AD8021 datasheet(19/32 Pages) AD

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AD5429/AD5439/AD5449

Rev. 0 | Page 19 of 32

REFERENCE SELECTION

When selecting a reference for use with the AD5429/AD5439/

AD5449 family of current output DACs, pay attention to the

reference’s output voltage temperature coefficient specification.

This parameter affects not only the full-scale error, but also

the linearity (INL and DNL) performance. The reference

temperature coefficient should be consistent with the system

accuracy specifications. For example, an 8-bit system required

to hold its overall specification to within 1 LSB over the

temperature range 0°C to 50°C dictates that the maximum

system drift with temperature should be less than 78 ppm/°C.

A 12-bit system with the same temperature range to overall

specification within 2 LSBs requires a maximum drift of

10 ppm/°C. By choosing a precision reference with low output

temperature coefficient, this error source can be minimized.

Table 7 lists some of the references available from Analog

Devices 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. The input offset voltage of an op amp is multiplied by

the variable gain (due to the code-dependent output resistance

of the DAC) 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, could cause the DAC to

be nonmonotonic. The input bias current of an op amp also

generates an offset at the voltage output as a result of the bias

current flowing in the feedback resistor RFB. Most op amps have

input bias currents low enough to prevent any significant errors

in 12-bit applications.

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. Most

op amps have adequate common-mode rejection for use at

8-, 10-, and 12-bit resolution.

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,

it is important to minimize capacitance at the VREF node

(voltage output node in this application) of the DAC. This is

done by using low input capacitance buffer amplifiers and

careful board design.

Most single-supply circuits include ground as part of the analog

signal range, which in turns requires an amplifier that can

handle rail-to-rail signals. Analog Devices supplies a large range

of single-supply amplifiers.

Table 7. Suitable ADI Precision References Recommended for Use with AD5429/AD5439/AD5449 DACs

Reference

Output Voltage

Initial Tolerance

Temperature Drift

0.1 Hz to 10 Hz Noise

Package

ADR01

10 V

0.1%

3 ppm/°C

20 µV p-p

SC70, TSOT, SOIC

ADR02

5 V

0.1%

3 ppm/°C

10 µV p-p

SC70, TSOT, SOIC

ADR03

2.5 V

0.2%

3 ppm/°C

10 µV p-p

SC70, TSOT, SOIC

ADR425

5 V

0.04%

3 ppm/°C

3.4 µV p-p

MSOP, SOIC

Table 8. Precision ADI Op Amps Suitable for Use with AD5429/AD5439/AD5449 DACs

Part No.

Max Supply Voltage (V)

VOS (max) µV

IB (max) nA

GBP MHz

Slew Rate V/µs

OP97

±20

0.1

0.9

0.2

OP1177

±18

1.3

0.7

AD8551

±6

0.05

1.5

0.4

Table 9. High Speed ADI Op Amps Suitable for Use with AD5429/AD5439/AD5449 DACs

Part No.

Max Supply Voltage (V)

BW @ ACL (MHz)

Slew Rate (V/µs)

VOS (max) µV

IB max (nA)

AD8065

±12

145

180

1500

0.01

AD8021

±12

200

100

1000

AD8038

±5

350

425

3000

0.75

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