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ADC0816 Просмотр технического описания (PDF) - National ->Texas Instruments

Номер в каталогеADC0816 National-Semiconductor
National ->Texas Instruments National-Semiconductor
Компоненты Описание8-Bit μP Compatible A/D Converters with 16-Channel Multiplexer
ADC0816 Datasheet PDF : 14 Pages
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Applications Information
OPERATION
1.0 Ratiometric Conversion
The ADC0816, ADC0817 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 ADC0816 is expressed by the equation
(1)
VIN = Input voltage into the ADC0816
Vfs = Full-scale voltage
VZ = Zero voltage
DX = Data point being measured
DMAX = Maximum data limit
DMIN = 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 ADC0816, ADC0817
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 in-
puts, (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 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 VCC = VREF = 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.
2.0 Resistor Ladder Limitations
The voltages from the resistor ladder are compared to the
selected input 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.
Figure 10 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 reference is used, the supply should be adjusted to the
same voltage within 0.1V.
FIGURE 9. Ratiometric Conversion System
527711
The ADC0816 needs less than a milliamp of supply current
so developing the supply from the reference is readily ac-
complished. In Figure 11 a ground references system is
shown which generates the supply from the reference. The
buffer shown can be an op amp of sufficient drive to supply
the milliamp of supply current and the desired bus drive, or if
a capacitive bus is driven by the outputs a large capacitor will
supply the transient supply current as seen in Figure 12. The
LM301 is overcompensated to insure stability when loaded by
the 10 μF output capacitor.
The top and bottom ladder voltages cannot exceed VCC and
ground, respectively, but they can be symmetrically less than
VCC and greater than ground. The center of the ladder voltage
should always be near the center of the supply. The sensitivity
of the converter can be increased, (i.e., size of the LSB steps
decreased) by using a symmetrical reference system. In Fig-
ure 13, a 2.5V reference is symmetrically centered about
VCC/2 since the same current flows in identical resistors. This
system with a 2.5V reference allows the LSB to be half the
size of the LSB in a 5V reference system.
9
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