Rainbow-electronics ADC0848 Manual de usuario Pagina 9
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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
XLLLLL H
ab
XLLLHL H
ba
XLLHLL H
ab
XLLHHL
£
H
ba
Differential
XLHLLL H
ab
XLHLHL H
ba
XLHHLL H
ab
XLHHHL H
ba
LHLLLL H
ab
LHLLHL H
ab
LHLHLL H
ab
LHLHHL
£
H
ab
Single-Ended
LHHLLL H
ab
LHHLHL H
ab
LHHHLL H
ab
LHHHHL H
ab
HHLLLL H
ab
HHL LHL H
ab
HHLHLL H
ab
Pseudo-
HHLHHL£H
ab
Differential
HHHL LL H
ab
HHHLHL H
ab
HHHHLL H
ab
XXXXXL£L Previous Channel Configuration
9
- General Description 1
- Features 1
- Key Specifications 1
- Block and Connection Diagrams 1
- Error vs 4
- Leakage Current Test Circuit 5
- Timing Diagrams 6
- TL/H/5016–11 7
- Functional Description 8
- Applications Information 8
- Figure 1 9
- Figure 2a 9
- Figure 2b 9
- TL/H/5016–16 10
- TL/H/5016–17 10
- zero) of the analog range 11
- Diodes are 1N914 12
- High Accuracy Comparators 13
- (Continued) 13
- A Stand Alone Circuit 14
- TL/H/5016–27 15
- 1 for CH2, etc 16
- Ordering Information 17
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