MCP6281 datasheet(14/32 Pages) MICROCHIP

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MCP6281/2/3/4/5

DS21811D-page 14

 2004 Microchip Technology Inc.

4.8.3

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.

4.8.3.1

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 4-9:

Isolating the Load with a

Buffer.

4.8.3.2

Cascaded Gain

Figure 4-10 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).

FIGURE 4-10:

Cascaded Gain Circuit

Configuration.

4.8.3.3

Difference Amplifier

Figure 4-11 shows op amp A configured as a difference

amplifier with Chip Select. In this configuration, it is

recommended to use well-matched resistors (e.g.,

0.1%) to increase the Common Mode Rejection Ratio

(CMRR). Op amp B can be used to provide additional

gain and isolate the load from the difference amplifier.

FIGURE 4-11:

Difference Amplifier Circuit.

MCP6285

VOUTB

Load

OUT

INGAGB

OSAGAGB

OSBGB

Where:

GA = op amp A gain

GB = op amp B gain

VOSA = op amp A input offset voltage

VOSB = op amp B input offset voltage

VIN

VOUT

MCP6285

VIN2

VIN1

VOUT

MCP6285

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