AD7846AP datasheet(18/24 Pages) AD

18 / 24 page

AD7846

Rev. G | Page 18 of 24

APPLICATION HINTS

NOISE

In high resolution systems, noise is often the limiting factor.

With a 10 V span, a 16-bit LSB is 152 μV (–96 dB). Thus, the

noise floor must stay below −96 dB in the frequency range of

interest. Figure 12 shows the noise spectral density for the

AD7846.

GROUNDING

As well as noise, the other prime consideration in high resolution

DAC systems is grounding. With an LSB size of 152 μV and a

load current of 5 mA, 1 LSB of error can be introduced by series

resistance of only 0.03 Ω.

Figure 30 shows recommended grounding for the AD7846 in a

typical application.

ANALOG SUPPLY

DIGITAL SUPPLY

–15V

+15V 0V

DGND

+5V

SIGNAL

GROUND

AD7846*

AD588*

VOUT

(+5V TO –5V)

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 30. AD7846 Grounding

R1 to R5 represent lead and track resistances on the printed

circuit board. R1 is the resistance between the analog power

supply ground and the signal ground. Because current flowing

in R1 is very low (bias current of AD588 sense amplifier), the

effect of R1 is negligible. R2 and R3 represent track resistance

between the AD588 outputs and the AD7846 reference inputs.

Because of the force and sense outputs on the AD588, these

resistances will also have a negligible effect on accuracy.

R4 is the resistance between the DAC output and the load. If RL

is constant, then R4 introduces a gain error only that can be

trimmed out in the calibration cycle. R5 is the resistance

between the load and the analog common. If the output voltage

is sensed across the load, R5 introduces a further gain error,

which can be trimmed out. If, on the other hand, the output

voltage is sensed at the analog supply common, R5 appears as

part of the load and therefore introduces no errors.

PRINTED CIRCUIT BOARD LAYOUT

Figure 31 shows the AD7846 in a typical application with the

AD588 reference, producing an output analog voltage in the

±10 V range. Full-scale and bipolar zero adjustment are

provided by Potentiometer R2 and Potentiometer R3. Latches

(2 × 74LS245) isolate the DAC digital inputs from the active

microprocessor bus and minimize digital feedthrough.

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