Microchip Technology AC164120 User Manual

Lesson Projects

 2004 Microchip Technology Inc.

DS51476A-page 23

3.4

LESSON 2 – PHOTODIODE

A simple photodiode circuit can be constructed from the schematic shown in 
Figure 3-3. D

1

 is a Panasonic PNZ334 PIN photodiode. U

1

 is Microchip Technology 

MCP6001U op amp. R

1

 was chosen to be 64.9 k

Ω so that V

OUT

≈ 4.5V at the maximum 

I

S

, and V

OUT

≈ 0V at the minimum I

S

. C

3

 is a 0.1

µF capacitor for good supply bypass 

performance. R

2

 and C

2

 form a low-pass filter for noise performance. Since the 

maximum sample rate of the Signal Analysis PICtail Daughter Board is 5000 samples 
per second (sps), the low-pass pole is set near 500 Hz. 

R

2

 = 10.0 k

Ω

C

2

 

≈ 1/(2π(500 Hz)(10.0 kΩ)) = 32 nF

The nearest standard value of 33 nF, giving

f

RnCn

 = 1/(2

π(10.0 kΩ)(33 nF)) = 482 kHz

Table 3-1 summarizes the final design values of the components in Figure 3-3. 
Figure 3-4 through Figure 3-6 show screen captures from data acquisitions from the 
photodiode placed 30 cm under a fluorescent desk lamp. The sample size was 1024 
and the sample speed was set at 1000.

FIGURE 3-3:

PHOTODIODE PICtail™ SCHEMATIC DIAGRAM

TABLE 3-1:

PHOTODIODE PICtail™ COMPONENTS

U

1

D

1

R

1

C

1

R

2

C

2

C

3

+5V

V

OUT

PNZ334

MCP6001U

0.1

µ

Component

Value/Part No.

Size/Package

C

1

18 pF

SMD 0805

C

2

33 nF

SMD 0805

C

3

0.1

µF

SMD 0805

D

1

PNZ334

Panasonic

Plastic Through-hole,

100 mil pitch

R

1

64.9 k

Ω, 1%

SMD 0805

R

2

10.0 k

Ω

SMD 0805

U

1

MCP6001U

SOT-23-5