ADC0838 Micro Linear Corporation, ADC0838 Datasheet - Page 15

• Manufacturer Part Number: ADC0838
• Description: Serial I/O 8-Bit A/D Converters with Multiplexer Options
• Manufacturer: Micro Linear Corporation

ZERO ERROR ADJUSTMENT
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 00000000 digital code for this minimum input
voltage by biasing any V
This utilizes the differential mode operation 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
measured by grounding the V
small magnitude positive voltage to the V
error is the difference between the actual DC input
voltage which is necessary to just cause an output digital
code transition from 00000000 to 00000001 and the ideal
1/2 LSB value (1/2 LSB = 9.8mV for V
FULL-SCALE ADJUSTMENT
The full-scale adjustment can be made by applying a
differential input voltage which is 1-1/2 LSB down from
the desired analog full-scale voltage range and then
adjusting the magnitude of the V
digital output code which is just changing from 11111110
to 11111111.
ADJUSTMENT FOR AN ARBITRARY ANALOG
INPUT VOLTAGE RANGE
If the analog zero voltage of the A/D is shifted away from
ground (for example, to accommodate an analog input
signal which does not go to ground), this new zero
reference should be properly adjusted first. A V
which equals this desired zero reference plus 1/2 LSB
CURRENT LIMITING
RESISTOR, I+ 15mA
12V
I +
Figure 8. Shunt Regulator
GND
V+
IN
– input at this V
28.8k
3.2k
3.2k
IN
– input and applying a
REF
input or V
REF
IN MIN
IN MIN
= 5.000V
IN
+ input. Zero
is not ground,
V
CC
IN
CC
value.
+ voltage
for a
DC
).
ML2281, ML2282, ML2284, ML2288
(where the LSB is calculated for the desired analog span,
1 LSB = analog span/256) is applied to selected “+” input
and the zero reference voltage at the corresponding “–”
input should then be adjusted to just obtain the 00000000
to 00000001 code transition.
The full-scale adjustment should be made by forcing a
voltage to the V
where V
The V
code change from 11111110 to 11111111.
SHUNT REGULATOR
A unique feature of ML2288 and ML2284 is the inclusion
of a shunt regulator connected from V+ terminal to
ground which also connects to the V
the actual converter supply) through a silicon diode as
shown in Figure 8. When the regulator is turned on, the
V+ voltage is clamped at 11V
ratio. The typical I-V of the shunt regulator is shown in
Figure 9. It should be noted that before V+ voltage is high
enough to turn on the shunt regulator (which occurs at
about 5.5V), 35k resistance is observed between V+ and
GND. When the shunt regulator is not used, V+ pin
should be either left floating or tied to GND. The
temperature coefficient of the regulator is –22mV/°C.
Figure 9. I-V Characteristic of the Shunt Regulator
REF
V
IN
V
or V
MAX
MIN
fs adjust
15mA
CC
= low end (offset zero) of the analog range
= high end of the analog input range
IN
voltage is then adjusted to provide a
I+
+ input which is given be:
SLOPE =
V
MAX
35k
1
BE
1 5
.
set by the internal resistor
5.5V
CC
(
V
MAX
terminal (which is
6.9V
256
V+
V
MIN
)
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