ADC08031CIWM datasheet(13/24 Pages) NSC

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Functional Description (Continued)

3.0 REFERENCE CONSIDERATIONS

The voltage applied to the reference input on these convert-

ers, V

REFIN, defines the voltage span of the analog input

(the difference between V

IN(MAX) and VIN(MIN) over which the

256 possible output codes apply. The devices can be used

either in ratiometric applications or in systems requiring ab-

solute accuracy. The reference pin must be connected to a

voltage source capable of driving the reference input resis-

tance which can be as low as 1.3k

Ω. This pin is the top of a

resistor divider string and capacitor array used for the suc-

cessive approximation conversion.

In a ratiometric system the analog input voltage is propor-

tional to the voltage used for the A/D reference. This voltage

is typically the system power supply, so the V

REFIN pin can

be tied to V

CC. This technique relaxes the stability require-

ments of the system reference as the analog input and A/D

reference move together maintaining the same output code

for a given input condition.

For absolute accuracy, where the analog input varies be-

tween very specific voltage limits, the reference pin can be

biased with a time and temperature stable voltage source.

For the ADC08034 and the ADC08038 a band-gap derived

reference voltage of 2.6V (Note 8) is tied to V

REFOUT. This

can be tied back to V

REFIN. Bypassing VREFOUT with a

100µF capacitor is recommended. The LM385 and LM336

reference diodes are good low current devices to use with

these converters.

The maximum value of the reference is limited to the V

supply voltage. The minimum value, however, can be quite

small (see Typical Performance Characteristics) to allow di-

rect conversions of transducer outputs providing less than a

5V output span. Particular care must be taken with regard to

noise pickup, circuit layout and system error voltage sources

when operating with a reduced span due to the increased

sensitivity of the converter (1 LSB equals V

REF/256).

8 Single-Ended

DS010555-48

8 Pseudo-Differential

DS010555-49

4 Differential

DS010555-50

Mixed Mode

DS010555-51

FIGURE 1. Analog Input Multiplexer Options for the ADC08038

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