MCP6281 datasheet(15/36 Pages) MICROCHIP

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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 VDD/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 VDD/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

VOUT

VIN

VDD/2

–

MCP6281

VOUT

VIN

VDD/2

VOUT

VIN

sRC

–

MCP6285

VOUTB

Load

OUT

INGAGB

OSAGAGB

OSBGB

Where:

op amp A gain

op amp B gain

VOSA

op amp A input offset voltage

VOSB

op amp B input offset voltage

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