ADC0808 National Semiconductor, ADC0808 Datasheet - Page 8

• Manufacturer Part Number: ADC0808
• Description: 8-Bit uP Compatible A/D Converters with 8-Channel Multiplexer
• Manufacturer: National Semiconductor

www.national.com
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
V
V
V
IN
fs
Z
= Zero voltage
= Full-scale voltage
= Input voltage into the ADC0808
FIGURE 6. Comparator I
t
t
1H
0H
, t
, t
H1
H0
(V
CC
DS005672-21
= V
DS005672-18
REF
= 5V)
IN
DS005672-16
vs V
IN
t
t
(1)
1H
0H
FIGURE 8.
, C
, C
L
L
8
= 10 pF
= 10 pF
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-
D
D
D
X
MAX
MIN
= Data point being measured
DS005672-19
DS005672-22
= Minimum data limit
= Maximum data limit
FIGURE 7. Multiplexer R
(V
CC
= V
REF
t
t
= 5V)
H1
H0
, C
, C
ON
L
L
= 50 pF
= 50 pF
vs V
DS005672-17
DS005672-20
DS005672-23
IN