Analog application information – Rainbow Electronics ADC12048 User Manual
Page 25
Analog Application Information
REFERENCE VOLTAGE
The ADC12048 has two reference inputs V
REF
a
and
V
REF
b
They define the zero to full-scale range of the ana-
log input signals over which 4095 positive and 4096 nega-
tive codes exist The reference inputs can be connected to
span the entire supply voltage range (V
REF
b
e
AGND
V
REF
a
e
V
A
a
) or they can be connected to different volt-
ages when other input spans are required The reference
inputs of the ADC12048 have transient capacitive switching
currents The voltage sources driving V
REF
a
and V
REF
b
must have very low output impedence and noise and must
be adequately bypassed The circuit in
Figure 8
is an exam-
ple of a very stable reference source
The ADC12048 can be used in either ratiometric or absolute
reference appplications In ratiometric systems the analog
TL H 12387 – 37
FIGURE 9 V
REF
Operating Range
input voltage is proportional to the voltage used for the
ADC’s reference voltage This technique relaxes the system
reference requirements because the analog input voltage
moves with the ADC’s reference The system power supply
can be used as the reference voltage by connecting the
V
REF
a
pin to V
A
a
and the V
REF
b
pin to AGND For abso-
lute accuracy where the analog input voltage varies be-
tween very specific voltage limits a time and temperature
stable voltage source can be connected to the reference
inputs Typically the reference voltage’s magnitude will re-
quire an initial adjustment to null reference voltage induced
full-scale errors
The reference voltage inputs are not fully differential The
ADC12048 will not generate correct conversions if V
REF
a
–
V
REF
b
is below 1V
Figure 9
shows the allowable relation-
ship between V
REF
a
and V
REF
b
OUTPUT DIGITAL CODE VERSUS ANALOG INPUT
VOLTAGE
The ADC12048’s fully differential 12-bit a sign ADC gener-
ates a two’s complement output that is found by using the
equation shown below
Output code e
(V
IN
a
b
V
IN
b
) (4096)
(V
REF
a
b
V
REF
b
)
Round off the result to the nearest integer value between
b
4096 and 4095
Tantalum
Ceramic
TL H 12387 – 36
FIGURE 8 Low Drift Extremely Stable Reference Circuit
Part Number
Output Voltage
Temperature
Tolerance
Coefficient
LM4041CI-Adj
g
0 5%
g
100ppm C
LM4040AI-4 1
g
0 1%
g
100ppm C
LM9140BYZ-4 1
g
0 5%
g
25ppm C
LM368Y-5 0
g
0 1%
g
20ppm C
Circuit of
Figure 8
Adjustable
g
2ppm C
25