Datasheet

© 2008 Microchip Technology Inc. DS21811E-page 15
MCP6281/1R/2/3/4/5
4.9 Application Circuits
4.9.1 SALLEN-KEY HIGH-PASS FILTER
The MCP6281/1R/2/3/4/5 op amps can be used in
active-filter applications. Figure 4-8 shows a second-
order Sallen-Key high-pass filter with a gain of 1. The
output bias voltage is set by the V
DD
/2 reference, which
can be changed to any voltage within the output voltage
range.
FIGURE 4-8: Sallen-Key High-Pass Filter.
This filter, and others, can be designed using
Microchip’s Design Aids; see Section 5.2 “FilterLab®
Software” and Section 5.3 “Mindi™ Circuit
Designer & Simulator”.
4.9.2 INVERTING MILLER INTEGRATOR
Analog integrators are used in filters, control loops and
measurement circuits. Figure 4-9 shows the most
common implementation, the inverting Miller integrator.
The non-inverting input is at V
DD
/2 so that the op amp
properly biases up. The switch (SW) is used to zero the
output in some applications. Other applications use a
feedback loop to keep the output within its linear range
of operation.
FIGURE 4-9: Miller Integrator.
4.9.3 CASCADED OP AMP
APPLICATIONS
The MCP6285 provides the flexibility of Low-power
mode for dual op amps in an 8-pin package. The
MCP6285 eliminates the added cost and space in
battery-powered applications by using two single op
amps with Chip Select lines or a 10-pin device with one
Chip Select line for both op amps. Since the two op
amps are internally cascaded, this device cannot be
used in circuits that require active or passive elements
between the two op amps. However, there are several
applications where this op amp configuration with
Chip Select line becomes suitable. The circuits below
show possible applications for this device.
4.9.3.1 Load Isolation
With the cascaded op amp configuration, op amp B can
be used to isolate the load from op amp A. In applica-
tions where op amp A is driving capacitive or low resis-
tance loads in the feedback loop (such as an integrator
circuit or filter circuit), the op amp may not have
sufficient source current to drive the load. In this case,
op amp B can be used as a buffer.
FIGURE 4-10: Isolating the Load with a
Buffer.
4.9.3.2 Cascaded Gain
Figure 4-11 shows a cascaded gain circuit configura-
tion with Chip Select. Op amps A and B are configured
in a non-inverting amplifier configuration. In this
configuration, it is important to note that the input offset
voltage of op amp A is amplified by the gain of
op amp A and B, as shown below:
Therefore, it is recommended to set most of the gain
with op amp A and use op amp B with relatively small
gain (e.g., a unity-gain buffer).
MCP6281
V
OUT
V
IN
V
DD
/2
R
2
R
1
C
2
C
1
+
–
MCP6281
V
OUT
V
IN
V
DD
/2
RC
V
OUT
V
IN
=
1
sRC
SW
+
–
A
B
MCP6285
CS
V
OUTB
Load
V
OUT
V
IN
G
A
G
B
V
OSA
G
A
G
B
V
OSB
G
B
++=
Where:
G
A
= op amp A gain
G
B
= op amp B gain
V
OSA
= op amp A input offset voltage
V
OSB
= op amp B input offset voltage