ADC08034 National Semiconductor, ADC08034 Datasheet - Page 13

• Manufacturer Part Number: ADC08034
• Description: 8-Bit High-Speed Serial I/O A/D Converters with Multiplexer Options/ Voltage Reference/ and Track/Hold Function
• Manufacturer: National Semiconductor

Functional Description
3.0 REFERENCE CONSIDERATIONS
The voltage applied to the reference input on these convert-
ers, V
(the difference between V
256 possible output codes apply. The devices can be used
either in ratiometric applications or in systems requiring ab-
solute accuracy. The reference pin must be connected to a
voltage source capable of driving the reference input resis-
tance which can be as low as 1.3k . This pin is the top of a
resistor divider string and capacitor array used for the suc-
cessive approximation conversion.
In a ratiometric system the analog input voltage is propor-
tional to the voltage used for the A/D reference. This voltage
is typically the system power supply, so the V
be tied to V
ments of the system reference as the analog input and A/D
reference move together maintaining the same output code
for a given input condition.
REF
IN, defines the voltage span of the analog input
CC
. This technique relaxes the stability require-
8 Single-Ended
4 Differential
IN(MAX)
FIGURE 1. Analog Input Multiplexer Options for the ADC08038
and V
DS010555-48
DS010555-50
IN(MIN)
(Continued)
over which the
REF
IN pin can
13
For absolute accuracy, where the analog input varies be-
tween very specific voltage limits, the reference pin can be
biased with a time and temperature stable voltage source.
For the ADC08034 and the ADC08038 a band-gap derived
reference voltage of 2.6V (Note 8) is tied to V
can be tied back to V
100µF capacitor is recommended. 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 V
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 V
8 Pseudo-Differential
Mixed Mode
REF
IN. Bypassing V
DS010555-49
DS010555-51
REF/
REF
REF
OUT with a
256).
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OUT. This
CC