Applications information – Rainbow Electronics ADC0848 User Manual
Page 9
Applications Information
(Continued)
ended or pseudo-differential
Figure 1
shows the three
modes using the 4-channel MUX ADC0844 The eight inputs
of the ADC0848 can also be configured in any of the three
modes In the differential mode the ADC0844 channel in-
puts are grouped in pairs CH1 with CH2 and CH3 with CH4
The polarity assignment of each channel in the pair is inter-
changeable The single-ended mode has CH1 – CH4 as-
signed as the positive input with the negative input being the
analog ground (AGND) of the device Finally in the pseudo-
differential mode CH1 – CH3 are positive inputs referenced
to CH4 which is now a pseudo-ground This pseudo-ground
input can be set to any potential within the input common-
mode range of the converter The analog signal conditioning
required in transducer-based data acquisition systems is
significantly simplified with this type of input flexibility One
converter package can now handle ground referenced in-
puts and true differential inputs as well as signals with some
arbitrary reference voltage
The analog input voltages for each channel can range from
50 mV below ground to 50 mV above V
CC
(typically 5V)
without degrading conversion accuracy
2 0 REFERENCE CONSIDERATIONS
The voltage applied to the reference input of these convert-
ers defines the voltage span of the analog input (the differ-
ence between V
IN(MAX)
and V
IN(MIN)
) over which the 256
possible output codes apply The devices can be used in
either ratiometric applications or in systems requiring abso-
lute accuracy The reference pin must be connected to a
voltage source capable of driving the minimum reference
input resistance of 1 1 kX This pin is the top of a resistor
divider string used for the successive approximation conver-
sion
In a ratiometric system (
Figure 2a
) the analog input voltage
is proportional to the voltage used for the A D reference
This voltage is typically the system power supply so the
V
REF
pin can be tied to V
CC
This technique relaxes the
stability requirements of the system reference as the analog
input and A D reference move together maintaining the
same output code for a given input condition
For absolute accuracy (
Figure 2b
) where the analog input
varies between very specific voltage limits the reference pin
can be biased with a time and temperature stable voltage
source The LM385 and LM336 reference diodes are good
low current devices to use with these converters
The maximum value of the reference is limited to the V
CC
supply voltage The minimum value however can be quite
small (see Typical Performance Characteristics) to allow di-
rect conversions of transducer outputs providing less than a
5V output span Particular care must be taken with regard to
noise pickup circuit layout and system error voltage sourc-
es when operating with a reduced span due to the in-
creased sensitivity of the converter (1 LSB equals
V
REF
256)
3 0 THE ANALOG INPUTS
3 1 Analog Differential Voltage Inputs and Common-
Mode Rejection
The differential input of these converters actually reduces
the effects of common-mode input noise a signal common
to both selected ‘‘a’’ and ‘‘b’’ inputs for a conversion (60
Hz is most typical) The time interval between sampling the
TABLE II ADC0848 MUX Addressing
MUX Address
CS
WR
RD
Channel
MUX
MA4
MA3
MA2
MA1
MA0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
CH8
AGND
Mode
X
L
L
L
L
L
H
a
b
X
L
L
L
H
L
H
b
a
X
L
L
H
L
L
H
a
b
X
L
L
H
H
L
H
b
a
Differential
X
L
H
L
L
L
H
a
b
X
L
H
L
H
L
H
b
a
X
L
H
H
L
L
H
a
b
X
L
H
H
H
L
H
b
a
L
H
L
L
L
L
H
a
b
L
H
L
L
H
L
H
a
b
L
H
L
H
L
L
H
a
b
L
H
L
H
H
L
H
a
b
Single-Ended
L
H
H
L
L
L
H
a
b
L
H
H
L
H
L
H
a
b
L
H
H
H
L
L
H
a
b
L
H
H
H
H
L
H
a
b
H
H
L
L
L
L
H
a
b
H
H
L
L
H
L
H
a
b
H
H
L
H
L
L
H
a
b
Pseudo-
H
H
L
H
H
L
H
a
b
Differential
H
H
H
L
L
L
H
a
b
H
H
H
L
H
L
H
a
b
H
H
H
H
L
L
H
a
b
X
X
X
X
X
L
L
Previous Channel Configuration
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