adc12032 National Semiconductor Corporation, adc12032 Datasheet - Page 33

• Manufacturer Part Number: adc12032
• Description: Self-calibrating 12-bit Plus Sign Serial I/o A/d Converters With Mux And Sample/hold
• Manufacturer: National Semiconductor Corporation

Application Hints
3.0 REFERENCE VOLTAGE
The difference in the voltages applied to the V
V
the voltage applied between two multiplexer inputs or the
voltage applied to one of the multiplexer inputs and analog
ground), over which 4095 positive and 4096 negative codes
exist. The voltage sources driving V
very low output impedance and noise. The circuit in Figure
16 is an example of a very stable reference appropriate for
use with the device.
The ADC 12030/2/4/8 can be used in either ratiometric or
absolute reference applications. In ratiometric systems, the
analog input voltage is proportional to the voltage used for
the ADC’s reference voltage. When this voltage is the sys-
tem power supply, the V
V
system reference stability requirements because the analog
input voltage and the ADC reference voltage move together.
This maintains the same output code for given input condi-
tions. For absolute accuracy, where the analog input voltage
*
Tantalum
REF
REF
−
−
defines the analog input span (the difference between
is connected to ground. This technique relaxes the
FIGURE 16. Low Drift Extremely
Stable Reference Circuit
REF
+
pin is connected to V
(Continued)
REF
+
or V
FIGURE 15. Fully Differential Biasing
REF
−
must have
REF
A
+
01135442
+
and
and
33
varies between very specific voltage limits, a time and tem-
perature stable voltage source can be connected to the
reference inputs. Typically, the reference voltage’s magni-
tude will require an initial adjustment to null reference volt-
age induced full-scale errors.
Below are recommended references along with some key
specifications.
The reference voltage inputs are not fully differential. The
ADC12030/2/4/8 will not generate correct conversions or
comparisons if V
sions result when V
at all times, between ground and V
mode range, (V
(0.6 x V
reference ladder should not go below 0.5V or above 3.0V.
Figure 17 is a graphic representation of the voltage restric-
tions on V
LM4041CI-Adj
LM4040AI-4.1
LM4120AI-4.1
LM4121AI-4.1
LM4050AI-4.1
LM4030AI-4.1
LM4031AI
LM4031AC
LM4140AC-4.1
Circuit of Figure 16
Part Number
A
+
REF
). Therefore, with V
+
REF
and V
REF
+
REF
+
+ V
REF
is taken below V
+
REF
and V
−
.
−
)/2 is restricted to (0.1 x V
Adjustable
Tolerance
REF
Voltage
±
Output
±
±
±
±
±
A
0.05%
±
±
±
+
0.5%
0.1%
0.2%
0.2%
0.1%
0.1
2.2
0.4
−
= 5V the center of the
differ by 1V and remain,
A
+
REF
. The V
01135450
−
. Correct conver-
Temperature
Coefficient
±
±
±
REF
±
±
±
±
±
±
100ppm/˚C
100ppm/˚C
3.0ppm/˚C
www.national.com
±
50ppm/˚C
50ppm/˚C
50ppm/˚C
10ppm/˚C
26ppm/˚C
46ppm/˚C
2ppm/˚C
common
A
+
) to