Rainbow Electronics ADC12138 - Manual (Page 32)

Application Hints
(Continued)
3 0 REFERENCE VOLTAGE
The difference in the voltages applied to the V REF a and
V
REF
b defines the analog input span (the difference be-
tween the voltage applied between two multiplexer inputs or
the voltage applied to one of the multiplexer inputs and ana-
log ground) over which 4095 positive and 4096 negative
codes exist The voltage sources driving V
REF a or V REF b
must have very low output impedance and noise The circuit
in
Figure 14 is an example of a very stable reference appro-
priate for use with the device
Tantalum
TL H 12079 – 43
FIGURE 14 Low Drift Extremely
Stable Reference Circuit
The ADC12130 2 8 can be used in either ratiometric or ab-
solute 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
REF
a pin is connected to V
A
a and
V
REF
b is connected to ground This technique relaxes the
system reference stability requirements because the analog
input voltage and the ADC reference voltage move togeth-
er This maintains the same output code for given input con-
ditions For absolute accuracy where the analog input volt-
age varies between very specific voltage limits a time and
temperature 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
Output
Temperature
Part Number Voltage
Coefficient
Tolerance
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 14 Adjustable g 2ppm C
The reference voltage inputs are not fully differential The
ADC12130 2 8 will not generate correct conversions or
comparisons if V
REF a is taken below V REF b Correct con-
versions result when V REF a and V REF b differ by 1V and
remain at all times between ground and V
A
a The V
REF
common mode range (V
REF
a a V
REF
b ) 2 is restricted to
(0 1 c V
A a ) to (0 6 c V
A a ) Therefore with V
A a e 5V
the center of the reference ladder should not go below 0 5V
or above 3 0V
Figure 15 is a graphic representation of the
voltage restrictions on V
REF
a and V
REF
b
TL H 12079 – 44
FIGURE 15 V
REF
Operating Range
4 0 ANALOG INPUT VOLTAGE RANGE
The ADC12130 2 8’s fully differential ADC generate a
two’s complement output that is found by using the equation
shown below
for (12-bit) resolution the Output Code e
(V IN a b V IN b ) (4096)
(V REF a b V REF b )
Round off to the nearest integer value between b4096 to
4095 if the result of the above equation is not a whole num-
ber
Examples are shown in the table below
Digital
V
REF
a V
REF
b V
IN
a V
IN
b Output
Code
a 2 5V a 1V a 1 5V 0V 0 1111 1111 1111
a 4 096V 0V a 3V 0V 0 1011 1011 1000
a 4 096V 0V a 2 499V a2 500V 1 1111 1111 1111
a 4 096V 0V 0V a 4 096V 1 0000 0000 0000
5 0 INPUT CURRENT
At the start of the acquisition window (t
A
) a charging current
flows into or out of the analog input pins (A DIN1 and
A DIN2) depending on the input voltage polarity The ana-
log input pins are CH0 – CH7 and COM when A DIN1 is tied
to MUXOUT1 and A DIN2 is tied to MUXOUT2 The peak
value of this input current will depend on the actual input
voltage applied the source impedance and the internal mul-
tiplexer switch on resistance
With MUXOUT1 tied to
A DIN1 and MUXOUT2 tied to A DIN2 the internal multi-
plexer switch on resistance is typically 1 6 kX The A DIN1
and A DIN2 mux on resistance is typically 750X
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