SNAS535H – OCTOBER 1999 – REVISED MARCH 2013
APPLICATIONS INFORMATION
OPERATION
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 necessarily related to an absolute standard. The voltage input to the ADC0808 is expressed by the
equation
•
•
•
•
•
•
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
(1)
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 the transducers can be connected directly across the supply and
their outputs connected directly into the multiplexer inputs, (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 smallest standard step is 1 LSB which is then 20 mV.
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 voltage 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 satisfied in ratiometric systems and can be easily met in ground referenced systems.
shows a ground referenced system with a separate supply and reference. In this system, the supply
must be trimmed to match the reference voltage. For instance, if a 5.12V is used, the supply should be adjusted
to the same voltage within 0.1V.
Figure 11. Ratiometric Conversion System
10
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