Datasheet

ADC0844, ADC0848
SNAS523D –JUNE 1999–REVISED MARCH 2013 www.ti.com
Table 1. ADC0844 MUX ADDRESSING
(1)
MUX Address Channel#
CS WR RD MUX Mode
MA3 MA2 MA1 MA0 CH1 CH2 CH3 CH4 AGND
X L L L L H + −
X L L H L NP H − +
Differential
X L H L L H + −
X L H H L H − +
L H L L L H + −
L H L H L NP H + −
Single-Ended
L H H L L H + −
L H H H L H + −
H H L L L H + −
H H L H L NP H + − Pseudo- Differential
H H H L L H + −
X X X X L NP L Previous Channel Configuration
(1) X = don't care, NP = negative pulse
4 Single-Ended 2 Differential
3 Pseudo-Differential Combined
Figure 12. Analog Input Multiplexer Options
REFERENCE CONSIDERATIONS
The voltage applied to the reference input of these converters defines the voltage span of the analog input (the
difference 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 absolute accuracy. The reference pin must be
connected to a voltage source capable of driving the minimum reference input resistance of 1.1 kΩ. This pin is
the top of a resistor divider string used for the successive approximation conversion.
In a ratiometric system (Figure 13), 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 14), 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 direct 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 sources when operating with a reduced span due to the increased sensitivity of the converter (1
LSB equals V
REF
/256).
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