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Preliminary Technical Data

AD5757/AD5737

Rev. PrD | Page 29 of 31

APPLICATIONS INFORMATION

PRECISION VOLTAGE REFERENCE SELECTION

To achieve the optimum performance from the

AD5757/AD5737 over its full operating temperature range, a

precision voltage reference must be used. Thought should be

given to the selection of a precision voltage reference. The

voltage applied to the reference inpus is used to provide a

buffered reference for the DAC cores. Therefore, any error in

the voltage reference is reflected in the outputs 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 refer-

ence could 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 ADR425, allows a system

designer to trim system errors out by setting the reference

voltage to a voltage other than the nominal. The trim adjust-

ment can also be used at 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’s output voltage

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

temperature coefficient specification should be chosen 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 needs to be considered.

Choosing a reference with as low an output noise voltage as

practical for the system resolution required is important.

Precision voltage references such as the ADR435 (XFET design)

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.

Table 40. Some Recommended Precision References

Part

No.

Initial

Accuracy

(mV

Max)

Long-Term

Drift (ppm

Typ)

Temp

Drift

(ppm/°C

Max)

0.1 Hz to 10

Hz Noise

(µV p-p

Typ)

ADR435

±6

3.4

ADR425

±6

3.4

ADR02

±5

ADR395

±6

AD586

±2.5

DRIVING INDUCTIVE LOADS

When driving inductive or poorly defined loads connect a

0.01µF capacitor between IOUT and GND. This will ensure

stability with loads beyond 50mH. There is no maximum

capacitance limit. The capacitive component of the load may

cause slower settling, though this may be masked by the settling

time of the AD5757/AD5737.

TRANSIENT VOLTAGE PROTECTION

The AD5757/AD5737 contains ESD protection diodes which

prevent damage from normal handling. The industrial control

environment can, however, subject I/O circuits to much higher

transients. In order to protect the AD5757/AD5737 from

excessively high voltag etransients , external power diodes and a

surge current limiting resistor may be required, as shown in

Figure 21. The constraint on the resistor value is that during

normal operation the output level at IOUT must remain within its

voltage compliance limit of AVDD – 2.5V and the two protection

diodes and resistor must have appropriate power ratings.

Figure 21. Output Transient Voltage Protection

MICROPROCESSOR INTERFACING

Microprocessor interfacing to the AD5757/AD5737 is via a serial

bus that uses a protocol compatible with microcontrollers and

DSP processors. The communications channel is a 3-wire

minimum interface consisting of a clock signal, a data signal,

and a latch signal. The AD5757/AD5737 require a 24-bit data-

word with data valid on the falling edge of SCLK.

The DAC output update is initiated on either the rising edge of

LDAC or, if LDAC is held low, on the rising edge of SYNC. The

contents of the registers can be read using the readback

function.

LAYOUT GUIDELINES

In any circuit where accuracy is important, careful consideration

of the power supply and ground return layout helps to ensure

the rated performance. The printed circuit board on which the

AD5757/AD5737 is mounted should be designed so that the

analog and digital sections are separated and confined to certain

areas of the board. If the AD5757/AD5737 is in a system where

multiple devices require an AGND-to-DGND connection, the

connection should be made at one point only. The star ground

point should be established as close as possible to the device.

IOUT

GND

AVDD

RLOAD

AVDD

AD5755

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