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ADC0801 Datasheet - Page 17

Download or read online National Semiconductor ADC0801 8-Bit Microprocessor Compatible A/D Converters pdf datasheet.
Also see for ADC0801: Datasheet #2 (41 pages)



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Functional Description
(Continued)
Add if V
to draw 3 mA to ground
a) Analog Input Signal Example
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
voltages of 2 4 V
nominal value initial errors of
DC
mV
will cause conversion errors of
1 LSB due to the
g
DC
gain of 2 of the V
2 input In reduced span applications
REF
the initial value and the stability of the V
REF
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
2 input becomes 5 mV As can be seen
REF
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
T
70 C Other temperature
s
A s
a
range parts are also available
2
1 V
with LM358
s
REF
DC
b) Accommodating an Analog Input from
0 5V (Digital Out
e e
(Digital Out
e
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
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
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
2
REF
of the first riser of the transfer function and can be mea-
10
g
sured by grounding the V
magnitude positive voltage to the V
is the difference between the actual DC input voltage that is
2 input voltage
necessary to just cause an output digital code transition
from 0000 0000 to 0000 0001 and the ideal
(
LSB
9 8 mV for V
e
2 5 2 Full-Scale
The full-scale adjustment can be made by applying a differ-
ential input voltage that is 1
analog full-scale voltage range and then adjusting the mag-
nitude of the V
2 input (pin 9 or the V
REF
not used) for a digital output code that is just changing from
1111 1110 to 1111 1111
17
TL H 5671 – 16
00
) to 3 5V
HEX
FF
)
HEX
is not ground
IN(MIN)
(
) input at this V
value (see
b
IN
IN(MIN)
(
) input and applying a small
b
IN
(
) input Zero error
a
IN
LSB value
2
2 500 V
)
e
REF
DC
LSB less than the desired
supply if pin 9 is
CC

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