Functional Description (Continued)
Notice that the reference voltage for the IC is either 1
⁄ of the
2
voltage applied to the VCC supply pin, or is equal to the volt-
age that is externally forced at the VREF/2 pin. This allows for
a ratiometric voltage reference using the VCC supply, a 5
VDC reference voltage can be used for the VCC supply or a
voltage less than 2.5 VDC can be applied to the VREF/2 input
for increased application flexibility. The internal gain to the
100Ω series resistor can be used to isolate this
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 kΩ. 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 VREF/2 for a proper full-scale reading — see section
2.5.2 on Full-Scale Adjustment) with the source resistance
and input bypass capacitor in place.
V
REF/2 input is 2, making the full-scale differential input volt-
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 VDC to 3.5 VDC, instead of 0V to 5 VDC, the
span would be 3V as shown in Figure 7. With 0.5 VDC ap-
plied to the VIN(−) pin to absorb the offset, the reference volt-
age can be made equal to 1
⁄ of the 3V span or 1.5 VDC. The
2
A/D now will encode the VIN(+) signal from 0.5V to 3.5 V with
the 0.5V input corresponding to zero and the 3.5 VDC input
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 VREF/2 volt-
2.4 Reference Voltage
2.4.1 Span Adjust
For maximum applications flexibility, these A/Ds have been
designed to accommodate a 5 VDC, 2.5 VDC or an adjusted
voltage reference. This has been achieved in the design of
the IC as shown in Figure 6.
±
ages of 2.4 VDC nominal value, initial errors of 10 mVDC will
±
cause conversion errors of 1 LSB due to the gain of 2 of the
REF/2 input. In reduced span applications, the initial value
V
and the stability of the VREF/2 input voltage become even
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
REF/2 input becomes 5 mV. As can be seen, this reduces
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≤TA≤+70˚C. Other temperature range
parts are also available.
DS005671-15
FIGURE 6. The VREFERENCE Design on the IC
21
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