MCP604-I Datasheet, PDF (15/34 Pages) Microchip Technology – 2.7V to 6.0V Single Supply CMOS Op Amps

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/R_G gets
larger.
V₁ +
MCP60X
–
R_G
R₂
R₂
R₃
R₄
–
MCP60X
+
R₃
R₄
V_OUT
–
MCP60X
V₂ +
V_REF
V_OUT
=
(V₁
–
V₂
)
⎛
⎝
1
+
2--_R--R_{--G--2-⎠⎞}
⎛
⎝
R^{-R---43-⎠⎞}
+ V_REF
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.
R_G
R₁
R₂
R₂
V_OUT
R₁
V_REF
V₂
-
MCP60X
+
-
MCP60X
+
V₁
V_OUT
=
(V₁
–
V₂
)
⎛
⎝
1
+
R_----1-
R₂
+
2--_R--R_{--G--1⎠⎞}
+ V_REF
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.
© 2007 Microchip Technology Inc.
MCP601/1R/2/3/4
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
amplifiers shown in Figure 4-12 and Figure 4-13. The
optional capacitor (C₂) 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.
C₂
R₂
I_D1
–
V_DD
D₁
Light
MCP60X
V_OUT
+
V_OUT = I_D1 R₂
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).
C₂
Light
I_D1
D₁
V_BIAS
R₂
–
V_DD
V_OUT
MCP60X
+
V_OUT = I_D1 R₂
V_BIAS < 0V
FIGURE 4-13:
Detector.
Photoconductive Mode
DS21314G-page 15