MCP3304 Datasheet, PDF (19/40 Pages) Microchip Technology – 13-Bit Differential Input, Low Power A/D Converter with SPI Serial Interface

10 kΩ
1 kΩ
MCP6021
-
V_IN
1 µF
+
1 MΩ
V_DD = 5V
0.1 µF
IN+
MCP330X
IN-
V_REF
1 µF
V_OUT V_IN
MCP1525
0.1 µF
FIGURE 6-5:
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 6-5.
10 kΩ
1 kΩ
MCP606
-
V_IN
+
1 µF
1 MΩ
V_DD = 5V
0.1 µF
IN+
MCP330X
IN- _VREF
10 kΩ
2.048V
1 µF
V_OUT V_IN
MCP1525
0.1 µF
FIGURE 6-6:
Circuit solution to overcome
amplifier output swing limitation.
MCP3302/04
6.4 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 6-7 and
Figure 6-8 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
DNL and INL. For characterization graphs that show
this performance relationship, see Figure 2-9 and
Figure 2-12.
V_DD = 5V
5
4
4.05V
3
2.8V
2
2.3V
1
0.95V
0
-1
0.25 1.0
2.5
V_REF (V)
4.0
5.0
FIGURE 6-7:
Common Mode Input Range
of Full Differential Input Signal versus V_REF.
V_DD = 5V
5
4
4.05V
3
2.8V
2
2.3V
1
0.95V
0
-1
0.25 0.5
1.25
V_REF (V)
2.0
2.5
FIGURE 6-8:
Common Mode Input Range
versus V_REF for Pseudo Differential Input.
 2002 Microchip Technology Inc.
DS21697B-page 19