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 1MΩ 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 90µs (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—
0. 35
t
R
≈
(1)
f
C
where:
t
R
is the rise time (10% to 90%)
f
C
is the –3dB bandwidth
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 100µA 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
F
and measured
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
F
and measurement bandwidth.
2
2
1MΩ
4
1MΩ
R
F
4
40pF
Gain Adjustment
+50%; –0%
75Ω
40pF
V+
100µA
1/2 REF200
100Ω
75Ω
λ
OPT301
8
500Ω
0.01µF
1
V+
3
V–
5
V
O
λ
OPT301
8
1
V+
3
V–
5
V
O
5kΩ
10kΩ
100Ω
100µA
1/2 REF200
FIGURE 4. Responsivity (Gain) Adjustment Circuit.
Adjust dark output for 0V.
Trim Range: ±7mV
V–
FIGURE 3. Dark Error (Offset) Adjustment Circuit.
®
OPT301
8
BURR-BROWN [ BURR-BROWN CORPORATION ]
