Functional Description
Continued
2.5 Errors and Reference Voltage Adjustments 2.5.1 Zero Error
The zero of the AD does not require adjustment. If the minimum analog input voltage value, V
INMIN
, 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 AD V
IN
− input at this V
INMIN
value see Applications section. This utilizes the differential mode op-
eration of the AD. The zero error of the AD converter relates to the location of
the first riser of the transfer function and can be measured by grounding the V
IN
− input and applying a small magnitude positive voltage to the V
IN
+ input. Zero error is the differ- ence 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
1
⁄
2
LSB value
1
⁄
2
LSB = 9.8 mV for V
REF
2=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
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.
2.5.3 Adjusting for an Arbitrary Analog Input Voltage Range
If the analog zero voltage of the AD is shifted away from ground for example, to accommodate an analog input signal
that does not go to ground this new zero reference should be properly adjusted first. A V
IN
+ voltage that equals this desired zero reference plus
1
⁄
2
LSB where the LSB is cal- culated for the desired analog span, 1 LSB=analog span
256 is applied to pin 6 and the zero reference voltage at pin 7 should then be adjusted to just obtain the 00
HEX
to 01
HEX
code transition. The full-scale adjustment should then be made with the
proper V
IN
− voltage applied by forcing a voltage to the V
IN
+ input which is given by:
where: V
MAX
=The high end of the analog input range and
V
MIN
=the low end the offset zero of the analog range. Both are ground referenced.
The V
REF
2 or V
CC
voltage is then adjusted to provide a code change from FE
HEX
to FF
HEX
. This completes the adjustment procedure.
2.6 Clocking Option
The clock for the AD can be derived from the CPU clock or an external RC can be added to provide self-clocking. The
CLK IN pin 4 makes use of a Schmitt trigger as shown in Figure 8.
DS005671-87
a Analog Input Signal Example
DS005671-88
Add if V
REF
2 ≤
1 V
DC
with LM358 to draw 3 mA to ground.
b Accommodating an Analog Input from 0.5V Digital Out = 00
HEX
to 3.5V Digital Out=FF
HEX
FIGURE 7. Adapting the AD Analog Input Voltages to Match an Arbitrary Input Signal Range
ADC0801ADC0802ADC0803ADC0804ADC0805
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