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 R2/RG gets
larger.
FIGURE 4-10:
Three-Op Amp
Instrumentation Amplifier.
The two-op amp instrumentation amplifier is shown in
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 F. 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 (R2) in the feedback loop of the
optional capacitor (C2) sometimes provides stability for
these circuits.
A photodiode configured in the Photovoltaic mode has
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
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
V1
MCP60X
V2
R2
R3
MCP60X
R4
R3
R4
VOUT
VREF
RG
+
–
+
–
+
V
OUT
V
1
V
2
–
() 1
2R
2
R
G
---------
+
R4
R
3
------
V
REF
+
=
MCP60X
V2
RG
R2
MCP60X
R1
VOUT
VREF
V1
R1
-
+
-
+
V
OUT
V
1
V
2
–
() 1
R
1
R
2
------
2R
1
R
G
---------
++
V
REF
+
=
D1
Light
VOUT
VDD
MCP60X
R2
C2
ID1
VOUT = ID1 R2
–
+
D1
Light
VOUT
VDD
MCP60X
R2
C2
ID1
VOUT = ID1 R2
VBIAS
VBIAS < 0V
–
+