8
®
OPT210
cosine of the incident angle). At a greater incident angle,
light is diffused by the side of the package. These effects are
shown in the typical performance curve, “Response vs
Incident Angle.”
LINEARITY PERFORMANCE
Photodiode current is very linear with radiant power
throughout its range. Nonlinearity remains below
approximately 0.01% up to 200µA. The anode buffer drive,
however, is limited to approximately 200µA. This produces
an abrupt limit to photodiode output current when radiant
power reaches approximately 450µW.
Best linearity is achieved with the photodiode uniformly
illuminated. A light source focused to a very small beam,
illuminating only a small percentage of the photodiode area,
may produce a higher nonlinearity.
NOISE PERFORMANCE
Noise performance of the OPT210 is determined by the op
amp characteristics in conjunction with the feedback
components, photodiode capacitance, and buffer performance.
The typical performance curve “Output Noise Voltage vs
Measurement Bandwidth” shows how the noise varies with
R
F
and measured bandwidth (0.1Hz to the indicated
frequency). The signal bandwidth of the OPT210 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), or Watts/√Hz to convey spectral information about
the noise. The typical performance curve “Output Noise
Voltage vs Measurement Bandwidth” is also scaled for NEP
on the right-hand side.
The OPT210 can be connected to operate from a single
power supply as shown in Figure 3. The non-inverting input
bias current flows through a zener diode to provide a bias
voltage. The output voltage is referenced to this bias point.
FIGURE 3. Single Power Supply Operation.
OPT210
(2)
8
1
+
(3)
2
1µF
(1)
3
(4)
5
(5)
(5.6V)
V
O
VO measured
relative to 5.6V
zener voltage.
ZD
1
ZD1: IN4626 5.6V
specified at I
Z
= 250µA
≈300µA
λ
R
F
+1
+15V
0.1µF
DARK ERRORS
The dark errors in the specification table include all sources
with R
F
= 1MΩ. The dominant error source is the input
offset voltage of the op amp. Photodiode dark current is
approximately 70pA and the combined input bias current of
the op amp and buffer is approximately 30pA. Photodiode
dark current and input bias current total approximately
100pA at 25°C and double for each 10°C above 25°C. At
70°C, the total error current is approximately 2nA. With
R
F
= 1MΩ, this would produce a 2mV offset voltage in
addition to the initial amplifier offset voltage (10mV max)
at 25°C. The dark output voltage can be trimmed to zero
with the optional circuit shown in Figure 2.
LIGHT SOURCE POSITIONING
The OPT210 is tested with a light source that uniformly
illuminates the full integrated circuit area, including the op
amp. Although all IC amplifiers are light sensitive to some
degree, the OPT210 op amp circuitry is designed to minimize
this effect. Sensitive junctions are shielded with metal where
possible. Furthermore, the photodiode area is very large
compared to the op amp circuitry making these effects
negligible.
If your light source is focused to a small area, be sure that
it is properly aimed to fall on the photodiode. If a narrowly
focused light source were to miss the photodiode and fall on
the op amp circuitry, the OPT210 would not perform properly.
The large photodiode area is clearly visible as a very dark
area slightly offset from the center of the IC.
The incident angle of the light source also affects the
apparent sensitivity in uniform irradiance. For small incident
angles, the loss in sensitivity is due to the smaller effective
light gathering area of the photodiode (proportional to the