Datasheet
2011 Microchip Technology Inc. DS21697F-page 19
MCP3302/04
Using an external operation amplifier on the input
allows for gain and also 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 swing. 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 Input Range
of Full Differential Input Signal versus V
REF
.
FIGURE 5-8: Common Mode Input Range
versus V
REF
for Pseudo Differential Input.
V
DD
= 5V
-
+
MCP6021
IN+
IN-
V
REF
MCP330X
1µF
MCP1525
V
IN
V
OUT
1k
10 k
1M
V
IN
0.1 µF
0.1 µF
1µF
1M
2.048V
V
DD
= 5V
-
+
0.1 µF
MCP606
IN+
IN-
V
REF
MCP330X
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)