ADC0808/ADC0809
TRI-STATE Test Circuits and Timing Diagrams
t
1H
, t
H1
t
0H
, t
H0
567218
567221
t
1H
, C
L
= 10 pF
t
0H
, C
L
= 10 pF
567219
567222
t
H1
, C
L
= 50 pF
t
H0
, C
L
= 50 pF
567220
567223
FIGURE 8.
Applications Information
OPERATION
1.0 RATIOMETRIC CONVERSION
The ADC0808, ADC0809 is designed as a complete Data
Acquisition System (DAS) for ratiometric conversion systems.
In ratiometric systems, the physical variable being measured
is expressed as a percentage of full-scale which is not nec-
essarily related to an absolute standard. The voltage input to
the ADC0808 is expressed by the equation
the transducers can be connected directly across the supply
and their outputs connected directly into the multiplexer in-
puts, (Figure
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 absolute
standard such as voltage or current. This means a system
reference must be used which relates the full-scale voltage to
the standard volt. For example, if V
CC
=V
REF
=5.12V, then the
full-scale range is divided into 256 standard steps. The small-
est standard step is 1 LSB which is then 20 mV.
2.0 RESISTOR LADDER LIMITATIONS
The voltages from the resistor ladder are compared to the
selected into 8 times in a conversion. These voltages are
coupled to the comparator via an analog switch tree which is
referenced to the supply. The voltages at the top, center and
bottom of the ladder must be controlled to maintain proper
operation.
The top of the ladder, Ref(+), should not be more positive than
the supply, and the bottom of the ladder, Ref(−), should not
be more negative than ground. The center of the ladder volt-
age must also be near the center of the supply because the
analog switch tree changes from N-channel switches to P-
channel switches. These limitations are automatically satis-
fied in ratiometric systems and can be easily met in ground
referenced systems.
(1)
V
IN
= Input voltage into the ADC0808
V
fs
= Full-scale voltage
V
Z
= Zero voltage
D
X
= Data point being measured
D
MAX
= Maximum data limit
D
MIN
= Minimum data limit
A good example of a ratiometric transducer is a potentiometer
used as a position sensor. The position of the wiper is directly
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 supply range so
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