AD5757 datasheet(39/44 Pages) AD

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AD5757

Rev. A | Page 39 of 44

APPLICATIONS INFORMATION

CURRENT OUTPUT MODE WITH INTERNAL RSET

When using the internal RSET resistor in current output mode,

the output is significantly affected by how many other channels

using the internal RSET are enabled and by the dc crosstalk from

these channels. The internal RSET specifications in Table 1 are

for all channels enabled with the internal RSET selected and

outputting the same code.

For every channel enabled with the internal RSET, the offset error

decreases. For example, with one current output enabled using

the internal RSET, the offset error is 0.075% FSR. This value

decreases proportionally as more current channels are enabled;

the offset error is 0.056% FSR on each of two channels, 0.029%

on each of three channels, and 0.01% on each of four channels.

Similarly, the dc crosstalk when using the internal RSET is propor-

tional to the number of current output channels enabled with

the internal RSET. For example, with the measured channel at

0x8000 and one channel going from zero to full scale, the dc

crosstalk is −0.011% FSR. With two channels going from zero to

full scale, it is −0.019% FSR, and with all three other channels

going from zero to full scale, it is −0.025% FSR.

For the full-scale error measurement in Table 1, all channels are

at 0xFFFF. This means that, as any channel goes to zero scale,

the full-scale error increases due to the dc crosstalk. For example,

with the measured channel at 0xFFFF and three channels at

zero scale, the full-scale error is 0.025%. Similarly, if only one

channel is enabled in current output mode with the internal

RSET, the full-scale error is 0.025% FSR + 0.075% FSR = 0.1% FSR.

PRECISION VOLTAGE REFERENCE SELECTION

To achieve the optimum performance from the AD5757 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

inputs 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 can 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 be used at any 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 depend-

ence of the DAC output voltage to ambient temperature.

In high accuracy applications, which have a relatively low noise

budget, reference output voltage noise must be considered.

Choosing a reference with as low an output noise voltage as practi-

cal 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.

DRIVING INDUCTIVE LOADS

When driving inductive or poorly defined loads, a capacitor

may be required between IOUT_x and AGND to ensure stability. A

0.01 μF capacitor between IOUT_x and AGND ensures stability of

a load of 50 mH. The capacitive component of the load may

cause slower settling, although this may be masked by the set-

tling time of the AD5757. There is no maximum capacitance

limit for the current output of the AD5757.

Table 34. Recommended Precision References

Part No.

Initial Accuracy

(mV Maximum)

Long-Term Drift

(ppm Typical)

Temperature Drift (ppm/°C Maximum)

0.1 Hz to 10 Hz Noise

(μV p-p Typical)

ADR445

±2

2.25

ADR02

±3

ADR435

±2

ADR395

±5

AD586

±2.5

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