ADC1038CIWM datasheet(11/13 Pages) NSC

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3.0 Analog Considerations (Continued)

3.3 REFERENCE AND INPUT

The two V

REF inputs are fully differential and define the zero

to full-scale input range of the A to D converter. This allows

the designer to easily vary the span of the analog input since

this range will be equivalent to the voltage difference be-

tween V

REF+ and VREF−. By reducing VREF (VREF = VREF+−

REF−) to less than 5V, the sensitivity of the converter can be

increased (i.e., if V

REF = 2V then 1 LSB = 1.95 mV). The

input/reference arrangement also facilitates ratiometric op-

eration and in many cases the chip power supply can be

used for transducer power as well as the V

REF source.

This reference flexibility lets the input span not only be varied

but also offset from zero. The voltage at V

REF− sets the input

level which produces a digital output of all zeros. Though V

is not itself differential, the reference design allows nearly

differential-input capability for many measurement applica-

tions.

Figure 3 shows some of the configurations that are

possible.

Power Supply Bypassing

External Reference 2.5V Full Scale

DS010556-20

Power Supply as Reference

DS010556-21

Input Not Referred to GND

DS010556-22

Note 14: *Current path must still exist from VIN(−) to ground

FIGURE 3. Analog Input Options

DS010556-23

DS010556-24

ADC1038CIWM Datasheet(PDF) 11 Page - National Semiconductor (TI)