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OPT209P Datasheet(PDF) 8 Page - Burr-Brown (TI) |
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OPT209P Datasheet(HTML) 8 Page - Burr-Brown (TI) |
8 / 11 page 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 1mV offset with RF = 10MΩ. The OPT209 is useful with 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: tR is the rise time (10% to 90%) fC is the –3dB bandwidth 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” shows how the noise varies with RF and measured 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 with RF and measurement bandwidth. t R ≈ 0. 35 f C (1) |
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