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ADC0844 Просмотр технического описания (PDF) - National ->Texas Instruments

Номер в каталогеADC0844 National-Semiconductor
National ->Texas Instruments National-Semiconductor
Компоненты Описание8-Bit μP Compatible A/D Converters with Multiplexer Options
ADC0844 Datasheet PDF : 24 Pages
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4 Single-Ended
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2 Differential
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3 Pseudo-Differential
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Combined
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FIGURE 1. Analog Input Multiplexer Options
2.0 REFERENCE CONSIDERATIONS
The voltage applied to the reference input of these converters
defines the voltage span of the analog input (the difference
between VIN(MAX) and VIN(MIN)) over which the 256 possible
output codes apply. The devices can be used in either ratio-
metric applications or in systems requiring absolute accuracy.
The reference pin must be connected to a voltage source ca-
pable of driving the minimum reference input resistance of 1.1
kΩ. This pin is the top of a resistor divider string used for the
successive approximation conversion.
In a ratiometric system (Figure 2a), the analog input voltage
is proportional to the voltage used for the A/D reference. This
voltage is typically the system power supply, so the VREF pin
can be tied to VCC. This technique relaxes the stability re-
quirements of the system reference as the analog input and
A/D reference move together maintaining the same output
code for a given input condition. For absolute accuracy (Fig-
ure 2b), where the analog input varies between very specific
voltage limits, the reference pin can be biased with a time and
temperature stable voltage source. The LM385 and LM336
reference diodes are good low current devices to use with
these converters.
The maximum value of the reference is limited to the VCC
supply voltage. The minimum value, however, can be quite
small (see Typical Performance Characteristics) to allow di-
rect conversions of transducer outputs providing less than a
5V output span. Particular care must be taken with regard to
noise pickup, circuit layout and system error voltage sources
when operating with a reduced span due to the increased
sensitivity of the converter (1 LSB equals VREF/256).
11
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