Datasheet

© 2009 Microchip Technology Inc. DS22196A-page 17
MCP6286
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.
FIGURE 4-6: Second-Order, Low-Pass
Butterworth Filter with Sallen-Key Topology.
FIGURE 4-7: Second-Order, Low-Pass
Butterwork Filter with Multiple-Feedback
Topology.
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 (R
2
) in the feedback loop of the
amplifiers shown in Figure 4-8 and Figure 4-9. 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 4-8). 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.
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).
FIGURE 4-9: Photoconductive Mode
Detector.
C
2
V
OUT
R
1
R
2
C
1
V
IN
47 nF
382 kΩ
641 kΩ
22 nF
G = +1 V/V
f
P
= 10 Hz
+
–
MCP6286
C
2
V
OUT
R
1
R
3
C
1
V
IN
R
2
V
DD
/2
G = -1 V/V
f
P
= 10 Hz
618 kΩ
618 kΩ
1.00 MΩ
8.2 nF
47 nF
–
+
MCP6286
D
1
Light
V
OUT
V
DD
R
2
C
2
I
D1
V
OUT
= I
D1
*R
2
–
+
MCP6286
D
1
Light
V
OUT
V
DD
R
2
C
2
I
D1
V
OUT
= I
D1
*R
2
V
BIAS
V
BIAS
< 0V
–
+
MCP6286