AD8021 datasheet(15/24 Pages) AD

15 / 24 page

Data Sheet

AD5545/AD5555

Rev. G | Page 15 of 24

REFERENCE SELECTION

When selecting a reference for use with the AD55xx series

of current output DACs, pay attention to the output voltage,

temperature coefficient specification of the reference. Choosing

a precision reference with a low output temperature coefficient

minimizes error sources. Table 10 lists some of the references

available from Analog Devices, Inc., that are suitable for use

with this range of current output DACs.

AMPLIFIER SELECTION

The primary requirement for the current-steering mode is an

amplifier with low input bias currents and low input offset voltage.

Because of the code-dependent output resistance of the DAC,

the input offset voltage of an op amp is multiplied by the variable

gain of the circuit. A change in this noise gain between two

adjacent digital fractions produces a step change in the output

voltage due to the amplifier’s input offset voltage. This output

voltage change is superimposed upon the desired change in

output between the two codes and gives rise to a differential

linearity error, which, if large enough, can cause the DAC to be

nonmonotonic.

The input bias current of an op amp also generates an offset at

the voltage output because of the bias current flowing in the

feedback resistor, RFB.

Common-mode rejection of the op amp is important in voltage-

switching circuits because it produces a code-dependent error

at the voltage output of the circuit.

Provided that the DAC switches are driven from true wideband

low impedance sources (VIN and AGND), they settle quickly.

Consequently, the slew rate and settling time of a voltage-switching

DAC circuit is determined largely by the output op amp. To obtain

minimum settling time in this configuration, minimize capacitance

at the VREF node (the voltage output node in this application) of

the DAC. This is done by using low input capacitance buffer

amplifiers and careful board design.

Analog Devices offers a wide range of amplifiers for both precision

dc and ac applications, as listed in Table 11 and Table 12.

Table 10. Suitable Analog Devices Precision References

Part No.

Output Voltage (V)

Initial Tolerance (%)

Maximum Temperature

Drift (ppm/°C)

ISS (mA)

Output Noise (µV p-p)

Package(s)

ADR01

0.05

SOIC-8

ADR01

0.05

TSOT-5, SC70-5

ADR02

5.0

0.06

SOIC-8

ADR02

5.0

0.06

TSOT-5, SC70-5

ADR03

2.5

0.1

SOIC-8

ADR03

2.5

0.1

TSOT-5, SC70-5

ADR06

3.0

0.1

SOIC-8

ADR06

3.0

0.1

TSOT-5, SC70-5

ADR420

2.048

0.05

0.5

1.75

SOIC-8, MSOP-8

ADR421

2.50

0.04

0.5

1.75

SOIC-8, MSOP-8

ADR423

3.00

0.04

0.5

SOIC-8, MSOP-8

ADR425

5.00

0.04

0.5

3.4

SOIC-8, MSOP-8

ADR431

2.500

0.04

0.8

3.5

SOIC-8, MSOP-8

ADR435

5.000

0.04

0.8

SOIC-8, MSOP-8

ADR391

2.5

0.16

0.12

TSOT-5

ADR395

5.0

0.10

0.12

TSOT-5

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