ADC0803, ADC0804
Such an adjusted reference voltage can accommodate a
reduced span or dynamic voltage range of the analog input
voltage. If the analog input voltage were to range from 0.5V to
3.5V, instead of 0V to 5V, the span would be 3V. With 0.5V
applied to the V
lN(-)
pin to absorb the offset, the reference
voltage can be made equal to
1
/
2
of the 3V span or 1.5V. The
A/D now will encode the V
lN(+)
signal from 0.5V to 3.5V with
the 0.5V input corresponding to zero and the 3.5V input
corresponding to full scale. The full 8 bits of resolution are
therefore applied over this reduced analog input voltage
range. The requisite connections are shown in Figure 13. For
expanded scale inputs, the circuits of Figures 14 and 15 can
be used.
V+
(V
REF
)
5V
(V
REF
)
R
2R
6
20
+
10µF
V
IN
±
10V
V
IN(+)
V+
2R
7
ADC0803-
ADC0804
V
IN(-)
FIGURE 14. HANDLING
±10V
ANALOG INPUT RANGE
5V
(V
REF
)
R
20
R
V
REF
/2
9
DIGITAL
CIRCUITS
V
IN
±5V
R
6
V
IN(+)
V+
20
+
10µF
ADC0803-
ADC0804
7
V
IN(-)
R
DECODE
ANALOG
CIRCUITS
FIGURE 15. HANDLING
±5V
ANALOG INPUT RANGE
Reference Accuracy Requirements
The converter can be operated in a pseudo-ratiometric mode
or an absolute mode. In ratiometric converter applications,
the magnitude of the reference voltage is a factor in both the
output of the source transducer and the output of the A/D
converter and therefore cancels out in the final digital output
code. In absolute conversion applicatIons, both the initial
value and the temperature stability of the reference voltage
are important accuracy factors in the operation of the A/D
converter. For V
REF
/2 voltages of 2.5V nominal value, initial
errors of
±10mV
will cause conversion errors of
±1
LSB due
to the gain of 2 of the V
REF
/2 input. In reduced span
applications, the initial value and the stability of the V
REF
/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 reduced from 20mV (5V span) to
10mV and 1 LSB at the V
REF
/2 input becomes 5mV. As can
be seen, this reduces the allowed initial tolerance of the
reference voltage and requires correspondingly less
absolute change with temperature 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 reference voltage will require an initial
adjustment. Errors due to an improper value of reference
voltage appear as full scale errors in the A/D transfer
AGND
8
DGND
10
FIGURE 12. THE V
REFERENCE
DESIGN ON THE IC
V
REF
(5V)
ICL7611
FS
ADJ.
“SPAN”/2
+
0.1µF
TO V
IN(-)
5V
300
TO V
REF
/2
-
ZERO SHIFT VOLTAGE
FIGURE 13. OFFSETTING THE ZERO OF THE ADC080X AND
PERFORMING AN INPUT RANGE (SPAN)
ADJUSTMENT
10
INTERSIL [ INTERSIL CORPORATION ]
