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ADC0816CCV View Datasheet(PDF) - National ->Texas Instruments

Part NameDescriptionManufacturer
ADC0816CCV 8-Bit P Compatible A/D Converters with 16-Channel Multiplexer National-Semiconductor
National ->Texas Instruments National-Semiconductor
ADC0816CCV Datasheet PDF : 14 Pages
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Applications Information (Continued)
A good example of a ratiometric transducer is a potentiom-
eter 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 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
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
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 automaticaly sat-
isfied in ratiometric systems and can be easily met in ground
referenced systems.
Figure 10 shows a ground referenced system with a sepa-
rate 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
DS005277-11
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 millliamp 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 Figure 13, a 2.5V reference is symmetrically
centered about VCC/2 since the same current flows in iden-
tical 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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