Rainbow Electronics ADC1038 - Manual (Page 12)

3 0 Analog Considerations
(Continued)
External Reference 2 5V Full Scale
TL H 10556 – 20
Power Supply as Reference
TL H 10556 – 21
Input Not Referred to GND
TL H 10556 – 22
Current path must still exist from V IN
( b ) to
ground
FIGURE 3 Analog Input Options
3 3 REFERENCE AND INPUT
The two V
REF
inputs of the ADC1031 4 8 are fully differen-
tial and define the zero to full-scale input range of the A to D
converter This allows the designer to easily vary the span
of the analog input since this range will be equivalent to the
voltage difference between V
REF a and V REF b By reduc-
ing V
REF
(V
REF e V
REF a b V
REF b ) to less than 5V the
sensitivity of the converter can be increased (i e if V
REF e
2V then 1 LSB e 1 95 mV) The input reference arrange-
ment also facilitates ratiometric operation and in many
cases the chip power supply can be used for transducer
power as well as the V
REF
source
This reference flexibility lets the input span not only be var-
ied but also offset from zero The voltage at V
REF b sets the
input level which produces a digital output of all zeros
Though V
IN
is not itself differential the reference design
allows nearly differential-input capability for many measure-
ment applications
Figure 3 shows some of the configura-
tions that are possible
The ADC1031 has no V REF b pin V REF b is internally tied to
GND
Power Supply Bypassing
TL H 10556 – 23 TL H 10556 – 24
12