AD768AR datasheet(7/20 Pages) AD

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AD768

REV. B

–7–

REFERENCE OUTPUT

The internal 2.5 V bandgap reference is provided for generation

of the IREFIN current, and must be compensated externally with

a capacitor of 0.1

µF or greater from REFOUT to REFCOM. If

an external reference is used, REFOUT should be tied directly

to the positive supply voltage, VDD. This effectively turns off the

internal reference, eliminating the need for the external capaci-

tor at REFOUT. The reference is specified to drive a nominal

load of 5 mA with a maximum of 15 mA. Operation with a

heavier load will result in degradation of supply rejection and

reference voltage accuracy. Therefore, the reference output

should be buffered with an amplifier when additional load cur-

rent is required. A properly sized pull-up resistor can also be

used to source additional current to the load. The resistors value

should be selected such that REFOUT will always source a

minimum of 5 mA to IREFIN and the additional load.

AD768

IREFIN

REFOUT

REFCOM

CREFCOMP

1µF

500

Ω

Figure 6. Typical Reference Hookup

TEMPERATURE CONSIDERATIONS

Note that the reference plays a key role in the overall tempera-

ture performance of the AD768. Any drift of IREFIN shows up

directly in IOUT. When the output is taken as a current, the drift

of IREFIN (which depends on both VREF and RREF) must be mini-

mized. This can be done by using the internal temperature com-

pensated reference for VREF and a low temperature coefficient

resistor for RREF. If the output is taken as a voltage, it is a func-

tion of a resistor ratio, not an absolute resistor value. By select-

ing resistors with matched temperature coefficients for RREF

and RLOAD, the drift in the resistor values will cancel, providing

optimal drift performance.

REFERENCE NOISE REDUCTION AND MULTIPLYING

BANDWIDTH

For application flexibility and multiplying capabilities, the refer-

ence amplifier is designed to offer adjustable bandwidth that can

be reduced by connecting an external capacitor from the NR

node to the negative supply pin, VEE. This capacitor limits the

bandwidth and acts as a filter to reduce the noise contribution

from the reference amplifier.

The noise reduction capacitor, CNR, is not required for stability

and does not affect the settling time of the DAC output. With-

out this capacitor, the IREFIN bandwidth is 15 MHz allowing

high frequency modulation of the DAC full-scale range through

the reference input node. Figure 7 shows the relationship be-

tween the external noise reduction capacitor and the –3 dB

bandwidth of the reference amplifier.

NOISE REDUCTION CAPACITOR – F

100M

10p

1µ

100p

10n

100n

10M

100k

10k

Figure 7. External Noise Reduction Capacitor vs. –3 dB

Bandwidth

The sensitivity of the NR node requires that care be taken in

capacitor placement. The capacitor should be located as physi-

cally close to the package pins as possible and lead lengths

should be minimized. For this purpose, the use of a chip

capacitor is recommended. For applications that do not require

high frequency modulation at IREFIN, it is recommended that

a capacitor on the order of 1

µF be connected from NR to V

EE.

If the reference input is purely dc, noise may be minimized with

multiple capacitors, such as 1

µF and 0.1 µF, to more effectively

filter both high and low frequency disturbances.

ANALOG OUTPUTS

The AD768 offers two analog outputs; IOUTA is trimmed for

optimal INL and DNL performance and has a full-scale output

when all bits are high. For applications that require the specified

dc accuracy, IOUTA should be used. IOUTB is the comple-

mentary output with full-scale output when all bits are low.

Both IOUTA and IOUTB provide similar dynamic perfor-

mance. Refer to Figures 8 and 9 for typical INL and DNL per-

formance curves. The outputs can also be used differentially.

Refer to the section “Applying the AD768” for examples of vari-

ous output configurations.

DIGITAL INPUT CODE – k

–8

065

–2

–4

–6

152535

4555

Figure 8. Typical INL Performance

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