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AD5764

Preliminary Technical Data

Rev. PrC 21-Oct-04| Page 24 of 28

APPLICATIONS INFORMATION

TYPICAL OPERATING CIRCUIT

Figure ?? shows the typical operating circuit for the AD5764.

The only external components needed for this precision 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 conneted to ±15 V,

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

±16.5 V. In Figure ??, 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

AD5764

/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

Force/Sense of AGND

Because of the extremelyhigh accuracy of this device, system

design issues such as grounding and contact resistance are very

important. The AD5764, with ±10 V output, has an LSB size of

305 µV. Therefore, series wiring and connector resistances of

very small values could cause voltage drops of an LSB. For this

reason, the AD5764 offers a Force/Sense output configuration.

Figure ?? shows how to connect the AD5764 to the Force/Sense

amplifier. Where accuracy of the output is important, an

amplifier such as the OP177 is ideal. The OP177 is ultraprecise

with offset voltages of 10 µV maximum at room temperature

and offset drift 0f 0.1 µV/°C maximum. Alternative

recommended amplifiers are the OP1177 and the OP77. For

applications where optimization of the circuit for settling time is

needed, the AD845 is recommended.

Figure ??. Driving REFGND and AGNDA using a force/sense

amplifier

Precision Voltage Reference Selection

To achieve the optimum performance from the AD5764 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 AD5764 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.

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