ADC0805 NSC [National Semiconductor], ADC0805 Datasheet - Page 21

• Manufacturer Part Number: ADC0805
• Description: 8-Bit uP Compatible A/D Converters
• Manufacturer: NSC [National Semiconductor]

Functional Description
100
capacitor — both the R and C are placed outside the feed-
back loop — from the output of an op amp, if used.
2.3.4 Noise
The leads to the analog inputs (pins 6 and 7) should be kept
as short as possible to minimize input noise coupling. Both
noise and undesired digital clock coupling to these inputs
can cause system errors. The source resistance for these in-
puts should, in general, be kept below 5 k . Larger values of
source resistance can cause undesired system noise
pickup. Input bypass capacitors, placed from the analog in-
puts to ground, will eliminate system noise pickup but can
create analog scale errors as these capacitors will average
the transient input switching currents of the A/D (see section
2.3.1.). This scale error depends on both a large source re-
sistance and the use of an input bypass capacitor. This error
can be eliminated by doing a full-scale adjustment of the A/D
(adjust V
2.5.2 on Full-Scale Adjustment) with the source resistance
and input bypass capacitor in place.
2.4 Reference Voltage
2.4.1 Span Adjust
For maximum applications flexibility, these A/Ds have been
designed to accommodate a 5 V
voltage reference. This has been achieved in the design of
the IC as shown in Figure 6 .
FIGURE 6. The V
series resistor can be used to isolate this
REF
/2 for a proper full-scale reading — see section
REFERENCE
DC
, 2.5 V
Design on the IC
(Continued)
DC
DS005671-15
or an adjusted
21
Notice that the reference voltage for the IC is either
voltage applied to the V
age that is externally forced at the V
a ratiometric voltage reference using the V
V
voltage less than 2.5 V
for increased application flexibility. The internal gain to the
V
age twice the voltage at pin 9.
An example of the use of an adjusted reference voltage is to
accommodate a reduced span — or dynamic voltage range
of the analog input voltage. If the analog input voltage were
to range from 0.5 V
span would be 3V as shown in Figure 7 . With 0.5 V
plied to the V
age can be made equal to
A/D now will encode the V
the 0.5V input corresponding to zero and the 3.5 V
corresponding to full-scale. The full 8 bits of resolution are
therefore applied over this reduced analog input voltage
range.
2.4.2 Reference Accuracy Requirements
The converter can be operated in a ratiometric mode or an
absolute mode. In ratiometric converter applications, the
magnitude of the reference voltage is a factor in both the out-
put of the source transducer and the output of the A/D con-
verter and therefore cancels out in the final digital output
code. The ADC0805 is specified particularly for use in ratio-
metric applications with no adjustments required. In absolute
conversion applications, both the initial value and the tem-
perature stability of the reference voltage are important fac-
tors in the accuracy of the A/D converter. For V
ages of 2.4 V
cause conversion errors of
V
and the stability of the V
more important. For example, if the span is reduced to 2.5V,
the analog input LSB voltage value is correspondingly re-
duced from 20 mV (5V span) to 10 mV and 1 LSB at the
V
the allowed initial tolerance of the reference voltage and re-
quires correspondingly less absolute change with tempera-
ture variations. Note that spans smaller than 2.5V place
even tighter requirements on the initial accuracy and stability
of the reference source.
In general, the magnitude of the reference voltage will re-
quire an initial adjustment. Errors due to an improper value
of reference voltage appear as full-scale errors in the A/D
transfer function. IC voltage regulators may be used for ref-
erences if the ambient temperature changes are not exces-
sive. The LM336B 2.5V IC reference diode (from National
Semiconductor) has a temperature stability of 1.8 mV typ
(6 mV max) over 0˚C T
parts are also available.
DC
REF
REF
REF
reference voltage can be used for the V
/2 input is 2, making the full-scale differential input volt-
/2 input. In reduced span applications, the initial value
/2 input becomes 5 mV. As can be seen, this reduces
IN
DC
(−) pin to absorb the offset, the reference volt-
nominal value, initial errors of
DC
to 3.5 V
DC
CC
A
REF
can be applied to the V
IN
1
supply pin, or is equal to the volt-
±
+70˚C. Other temperature range
⁄
(+) signal from 0.5V to 3.5 V with
2
1 LSB due to the gain of 2 of the
/2 input voltage become even
of the 3V span or 1.5 V
DC
, instead of 0V to 5 V
REF
/2 pin. This allows for
CC
CC
±
10 mV
supply, a 5
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supply or a
REF
REF
1
DC
DC
⁄
/2 input
2
/2 volt-
DC
DC
DC
of the
. The
input
, the
ap-
will