AD5744CSU datasheet(23/27 Pages) AD

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Preliminary Technical Data

Rev. PrA 15-Nov-04| Page 23 of 27

APPLICATIONS INFORMATION

TYPICAL OPERATING CIRCUIT

Figure 11 shows the typical operating circuit for the

AD5744/64. The only external components needed for this

precision 14/16-bit DAC are decoupling capacitors on the

supply pins, R-C connection from REFOUT to REFAB and

REFCD and a short circuit current setting resistor. Because the

device incorporates a voltage reference, and reference buffers, it

eliminates the need for an external bipolar reference and

associated buffers. This leads to an overall saving in both cost

and board space.

In the circuit below, VDD and VSS are both connected to ±15 V,

but VDD and VSS can operate with supplies from ±11.4 V to

±16.5 V. In Figure 11, AGNDA is connected to REFGND, but

the option of Force/Sense is included on this device, if required

by the user.

10 11

13 14 15

31 30

29 28 27 26

AD5744/64

SYNC

SCLK

SDIN

SDO

LDAC

CLR

VOUTA

VOUTB

AGNDB

VOUTD

VOUTC

AGNDC

AGNDA

AGNDD

SYNC

SCLK

SDIN

SDO

LDAC

RSTOUT

RSTIN

BIN/2SCOMP

+5V

+5V +15V -15V

+15V -15V

VOUTA

VOUTB

VOUTC

VOUTD

Ω

100 nF

10 µF

100 nF

10 µF

TEMP

Figure 11. Typical operating circuit

Precision Voltage Reference Selection

To achieve the optimum performance from the AD5744/64 over

it’s full operating temperature range an external voltage

reference must be used. Thought should be given to the

selection of a precision voltage reference. The AD5744/64 has

two reference inputs, REFAB and REFCD. The voltages applied

to the reference inputs are used tomprovide a buffered positiver

and negative 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

reference could lead to a full-scale error in the DAC. Therefore,

to minimize these errors, a reference with low initial accuracy

error specification is preferred. Also, 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

adjustment can also be used at 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 lon-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

tempaerature coefficient specifiaction should be chosen to

reduce the dependence of the DAC output voltage on ambient

conditions.

In high accuracy applications, which have a relatively low noise

budget, reference output voltage noise needs to be considered.

Choosing a reference waith 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 Hx to 10 Hz region.

However, as the circuit bandwidth increases, filtering the output

of the reference may be required to minimise the output noise.

Table 20. Partial List of Precision References Recommended for

use with the AD5744/64

Part No.

Initial

Accuracy

(mV max)

Long-Term

Drift

(ppm typ)

Temp Drift

(ppm/

°C max)

0.1 Hz to

10 Hz Noise

(µV p-p typ)

ADR435

± 6

3.4

ADR425

± 6

3.4

ADR02

± 5

ADR395

± 6

AD586

± 2.5

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