A REFERENCE FOR THE AD5624/AD5664

To achieve the optimum performance from the AD5624/

AD5664, give thought to the choice of a precision voltage

reference. The AD5624/AD5664 have only one reference input,

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. Choosing a reference with an output trim adjustment,

such as the ADR423, allows a system designer to trim out system

errors 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

specification 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. Choose a reference with a tight temperature

coefficient specification 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 to10 Hz range. Examples of recom-

mended precision references for use as supply to the AD5624/

AD5664 are shown in the Table 14.

USING A REFERENCE AS A POWER SUPPLY FOR

THE AD5624/AD5664

Because the supply current required by the AD5624/AD5664 is

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

to supply the required voltage to the device (see Figure 40). 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 AD5624/AD5664 (see Table 14 for a suitable

reference). If the low dropout REF195 is used, it must supply

450 μA of current to the AD5624/AD5664, 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

450 μA + (5 V/5 kΩ) = 1.45 mA

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

results in a 2.9 ppm (14.5 μV) error for the 1.45 mA current

drawn from it. This corresponds to a 0.191 LSB error.

AD5624/

AD5664

3-WIRE

SERIAL

INTERFACE

SYNC

SCLK

DIN

15V

5V

500mA

REF195

Figure 40. REF195 as Power Supply to the AD5624/AD5664

Table 14. Partial List of Precision References for Use with the AD5624/AD5664

Part No.

Initial Accuracy (mV max)

Temp Drift (ppm

°C max)

0.1 Hz to 10 Hz Noise (µV p-p typ)

ADR425

±2

3

3.4

5

ADR395

±6

25

5

5

REF195

±2

5

50

5

AD780

±2

3

4

2.5/3

ADR423

±2

3

3.4

3