Datasheet

© 2009 Microchip Technology Inc. DS11177F-page 19
MCP606/7/8/9
4.8.4 THREE OP AMP
INSTRUMENTATION AMPLIFIER
A classic, three op amp instrumentation amplifier is
illustrated in Figure 4-12. The two input op amps
provide differential signal gain and a common mode
gain of +1. The output op amp is a difference amplifier,
which converts its input signal from differential to a sin-
gle ended output; it rejects common mode signals at its
input. The gain of this circuit is simply adjusted with one
resistor (R
G
). The reference voltage (V
REF
) is typically
referenced to mid-supply (V
DD
/2) in single-supply
applications.
FIGURE 4-12: Three Op Amp
Instrumentation Amplifier.
4.8.5 PRECISION GAIN WITH GOOD
LOAD ISOLATION
In Figure 4-13, the MCP606 op amps, R
1
and R
2
provide a high gain to the input signal (V
IN
). The
MCP606’s low offset voltage makes this an accurate
circuit.
The MCP601 is configured as a unity-gain buffer. It
isolates the MCP606’s output from the load, increasing
the high-gain stage’s precision. Since the MCP601 has
a higher output current, with the two amplifiers being
housed in separate packages, there is minimal change
in the MCP606’s offset voltage due to loading effect.
FIGURE 4-13: Precision Gain with Good
Load Isolation.
V
OUT
V
1
V
2
–()1
2R
2
R
G
---------+
⎝⎠
⎜⎟
⎛⎞
R
4
R
3
------
⎝⎠
⎜⎟
⎛⎞
V
REF
+=
R
2
V
REF
V
1
R
4
R
3
R
2
R
G
V
OUT
V
2
R
4
R
3
½
MCP607
½
MCP607
MCP606
V
OUT
V
IN
1R
2
R
1
⁄ +()=
R
2
R
1
V
OUT
V
IN
MCP601
MCP606