User Manual

Functional Description (Continued)
*Add if V
REF
/2
s
1V
DC
with LM358
to draw 3 mA to ground.
TL/H/5671–16
a) Analog Input Signal Example b) Accommodating an Analog Input from
0.5V (Digital Out
ee
00
HEX
) to 3.5V
(Digital Out
e
FF
HEX
)
FIGURE 5. Adapting the A/D Analog Input Voltages to Match an Arbitrary Input Signal 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
output of the source transducer and the output of the A/D
converter 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 abso-
lute conversion applications, both the initial value and the
temperature stability of the reference voltage are important
factors in the accuracy of the A/D converter. For V
REF
/2
voltages of 2.4 V
DC
nominal value, initial errors of
g
10
mV
DC
will cause conversion errors of
g
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 cor-
respondingly reduced 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 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 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
s
T
A
s
a
70
§
C. Other temperature
range parts are also available.
2.5 Errors and Reference Voltage Adjustments
2.5.1 Zero Error
The zero of the A/D does not require adjustment. If the
minimum analog input voltage value, V
IN(MIN)
, is not ground,
a zero offset can be done. The converter can be made to
output 0000 0000 digital code for this minimum input voltage
by biasing the A/D V
IN
(
b
) input at this V
IN(MIN)
value (see
Applications section). This utilizes the differential mode op-
eration of the A/D.
The zero error of the A/D converter relates to the location
of the first riser of the transfer function and can be mea-
sured by grounding the V
IN
(
b
) input and applying a small
magnitude positive voltage to the V
IN
(
a
) input. Zero error
is the difference between the actual DC input voltage that is
necessary to just cause an output digital code transition
from 0000 0000 to 0000 0001 and the ideal (/2 LSB value
((/2 LSB
e
9.8 mV for V
REF
/2
e
2.500 V
DC
).
2.5.2 Full-Scale
The full-scale adjustment can be made by applying a differ-
ential input voltage that is 1(/2 LSB less than the desired
analog full-scale voltage range and then adjusting the mag-
nitude of the V
REF
/2 input (pin 9 or the V
CC
supply if pin 9 is
not used) for a digital output code that is just changing from
1111 1110 to 1111 1111.
17