Datasheet

© 2007 Microchip Technology Inc. DS21314G-page 15
MCP601/1R/2/3/4
4.8.2 INSTRUMENTATION AMPLIFIER
CIRCUITS
Instrumentation amplifiers have a differential input that
subtracts one input voltage from another and rejects
common mode signals. These amplifiers also provide a
single-ended output voltage.
The three-op amp instrumentation amplifier is illustrated
in Figure 4-10. One advantage of this approach is unity-
gain operation, while one disadvantage is that the
common mode input range is reduced as R
2
/R
G
gets
larger.
FIGURE 4-10: Three-Op Amp
Instrumentation Amplifier.
The two-op amp instrumentation amplifier is shown in
Figure 4-11. While its power consumption is lower than
the three-op amp version, its main drawbacks are that
the common mode range is reduced with higher gains
and it must be configured in gains of two or higher.
FIGURE 4-11: Two-Op Amp
Instrumentation Amplifier.
Both instrumentation amplifiers should use a bulk
bypass capacitor of at least 1 µF. The CMRR of these
amplifiers will be set by both the op amp CMRR and
resistor matching.
4.8.3 PHOTO DETECTION
The MCP601/1R/2/3/4 op amps can be used to easily
convert the signal from a sensor that produces an
output current (such as a photo diode) into a voltage (a
transimpedance amplifier). This is implemented with a
single resistor (R
2
) in the feedback loop of the
amplifiers shown in Figure 4-12 and Figure 4-13. The
optional capacitor (C
2
) sometimes provides stability for
these circuits.
A photodiode configured in the Photovoltaic mode has
zero voltage potential placed across it (Figure 4-12). In
this mode, the light sensitivity and linearity is
maximized, making it best suited for precision
applications. The key amplifier specifications for this
application are: low input bias current, low noise,
common mode input voltage range (including ground),
and rail-to-rail output.
FIGURE 4-12: Photovoltaic Mode Detector.
In contrast, a photodiode that is configured in the
Photoconductive mode has a reverse bias voltage
across the photo-sensing element (Figure 4-13). This
decreases the diode capacitance, which facilitates
high-speed operation (e.g., high-speed digital
communications). The design trade-off is increased
diode leakage current and linearity errors. The op amp
needs to have a wide Gain Bandwidth Product
(GBWP).
FIGURE 4-13: Photoconductive Mode
Detector.
MCP60X
V
1
MCP60X
V
2
R
2
R
2
R
3
MCP60X
R
4
R
3
R
4
V
OUT
V
REF
R
G
+
–
–
+
–
+
V
OUT
V
1
V
2
–()1
2R
2
R
G
---------+
⎝⎠
⎛⎞
R
4
R
3
------
⎝⎠
⎛⎞
V
REF
+=
MCP60X
V
2
R
G
R
2
R
2
MCP60X
R
1
V
OUT
V
REF
V
1
R
1
-
+
-
+
V
OUT
V
1
V
2
–()1
R
1
R
2
------
2R
1
R
G
---------++
⎝⎠
⎛⎞
V
REF
+=
D
1
Light
V
OUT
V
DD
MCP60X
R
2
C
2
I
D1
V
OUT
= I
D1
R
2
–
+
D
1
Light
V
OUT
V
DD
MCP60X
R
2
C
2
I
D1
V
OUT
= I
D1
R
2
V
BIAS
V
BIAS
< 0V
–
+