ADC10158CIN National Semiconductor, ADC10158CIN Datasheet - Page 18

• Manufacturer Part Number: ADC10158CIN
• Description: 10-Bit Plus Sign 4 s ADCs with 4- or 8-Channel MUX/ Track/Hold and Reference
• Manufacturer: National Semiconductor

www.national.com
2.0 Applications Information
2.2 REFERENCE CONSIDERATIONS
The voltage difference between the V
defines the analog input voltage span (the difference be-
tween V
the programmed resolution) possible output codes apply. In
the pseudo-differential and differential modes the actual volt-
age applied to V
the AV
When using the single-ended multiplexer mode the voltage
at V
the “zero” reference voltage and, with V
age span.
The value of the voltage on the V
anywhere between AV
V
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 k .
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 tanta-
lum capacitor. The reference output is unstable with capaci-
tive loads greater than 100 pF and less than 100 µF. Any
capacitive loads 100 pF or 100 µF will not cause the refer-
ence to oscillate. Lower output noise can be obtained by in-
creasing the output capacitance. The 330 µF capacitor will
yield a typical noise floor of 200 nVrms/
REF
REF
+
+
is greater than V
−
and V
IN
has a dual function. It simultaneously determines
(Max) and V
−
. Only the difference voltage is of importance.
REF
3 Pseudo-Differential
+
4 Single-Ended
and V
+
IN
REF
+ 50 mV and V
(Min)) over which the 2
−
. The ADC10154 and ADC10158
REF
−
REF
can lie anywhere between
DS011225-42
DS011225-40
+
REF
or V
−
REF
FIGURE 4. Analog Input Multiplexer Options
− 50 mV, so long as
+
REF
and V
+
, the analog volt-
.
−
inputs can be
n
REF
(where n is
(Continued)
− inputs
18
The 2.5V reference output is referred to the negative supply
pin (V
2.5V greater than the voltage applied to V
voltage to V
voltage span of 2.5V. In bipolar operation the voltage at
VREFOut will be at −2.5V when V
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
erence 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
can also be tied to AV
requirements of the system reference as the analog input
and A/D reference move together maintaining the same out-
put 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.
−
−
). Therefore, the voltage at VREFOut will always be
2 Single Ended and 1 Differential
pin will affect the conversion result. The bandgap ref-
REF
+
with V
2 Differential
+
. This technique relaxes the stablity
REF
−
tied to V
DS011225-41
DS011225-43
−
is tied to −5V. For the
−
will yield an analog
−
. Applying this
REF
−
+