8

®

OPT209

DARK ERRORS

The dark errors in the specification table include all sources.

The dominant error source is the input offset voltage of the

op amp. Photodiode dark current and input bias current of

the op amp are in the 2pA range and contribute virtually no

offset error at room temperature. Dark current and input bias

current double for each 10

°C above 25°C. At 70°C, the error

current can be approximately 100pA. This would produce a

feedback resistors of 100M

Ω or greater at room temperature.

The dark output voltage can be trimmed to zero with the

optional circuit shown in Figure 3.

When used with very large feedback resistors, tiny leakage

currents on the circuit board can degrade the performance of

the OPT209. Careful circuit board design and clean assembly

procedures will help achieve best performance. A “guard

ring” 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.

FIGURE 3. Dark Error (Offset) Adjustment Circuit.

LINEARITY PERFORMANCE

Current output of the photodiode is very linear with radiant

power throughout a wide range. Nonlinearity remains below

approximately 0.01% up to 100

µA photodiode current. The

photodiode can produce output currents of 10mA or greater

with high radiant power, but nonlinearity increases to several

percent in this region.

This very linear performance 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.

DYNAMIC RESPONSE

Using the internal 1M

Ω resistor, the dynamic response of

the photodiode/op amp combination can be modeled as a

1M

Ω

OPT209

10pF

175

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

OPT209

10pF

175

Ω

2

83

5

4

1

V

O

V–

λ

V+

10k

Ω

5k

Ω

Gain Adjustment

+50%; –0%

R

F

simple R/C circuit with a –3dB cutoff frequency of 16kHz.

This yields a rise time of approximately 22

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:

NOISE PERFORMANCE

Noise performance of the OPT209 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 (1Hz to the indicated frequency). The signal

bandwidth of the OPT209 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

t

R

≈ 0. 35

f

C

(1)