8

®

OPT301

When used with very large feedback resistors, tiny leakage

currents on the circuit board can degrade the performance of

the OPT301. Careful circuit board design and clean assembly

procedures will help achieve best performance. A “guard

trace” on the circuit board can help minimize leakage to the

critical non-inverting input (pin 2). This guard ring should

encircle pin 2 and connect to Common, pin 8.

DYNAMIC RESPONSE

Using the internal 1M

Ω resistor, the dynamic response of

the photodiode/op amp combination can be modeled as a

simple R/C circuit with a –3dB cutoff frequency of 4kHz.

This yields a rise time of approximately 90

µs (10% to 90%).

Dynamic response is not limited by op amp slew rate. This

is demonstrated by the dynamic response oscilloscope

photographs showing virtually identical large-signal and

small-signal response.

Dynamic response will vary with feedback resistor value as

shown in the typical performance curve “Voltage Output

Responsivity vs Frequency.” Rise time (10% to 90%) will

vary according to the –3dB bandwidth produced by a given

feedback resistor value—

where:

t

f

LINEARITY PERFORMANCE

Current output of the photodiode is very linear with radiant

power throughout a wide range. Nonlinearity remains below

approximately 0.02% up to 100

µA photodiode current. The

photodiode can produce output currents of 1mA or greater

with high radiant power, but nonlinearity increases to several

percent in this region.

This excellent linearity at high radiant power assumes that

the full photodiode area is uniformly illuminated. If the light

source is focused to a small area of the photodiode,

nonlinearity will occur at lower radiant power.

NOISE PERFORMANCE

Noise performance of the OPT301 is determined by the op

amp characteristics in conjunction with the feedback

components and photodiode capacitance. The typical

performance curve “Output Noise Voltage vs Measurement

Bandwidth” shows how the noise varies with R

bandwidth (1Hz to the indicated frequency). The signal

bandwidth of the OPT301 is indicated on the curves. Noise

can be reduced by filtering the output with a cutoff frequency

equal to the signal bandwidth.

Output noise increases in proportion to the square-root of the

feedback resistance, while responsivity increases linearly

with feedback resistance. So best signal-to-noise ratio is

achieved with large feedback resistance. This comes with

the trade-off of decreased bandwidth.

The noise performance of a photodetector is sometimes

characterized by Noise Effective Power (NEP). This is the

radiant power which would produce an output signal equal

to the noise level. NEP has the units of radiant power

(watts). The typical performance curve “Noise Effective

Power vs Measurement Bandwidth” shows how NEP varies

with R

t

R

≈ 0. 35

f

C

(1)

FIGURE 3. Dark Error (Offset) Adjustment Circuit.

1M

Ω

OPT301

40pF

75

Ω

2

83

5

4

1

V

O

V–

λ

V+

0.01µF

500

Ω

V+

V–

100µA

1/2 REF200

100

Ω

100

Ω

100µA

1/2 REF200

Adjust dark output for 0V.

Trim Range: ±7mV

FIGURE 4. Responsivity (Gain) Adjustment Circuit.

1M

Ω

OPT301

40pF

75

Ω

2

83

5

4

1

V

O

V–

λ

V+

10k

Ω

5k

Ω

Gain Adjustment

+50%; –0%

R

F