Applications Information
(Continued)
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
and V
) over which the 256
possible output codes apply. The devices can be used in ei-
ther 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 resis-
tance of 1.1 k
. This pin is the top of a resistor divider string
used for the successive approximation conversion.
In a ratiometric system (Figure 2a), the analog input voltage
is proportional to the voltage used for theA/D reference. This
voltage is typically the system power supply, so the V
pin
can be tied to V
. This technique relaxes the stability re-
quirements 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
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 sources
when operating with a reduced span due to the increased
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 “+” and “” inputs for a conversion (60 Hz is
most typical). The time interval between sampling the “+” in-
put and then the “” inputs is
change in the common-mode voltage during this short time
interval can cause conversion errors. For a sinusoidal
common-mode signal this error is:
1
2
of a clock period. The
where f
is the frequency of the common-mode signal,
V
peak
is its peak voltage value and t
C
is the conversion time.
For a 60 Hz common-mode signal to generate a
1
4
LSB error
(
≈
5 mV) with the converter running at 40 μS, its peak value
would have to be 5.43V. This large a common-mode signal is
much greater than that generally found in a well designed
data acquisition system.
4 Single-Ended
DS005016-12
2 Differential
DS005016-13
3 Pseudo-Differential
DS005016-14
Combined
DS005016-15
FIGURE 1. Analog Input Multiplexer Options
DS005016-38
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