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AD645KN Scheda tecnica(PDF) 7 Page - Analog Devices |
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AD645KN Scheda tecnica(HTML) 7 Page - Analog Devices |
7 / 8 page AD645 –7– REV. B AD645 2 3 6 8 FILTERED OUTPUT OPTIONAL 26Hz FILTER PHOTODIODE GUARD OUTPUT 10pF 10 Ω 9 Figure 30. The AD645 Used as a Sensitive Preamplifier Preamplifier Applications The low input current and offset voltage levels of the AD645 to- gether with its low voltage noise make this amplifier an excellent choice for preamplifiers used in sensitive photodiode applica- tions. In a typical preamp circuit, shown in Figure 30, the out- put of the amplifier is equal to: VOUT = ID (Rf) = Rp (P) Rf where: ID = photodiode signal current (Amps) Rp = photodiode sensitivity (Amp/Watt) Rf = the value of the feedback resistor, in ohms. P = light power incident to photodiode surface, in watts. An equivalent model for a photodiode and its dc error sources is shown in Figure 31. The amplifier’s input current, IB, will con- tribute an output voltage error which will be proportional to the value of the feedback resistor. The offset voltage error, VOS, will cause a “dark” current error due to the photodiode’s finite shunt resistance, Rd. The resulting output voltage error, VE, is equal to: VE = (1 + Rf/Rd) VOS + Rf IB A shunt resistance on the order of 10 9 ohms is typical for a small photodiode. Resistance Rd is a junction resistance which will typically drop by a factor of two for every 10 °C rise in tem- perature. In the AD645, both the offset voltage and drift are low, this helps minimize these errors. PHOTODIODE OUTPUT 10pF 10 Ω 9 ID OS V IB Rd 50pF Cd Cf Rf Figure 31. A Photodiode Model Showing DC Error Sources Minimizing Noise Contributions The noise level limits the resolution obtainable from any pream- plifier. The total output voltage noise divided by the feedback resistance of the op amp defines the minimum detectable signal current. The minimum detectable current divided by the photo- diode sensitivity is the minimum detectable light power. Sources of noise in a typical preamp are shown in Figure 32. The total noise contribution is defined as: V OUT = in2 + i f 2 + is 2 Rf 1 + s (Cf ) Rf 2 + en 2 1 + Rf Rd 1 + s (Cd ) Rd 1 + s (Cf ) Rf 2 Figure 33, a spectral density versus frequency plot of each source’s noise contribution, shows that the bandwidth of the amplifier’s input voltage noise contribution is much greater than its signal bandwidth. In addition, capacitance at the summing junction results in a “peaking” of noise gain in this configura- tion. This effect can be substantial when large photodiodes with large shunt capacitances are used. Capacitor Cf sets the signal bandwidth and also limits the peak in the noise gain. Each source’s rms or root-sum-square contribution to noise is ob- tained by integrating the sum of the squares of all the noise sources and then by obtaining the square root of this sum. Mini- mizing the total area under these curves will optimize the preamplifier’s overall noise performance. PHOTODIODE OUTPUT 10 Ω 9 50pF iS iS Rd Cd 10pF Cf Rf i f in en Figure 32. Noise Contributions of Various Sources FREQUENCY – Hz 100 1k 10k 100k 10 1 10nV 100nV 1 µV 10 µV SIGNAL BANDWIDTH NO FILTER WITH FILTER e n is &if in en Figure 33. Voltage Noise Spectral Density of the Circuit of Figure 32 With and Without an Output Filter An output filter with a passband close to that of the signal can greatly improve the preamplifier’s signal to noise ratio. The pho- todiode preamplifier shown in Figure 32—without a bandpass filter—has a total output noise of 50 µV rms. Using a 26 Hz single pole output filter, the total output noise drops to 23 µV rms, a factor of 2 improvement with no loss in signal bandwidth. Using a “T” Network A “T” network, shown in Figure 34, can be used to boost the ef- fective transimpedance of an I to V converter, for a given feed- back resistor value. Unfortunately, amplifier noise and offset voltage contributions are also amplified by the “T” network gain. A low noise, low offset voltage amplifier, such as the AD645, is needed for this type of application. |
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