ADC08138CIWM datasheet(14/21 Pages) NSC

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Functional Description (Continued)

The stored data in the successive approximation register

is loaded into an internal shift register. If the programmer

prefers the data can be provided in an LSB first format

[this makes use of the shift enable (SE) control line]. On

the ADC08138 the SE line is brought out and if held high

the value of the LSB remains valid on the DO line. When

SE is forced low the data is clocked out LSB first. On de-

vices which do not include the SE control line, the data,

LSB first, is automatically shifted out the DO line after

the MSB first data stream. The DO line then goes low

and stays low until CS is returned high. The ADC08131

is an exception in that its data is only output in MSB first

format.

All internal registers are cleared when the CS line is high

and the t

SELECT requirement is met. See Data Input Tim-

ing under Timing Diagrams. If another conversion is de-

sired CS must make a high to low transition followed by

address information.

The DI and DO lines can be tied together and controlled

through a bidirectional processor I/O bit with one wire.

This is possible because the DI input is only “looked-at”

during the MUX addressing interval while the DO line is

still in a high impedance state.

REFERENCE CONSIDERATIONS

The V

REFIN pin on these converters is the top of a resistor

divider string and capacitor array used for the successive ap-

proximation conversion. The voltage applied to this refer-

ence input 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 reference source must be

capable of driving the reference input resistance, which can

be as low as 1.3 k

Ω.

For absolute accuracy, where the analog input varies be-

tween specific voltage limits, the reference input must be bi-

ased with a stable voltage source. The ADC08134 and the

ADC08138 provide the output of a 2.5V band-gap reference

at V

REFOUT. This voltage does not vary appreciably with

temperature, supply voltage, or load current (see Reference

Characteristics in the Electrical Characteristics tables) and

can be tied directly to V

REFIN for an analog input span of 0V

to 2.5V. This output can also be used to bias external circuits

and can therefore be used as the reference in ratiometric ap-

plications. Bypassing V

REFOUT with a 100 µF capacitor is

recommended.

For the ADC08131, the output of the on-board reference is

internally tied to the reference input. Consequently, the ana-

log input span for this device is set at 0V to 2.5V. The pin

REFC is provided for bypassing purposes and biasing exter-

nal circuits as suggested above.

The maximum value of the reference is limited to the V

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

8 Single-Ended

DS010749-41

4 Differential

DS010749-43

8 Psuedo-Differential

DS010749-42

Mixed Mode

DS010749-44

FIGURE 1. Analog Input Multiplexer Options for the ADC08138

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