MCP609-ISLASHP datasheet(18/42 Pages) MICROCHIP

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MCP606/7/8/9

DS11177F-page 18

4.8.2

PHOTODIODE AMPLIFIERS

Sensors that produce an output current and have high

output

impedance

can

connected

transimpedance

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.

4.8.2.1

Photo-Voltaic Mode

Figure 4-9 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.

FIGURE 4-9:

Photodiode (in Photo-voltaic

mode) and Transimpedance Amplifier.

4.8.2.2

Photo-Conductive Mode

Figure 4-9 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

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.

FIGURE 4-10:

Photodiode (in Photo-

conductive mode) and Transimpedance

Amplifier.

4.8.3

TWO OP AMP INSTRUMENTATION

AMPLIFIER

The two op amp instrumentation amplifier shown in

Figure 4-11 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.

FIGURE 4-11:

Two Op Amp

Instrumentation Amplifier.

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.

OUT

D1R2

VOUT

Light

VDD

ID1

MCP606

OUT

D1R2

VOUT

Light

VDD

ID1

MCP606

OUT

–

() 1

------

----------

⎝⎠

⎜⎟

⎛⎞

REF

VOUT

VREF

MCP607

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