MCP6281 datasheet(13/32 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21811D-page 13

MCP6281/2/3/4/5

FIGURE 4-6:

Example Guard Ring Layout

for Inverting Gain.

For

Inverting

Gain

and

Transimpedance

Amplifiers (convert current to voltage, such as

photo detectors):

Connect the guard ring to the non-inverting

input pin (VIN+). This biases the guard ring

to the same reference voltage as the op

amp (e.g., VDD/2 or ground).

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

Non-inverting Gain and Unity-Gain Buffer:

Connect the non-inverting pin (VIN+) to the

input with a wire that does not touch the

PCB surface.

Connect the guard ring to the inverting input

pin (VIN–). This biases the guard ring to the

common mode input voltage.

4.8

Application Circuits

4.8.1

SALLEN-KEY HIGH-PASS FILTER

The MCP6281/2/3/4/5 op amps can be used in active-

filter applications. Figure 4-7 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 4-7:

Sallen-Key High-Pass Filter.

This filter, and others, can be designed using

Microchip’s FilterLab® software, which is available on

our web site (www.microchip.com).

4.8.2

INVERTING MILLER INTEGRATOR

Analog integrators are used in filters, control loops and

measurement circuits. Figure 4-8 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 4-8:

Miller Integrator.

Guard Ring

VSS

VIN–VIN+

MCP6281

VOUT

VIN

VDD/2

–

MCP6281

VOUT

VIN

VDD/2

VOUT

VIN

sRC

–

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