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ADC08131 Просмотр технического описания (PDF) - National ->Texas Instruments

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ADC08131 8-Bit High-Speed Serial I/O A/D Converters with Multiplexer Options, Voltage Reference, and Track/Hold Function National-Semiconductor
National ->Texas Instruments National-Semiconductor
ADC08131 Datasheet PDF : 22 Pages
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Functional Description (Continued)
7. The stored data in the successive approximation register
is loaded into an internal shift register. If the programmer
prefers the data can be provided in an LSB first format
[this makes use of the shift enable (SE) control line]. On
the ADC08138 the SE line is brought out and if held high
the value of the LSB remains valid on the DO line. When
SE is forced low the data is clocked out LSB first. On
devices which do not include the SE control line, the
data, LSB first, is automatically shifted out the DO line
after the MSB first data stream. The DO line then goes
low and stays low until CS is returned high. The
8 Single-Ended
ADC08131 is an exception in that its data is only output
in MSB first format.
8. All internal registers are cleared when the CS line is high
and the tSELECT requirement is met. See Data Input
Timing under Timing Diagrams. If another conversion is
desired CS must make a high to low transition followed
by address information.
The DI and DO lines can be tied together and controlled
through a bidirectional processor I/O bit with one wire.
This is possible because the DI input is only “looked-at”
during the MUX addressing interval while the DO line is
still in a high impedance state.
4 Differential
8 Psuedo-Differential
Mixed Mode
FIGURE 1. Analog Input Multiplexer Options for the ADC08138
The VREFIN pin on these converters is the top of a resistor
divider string and capacitor array used for the successive
approximation conversion. The voltage applied to this refer-
ence input 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 reference source must be
capable of driving the reference input resistance, which can
be as low as 1.3 k.
For absolute accuracy, where the analog input varies be-
tween specific voltage limits, the reference input must be
biased with a stable voltage source. The ADC08134 and the
ADC08138 provide the output of a 2.5V band-gap reference
at VREFOUT. This voltage does not vary appreciably with
temperature, supply voltage, or load current (see Reference
Characteristics in the Electrical Characteristics tables) and
can be tied directly to VREFIN for an analog input span of 0V
to 2.5V. This output can also be used to bias external circuits
and can therefore be used as the reference in ratiometric
applications. Bypassing VREFOUT with a 100 µF capacitor is
For the ADC08131, the output of the on-board reference is
internally tied to the reference input. Consequently, the ana-
log input span for this device is set at 0V to 2.5V. The pin
VREFC is provided for bypassing purposes and biasing ex-
ternal circuits as suggested above.
The maximum value of the reference is limited to the VCC
supply voltage. The minimum value, however, can be quite
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