Datasheet

MCP3301
DS21700E-page 16  2001-2011 Microchip Technology Inc.
Using an external operational amplifier on the input
allows for gain and buffers the input signal from the
input to the ADC, allowing for a higher source
impedance. This circuit is shown in Figure 5-5.
FIGURE 5-5: Adding an amplifier allows
for gain and also buffers the input from any high
impedance sources.
This circuit shows that some headroom will be lost due
to the amplifier output not being able to swing all the
way to the rail. An example would be for an output
swing of 0V to 5V. This limitation can be overcome by
supplying a V
REF
that is slightly less than the common
mode voltage. Using a 2.048V reference for the A/D
converter, while biasing the input signal at 2.5V solves
the problem. This circuit is shown in Figure 5-6.
FIGURE 5-6: Circuit solution to overcome
amplifier output swing limitation.
5.4 Common Mode Input Range
The common mode input range has no restriction and
is equal to the absolute input voltage range: V
SS
-0.3V
to V
DD
+ 0.3V. However, for a given V
REF
, the common
mode voltage has a limited swing if the entire range of
the A/D converter is to be used. Figure 5-7 and
Figure 5-8 show the relationship between V
REF
and the
common mode voltage. A smaller V
REF
allows for wider
flexibility in a common mode voltage. V
REF
levels,
down to 400 mV, exhibit less than 0.1 LSB change in
INL and DNL.
For characterization graphs that show this performance
relationship, see Figure 2-2 and Figure 2-6.
FIGURE 5-7: Common Mode Range of
Full Differential input signal versus V
REF
.
FIGURE 5-8: Common Mode Range
versus V
REF
for Pseudo Differential Input.
V
DD
= 5V
-
+
0.1 µF
MCP6022
IN+
IN-
V
REF
MCP3301
1µF
MCP1525
V
IN
V
OUT
0.1 µF
1k
10 k
1M
1µF
V
IN
1M
2.048V
V
DD
= 5V
-
+
0.1 µF
MCP606
IN+
IN-
V
REF
MCP3301
1µF
MCP1525
V
IN
V
OUT
0.1 µF
1k
10 k
1µF
V
IN
10 k
V
REF
(V)
0.25
V
DD
= 5V
5.0
1.0
2.5
4.0
-1
0
1
2
3
4
5
4.05V
2.8V
2.3V
0.95V
Common Mode Range (V)
V
REF
(V)
0.25
V
DD
= 5V
2.5
0.5
1.25 2.0
-1
0
1
2
3
4
5
4.05V
2.8V
2.3V
0.95V
Common Mode Range (V)