ADC0834CCN National Semiconductor, ADC0834CCN Datasheet - Page 18

• Manufacturer Part Number: ADC0834CCN
• Description: 8-BIT SERIAL I/O,A/D,4-CH MUX
• Manufacturer: National Semiconductor

Specifications of ADC0834CCN

• Rohs Compliant: NO

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Functional Description
resistance of the analog signal source. Bypass capacitors
should not be used if the source resistance is greater than 1
kΩ.
This source resistance limitation is important with regard to
the DC leakage currents of input multiplexer as well. The
worst-case leakage current of
create a 1 mV input error with a 1 kΩ source resistance. An
op amp RC active low pass filter can provide both imped-
ance buffering and noise filtering should a high impedance
signal source be required.
5.0 Optional Adjustments
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 any V
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
positive voltage to the V
ence between the actual DC input voltage which is neces-
sary to just cause an output digital code transition from 0000
0000 to 0000 0001 and the ideal
mV for V
5.2 Full-Scale
The full-scale adjustment can be made by applying a differ-
ential input voltage which is 1
analog full-scale voltage range and then adjusting the mag-
nitude of the V
digital output code which is just changing from 1111 1110 to
1111 1111.
5.3 Adjusting 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
equals this desired zero reference plus
LSB is calculated for the desired analog span, using 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 00
transition.
The full-scale adjustment should be made [with the proper
V
input which is given by:
IN
(−) voltage applied] by forcing a voltage to the V
REF
=5.000 V
REF
IN
(−) input and applying a small magnitude
IN
input (or V
(−) input at this V
DC
).
IN
(+) input. Zero error is the differ-
1
⁄
2
±
CC
1 µA over temperature will
LSB down from the desired
1
⁄
2
for the ADC0832) for a
LSB value (
IN(MIN)
IN
HEX
1
(Continued)
IN(MIN)
⁄
(+) voltage which
2
LSB (where the
to 01
, is not ground
1
value. This
⁄
2
HEX
LSB=9.8
code
IN
(+)
18
where:
and
The V
change from FE
ment procedure.
6.0 Power Supply
A unique feature of the ADC0838 and ADC0834 is the inclu-
sion of a zener diode connected from the V
ground which also connects to the V
actual converter supply) through a silicon diode, as shown in
Figure 3. (Note 3)
This zener is intended for use as a shunt voltage regulator to
eliminate the need for any additional regulating components.
This is most desirable if the converter is to be remotely
located from the system power source.Figure 4 and Figure 5
illustrate two useful applications of this on-board zener when
an external transistor can be afforded.
An important use of the interconnecting diode between V
and V
is used as a rectifier to allow the V
to be derived from the clock. The low current requirements of
the A/D and the relatively high clock frequencies used (typi-
cally in the range of 10k–400 kHz) allows using the small
value filter capacitor shown to keep the ripple on the V
to well under
also be used in this mode. This requires a clock voltage
swing which is in excess of V
needed, either built into the clock generator or a resistor can
be used from the CLK pin to the V
V
V
FIGURE 3. An On-Chip Shunt Regulator Diode
MAX
MIN
REF
CC
is shown in Figure 6 and Figure 7. Here, this diode
= the low end (the offset zero) of the analog
(or V
= the high end of the analog input range
range.
(Both are ground referenced.)
1
CC
⁄
4
HEX
of an LSB. The shunt zener regulator can
) voltage is then adjusted to provide a code
to FF
HEX
Z
. A current limit for the zener is
. This completes the adjust-
CC
+
CC
supply for the converter
pin.
terminal (which is the
00558311
+
terminal to
CC
line
+