ADC08131 Datasheet, PDF (14/21 Pages) National Semiconductor (TI) – 8-Bit High-Speed Serial I/O A/D Converters with Multiplexer Options, Voltage Reference, and Track/Hold Function

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 de-
vices 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 ADC08131
8 Single-Ended
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 t_SELECT requirement is met. See Data Input Tim-
ing under Timing Diagrams. If another conversion is de-
sired 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
DS010749-41
8 Psuedo-Differential
DS010749-43
Mixed Mode
DS010749-42
FIGURE 1. Analog Input Multiplexer Options for the ADC08138
DS010749-44
REFERENCE CONSIDERATIONS
The V_REFIN pin on these converters is the top of a resistor
divider string and capacitor array used for the successive ap-
proximation conversion. The voltage applied to this refer-
ence input defines the voltage span of the analog input (the
difference between V_IN(MAX) and V_IN(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 kΩ.
For absolute accuracy, where the analog input varies be-
tween specific voltage limits, the reference input must be bi-
ased with a stable voltage source. The ADC08134 and the
ADC08138 provide the output of a 2.5V band-gap reference
at V_REFOUT. 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 V_REFIN 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 ap-
plications. Bypassing V_REFOUT with a 100 µF capacitor is
recommended.
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
V_REFC is provided for bypassing purposes and biasing exter-
nal circuits as suggested above.
The maximum value of the reference is limited to the V_CC
supply voltage. The minimum value, however, can be quite
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