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MCP6286T-E/OT Datasheet PDF : 28 Pages
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4.6 Application Circuits
4.6.1 ACTIVE LOW-PASS FILTER
The MCP6286 op amp’s low input bias current makes
it possible for the designer to use larger resistors and
smaller capacitors for active low-pass filter
applications. However, as the resistance increases, the
noise generated also increases. Parasitic capacitances
and the large value resistors could also modify the
frequency response. These trade-offs need to be
considered when selecting circuit elements.
Figure 4-6 and Figure 4-7 show low-pass,
second-order, Butterworth filters with a cut-off
frequency of 10 Hz. The filter in Figure 4-6 has a
non-inverting gain of +1 V/V, and the filter in Figure 4-7
has an inverting gain of -1 V/V.
C1
47 nF
R1
R2
382 kΩ 641 kΩ
VIN
C2
22 nF
G = +1 V/V
fP = 10 Hz
+
MCP6286
–
VOUT
FIGURE 4-6:
Second-Order, Low-Pass
Butterworth Filter with Sallen-Key Topology.
R2
618 kΩ
G = -1 V/V
fP = 10 Hz
R1
R3
618 kΩ 1.00 MΩ
VIN
C2
47 nF
VDD/2
C1
8.2 nF
–
MCP6286
+
VOUT
FIGURE 4-7:
Second-Order, Low-Pass
Butterwork Filter with Multiple-Feedback
Topology.
MCP6286
4.6.2 PHOTO DETECTION
The MCP6286 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 4-8 and Figure 4-9. 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 4-8). 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
ID1
–
VDD
D1
Light
MCP6286
+
VOUT
VOUT = ID1*R2
FIGURE 4-8:
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 4-9). 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
ID1
D1
VBIAS
R2
VDD
–
VOUT
MCP6286
+
VOUT = ID1*R2
VBIAS < 0V
FIGURE 4-9:
Detector.
Photoconductive Mode
© 2009 Microchip Technology Inc.
DS22196A-page 17
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