Rainbow Electronics ADC12038 - Manual (Page 33)

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 11354 – 42
FIGURE 14 Low Drift Extremely
Stable Reference Circuit
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
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
ADC12030 2 4 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 11354 – 45
FIGURE 15 V
REF
Operating Range
4 0 ANALOG INPUT VOLTAGE RANGE
The ADC12030 2 4 8’s fully differential ADC generate a
two’s complement output that is found by using the equa-
tions 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 )
for (8-bit) resolution the Output Code e
(V
IN
a b V
IN
b ) (256)
(V REF a b V REF b )
Round off to the nearest integer value between b4096 to
4095 for 12-bit resolution and between b256 to 255 for
8-bit resolution if the result of the above equation is not a
whole number
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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