MCP601 datasheet(13/28 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21314F-page 13

MCP601/2/3/4

3.7.2

INSTRUMENTATION AMPLIFIER

CIRCUITS

Instrumentation amplifiers have a differential input that

subtracts one input voltage from another and rejects

common mode signals. These amplifiers also provide a

single-ended output voltage.

The three-op amp instrumentation amplifier is illustrated

in Figure 3-7. One advantage of this approach is unity-

gain operation, while one disadvantage is that the

common mode input range is reduced as R2/RG gets

larger.

FIGURE 3-7:

Three-Op Amp

Instrumentation Amplifier.

The two-op amp instrumentation amplifier is shown in

Figure 3-8. While its power consumption is lower than

the three-op amp version, its main drawbacks are that

the common mode range is reduced with higher gains

and it must be configured in gains of two or higher.

FIGURE 3-8:

Two-Op Amp

Instrumentation Amplifier.

Both instrumentation amplifiers should use a bulk

bypass capacitor of at least 1 µF. The CMRR of these

amplifiers will be set by both the op amp CMRR and

resistor matching.

3.7.3

PHOTO DETECTION

The MCP601/2/3/4 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 (R2) in the feedback loop of the

amplifiers shown in Figure 3-9 and Figure 3-10. The

optional capacitor (C2) sometimes provides stability for

these circuits.

A photodiode configured in the Photovoltaic mode has

zero voltage potential placed across it (Figure 3-9). 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 3-9:

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 3-10). 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 3-10:

Photoconductive Mode

Detector.

MCP60X

VOUT

VREF

–

VOUT

V1 V2

–

() 1

2R2

---------



 R4

------



 V

REF

MCP60X

VOUT

VREF

VOUT

V1 V2

–

() 1

------

2R1

---------



 V

REF

Light

VOUT

VDD

MCP60X

ID1

VOUT = ID1 R2

–

Light

VOUT

VDD

MCP60X

ID1

VOUT = ID1 R2

VBIAS

VBIAS < 0V

–

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