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Data Sheet

AD5737

Rev. B | Page 39 of 44

APPLICATIONS INFORMATION

CURRENT OUTPUT MODE WITH INTERNAL RSET

When using the internal RSET resistor, the current 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

four 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% FSR on

each of three channels, and 0.01% FSR 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 another channel going from zero to full scale, the dc crosstalk

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

scale, the dc crosstalk 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% FSR. Similarly, if only

one channel is enabled 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 AD5737 over its

full operating temperature range, a precision voltage reference

must be used. Care should be taken with the selection of the

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 AD5737.

Four possible sources of error must be considered when choosing

a voltage reference for high accuracy applications: initial accuracy,

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

and output voltage noise.

Initial accuracy error on the output voltage of an external ref-

erence can lead to a full-scale error in the DAC. Therefore, to

minimize these errors, a reference with a low initial accuracy

error specification is preferred. Choosing a reference with an

output trim adjustment, such as the ADR435, allows a system

designer to trim out system errors 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 the reference output voltage affects

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

ficient specification should be chosen to reduce the dependence

of the DAC output voltage on ambient temperature.

In high accuracy applications, which have a relatively low noise

budget, reference output voltage noise must be considered. Choos-

ing 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 bandwidth. 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 the IOUT_x pin and the AGND pin 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 settling time of the AD5737. There is no maxi-

mum capacitance limit for the current output of the AD5737.

Table 37. Recommended Precision Voltage References

Part No.

Initial Accuracy

(mV Maximum)

Long-Term Drift

(ppm Typical)

Temperature Coefficient

(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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