ADP2302 datasheet(45/48 Pages) AD

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Data Sheet

AD5735

Rev. B | Page 45 of 48

APPLICATIONS INFORMATION

VOLTAGE AND CURRENT OUTPUT PINS ON THE

SAME TERMINAL

When using a channel of the AD5735, the current and voltage

output pins can be connected to two separate terminals or tied

together and connected to a single terminal. The two output

pins can be tied together because only the voltage output or the

current output can be enabled at any one time. When the current

output is enabled, the voltage output is in tristate mode, and when

the voltage output is enabled, the current output is in tristate mode.

When the two output pins are tied together, the POC pin must

be tied low and the POC bit in the main control register set to 0,

or, if the POC pin is tied high, the POC bit in the main control

As shown in the Absolute Maximum Ratings section, the output

tolerances are the same for both the voltage and current output

pins. The +VSENSE_x and −VSENSE_x connections are buffered so

that current leakage into these pins is negligible when the part

is operated in current output mode.

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 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 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 AD5735 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 AD5735.

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.

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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