Typical Performance Characteristics
DS005672-16
FIGURE 6. Comparator IIN vs VIN
=
=
(VCC VREF 5V)
DS005672-17
FIGURE 7. Multiplexer RON vs VIN
=
=
(VCC VREF 5V)
TRI-STATE Test Circuits and Timing Diagrams
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t1H, tH1
=
t1H, CL 10 pF
tH1, CL 50 pF
DS005672-18
DS005672-20
DS005672-19
=
t0H, tH0
t0H, CL 10 pF
=
tH0, CL 50 pF
DS005672-23
DS005672-22
DS005672-21
FIGURE 8.
=
DX Data point being measured
Applications Information
OPERATION
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DMAX Maximum data limit
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DMIN Minimum data limit
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).
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)
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-
=
VIN Input voltage into the ADC0808
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Vfs Full-scale voltage
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VZ Zero voltage
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