Typical Performance Characteristics

TRI-STATE Test Circuits and Timing Diagrams

Applications Information

OPERATION

1.0 RATIOMETRIC CONVERSION

The ADC0808, ADC0809 is designed as a complete Data

Acquisition System (DAS) for ratiometric conversion sys-

tems. In ratiometric systems, the physical variable being

measured is expressed as a percentage of full-scale which is

not necessarily related to an absolute standard. The voltage

input to the ADC0808 is expressed by the equation

(1)

V

V

V

D

D

D

A good example of a ratiometric transducer is a potentiom-

eter used as a position sensor. The position of the wiper is di-

rectly proportional to the output voltage which is a ratio of the

full-scale voltage across it. Since the data is represented as

a proportion of full-scale, reference requirements are greatly

reduced, eliminating a large source of error and cost for

many applications. A major advantage of the ADC0808,

ADC0809 is that the input voltage range is equal to the sup-

ply range so the transducers can be connected directly

across the supply and their outputs connected directly into

the multiplexer inputs, (

Figure 9).

Ratiometric transducers such as potentiometers, strain

gauges, thermistor bridges, pressure transducers, etc., are

suitable for measuring proportional relationships; however,

many types of measurements must be referred to an abso-

lute standard such as voltage or current. This means a sys-

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FIGURE 6. Comparator I

(V

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FIGURE 7. Multiplexer R

(V

t

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t

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t

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t

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t

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t

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FIGURE 8.

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