Datasheet

© 2006 Microchip Technology Inc. DS21945E-page 49
MCP4021/2/3/4
8.2 Operational Amplifier
Applications
Figure 8-3, Figure 8-4 and Figure 8-5 illustrate typical
amplifier circuits that could replace fixed resistors with
the MCP4021/2/3/4 to achieve digitally-adjustable
analog solutions.
Figure 8-4 shows a circuit that allows a non-inverting
amplifier to have its’ offset and gain to be independently
trimmed. The MCP4021 is used along with resistors R1
and R2 to set the offset voltage. The sum of R1 + R2
resistance should be significantly greater (> 100 times)
the resistance value of the MCP4021. This allows each
increment or decrement in the MCP4021 to be a fine
adjustment of the offset voltage. The input voltage of
the op amp (V
IN
) should be centered at the op amps V
W
voltage. The gain is adjusted by the MCP4022. If the
resistance value of the MCP4022 is small compared to
the resistance value of R3, then this is a fine
adjustment of the gain. If the resistance value of the
MCP4022 is equal (or large) compared to the
resistance value of R3, then this is a course adjustment
of the gain. In gerneral, trim the course adjustments
first and then trim the fine adjustments.
FIGURE 8-3: Trimming Offset and Gain in
an Inverting Amplifier.
FIGURE 8-4: Trimming Offset and Gain in
a Non-Inverting Amplifier.
FIGURE 8-5: Programmable Filter.
Op Amp
V
IN
V
OUT
B
A
W
+
–
MCP4021
R
1
R
2
B
A
V
DD
W
R
3
R
4
MCP402X
MCP6001
Op Amp
V
IN
V
OUT
–
+
R
1
R
2
B
A
V
DD
W
R
3
MCP6291
MCP4021
MCP4022
W
A
V
DD
V
W
Op Amp
V
IN
V
OUT
B
A
W
+
–
MCP4021
R
1
R
2
B
A
V
DD
W
MCP4022
R
3
R
4
f
c
1
2
π R
Eq
C ⋅⋅
-----------------------------=
Pot
1
Pot
2
R
Eq
R
1
R
AB
R
WB
–+()R
2
R
WB
+()R
w
+
||
=
Thevenin
Equivalent
MCP6021