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MCP607

2.5V to 5.5V Micropower CMOS Op Amps

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4.7.2 PHOTODIODE AMPLIFIERS

Sensors that produce an output current and have high

output impedance can be connected to a transimped-

ance amplifier. The transimpedance amplifier converts

the current into voltage. Photodiodes are one sensor

that produce an output current.

The key op amp characteristics that are needed for

these circuits are: low input offset voltage, low input

bias current, high input impedance and an input

common mode range that includes ground. The low

input offset voltage and low input bias current support

a very low voltage drop across the photodiode; this

gives the best photodiode linearity. Since the

photodiode is biased at ground, the op amp’s input

needs to function well both above and below ground.

4.7.2.1 Photo-Voltaic Mode

Figure 4-6 shows a transimpedance amplifier with a

photodiode (D1) biased in the Photo-voltaic mode (0V

across D1), which is used for precision photodiode

sensing.

As light impinges on D1, charge is generated, causing

a current to flow in the reverse bias direction of D1. The

op amp’s negative feedback forces the voltage across

the D1 to be nearly 0V. Resistor R2 converts the current

into voltage. Capacitor C2 limits the bandwidth and

helps stabilize the circuit when D1’s junction

capacitance is large.

VOUT = ID1R2

C2

ID1

Light

R2

VDD

VOUT

D1 MCP606

FIGURE 4-6:

Photodiode (in Photo-voltaic

mode) and Transimpedance Amplifier.

4.7.2.2 Photo-Conductive Mode

Figure 4-6 shows a transimpedance amplifier with a

photodiode (D1) biased in the Photo-conductive mode

(D1 is reverse biased), which is used for high-speed

applications.

As light impinges on D1, charge is generated, causing

a current to flow in the reverse bias direction of D1.

Placing a negative bias on D1 significantly reduces its

junction capacitance, which allows the circuit to

MCP606/7/8/9

operate at a much higher speed. This reverse bias also

increases the dark current and current noise, however.

Resistor R2 converts the current into voltage. Capacitor

C2 limits the bandwidth and helps stabilize the circuit

when D1’s junction capacitance is large.

VB < 0

VOUT = ID1R2

C2

ID1

Light

R2

VDD

VOUT

D1 MCP606

VB

FIGURE 4-7:

Photodiode (in Photo-

conductive mode) and Transimpedance

Amplifier.

4.7.3

TWO OP AMP INSTRUMENTATION

AMPLIFIER

The two op amp instrumentation amplifier shown in

Figure 4-8 serves the function of taking the difference

of two input voltages, level-shifting it and gaining it to

the output. This configuration is best suited for higher

gains (i.e., gain > 3 V/V). The reference voltage (VREF)

is typically at mid-supply (VDD/2) in a single-supply

environment.

VO U T

=

(V1

–

⎛

V2

)

⎜1

⎝

+

R----1--

R2

+

2----R----1-

RG

⎞

⎟

⎠

+

VREF

RG

R1

R2

R2

R1

VREF

VOUT

V2

½

½

MCP607

MCP607

V1

FIGURE 4-8:

Amplifier.

Two op amp Instrumentation

The key specifications that make the MCP606/7/8/9

family appropriate for this application circuit are low

input bias current, low offset voltage and high common-

mode rejection.

© 2005 Microchip Technology Inc.

DS11177D-page 13

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