Datasheet

2.0 Applications Information (Continued)
2.2 REFERENCE CONSIDERATIONS
The voltage difference between the V
REF
+
and V
REF
− inputs
defines the analog input voltage span (the difference be-
tween V
IN
(Max) and V
IN
(Min)) over which the 2
n
(where n is
the programmed resolution) possible output codes apply. In
the pseudo-differential and differential modes the actual volt-
age applied to V
REF
+
and V
REF
−
can lie anywhere between
the AV
+
and V
−
. Only the difference voltage is of importance.
When using the single-ended multiplexer mode the voltage
at V
REF
−
has a dual function. It simultaneously determines
the “zero” reference voltage and, with V
REF
+
, the analog
voltage span.
The value of the voltage on the V
REF
+
or V
REF
−
inputs can be
anywhere between AV
+
+ 50 mV and V
−
− 50 mV, so long as
V
REF
+
is greater than V
REF
−
. The ADC10154 and ADC10158
can be used in either ratiometric applications or in systems
requiring absolute accuracy. The reference pins must be
connected to a voltage source capable of driving the mini-
mum reference input resistance of 4.5 kΩ.
The internal 2.5V bandgap reference in the ADC10154 and
ADC10158 is available as an output on the VREFOut pin. To
ensure optimum performance this output needs to be by-
passed to ground with 330 µF aluminum electrolytic or tan-
talum capacitor. The reference output is unstable with ca-
pacitive loads greater than 100 pF and less than 100 µF. Any
capacitive loads ≤100 pF or ≥100 µF will not cause the
reference to oscillate. Lower output noise can be obtained by
increasing the output capacitance. The 330 µF capacitor will
yield a typical noise floor of 200 nVrms/
.
The 2.5V reference output is referred to the negative supply
pin (V
−
). Therefore, the voltage at VREFOut will always be
2.5V greater than the voltage applied to V
−
. Applying this
voltage to V
REF
+
with V
REF
−
tied to V
−
will yield an analog
voltage span of 2.5V. In bipolar operation the voltage at
VREFOut will be at −2.5V when V
−
is tied to −5V. For the
single-ended multiplexer mode the analog input voltage
range will be from −5V to −2.5V. The pseudo-differential and
differential multiplexer modes allow for more flexibility in the
analog input voltage range since the “zero” reference volt-
age is set by the actual voltage applied to the assigned
negative input pin. The drawback of using the internal refer-
ence in the bipolar mode is that any noise on the −5V tied to
the V
−
pin will affect the conversion result. The bandgap
reference is specified and tested in unipolar operation with
V
−
tied to the system ground.
In a ratiometric system (
Figure 5 (a)), the analog input volt-
age is proportional to the voltage used for the A/D reference.
This voltage may also be the system power supply, so V
REF
+
can also be tied to AV
+
. This technique relaxes the stablity
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 5 (b)), where the analog input
varies between very specific voltage limits, the reference pin
can be biased with a time- and temperature-stable voltage
source that has excellent initial accuracy. The LM4040 and
LM185 references are suitable for use with the ADC10154
and ADC10158.
4 Single-Ended
DS011225-40
2 Differential
DS011225-41
3 Pseudo-Differential
DS011225-42
2 Single Ended and 1 Differential
DS011225-43
FIGURE 4. Analog Input Multiplexer Options
ADC10154/ADC10158
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