Datasheet ADA4001-2 (Analog Devices) - 10

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ADA4001-2 Data Sheet APPLICATIONS INFORMATION Cf TOTAL NOISE INCLUDING SOURCE RESISTORS
The low input current noise and input bias current of the
R2
ADA4001-2 makes it the ideal amplifier for circuits with
VEE
substantial input source resistance. Input offset voltage increases by less than 15 nV per 500 Ω of source resistance
4 2
at room temperature. The total noise density of the circuit is
1 ADA4001-2 3
e  e 2  i R 2  4kTR
Rd Ct 8
nTOTAL n  n S S -034 where:
VCC
75 103 en is the input voltage noise density of the part. Figure 28. Equivalent Preamplifier Photodiode Circuit in is the input current noise density of the part. R A larger signal bandwidth can be attained at the expense of S is the source resistance at the noninverting terminal. k is Boltzmann’s constant (1.38 × 10–23 J/K). additional output noise. The total input capacitance (Ct) T is the ambient temperature in Kelvin (T = 273 + °C). consists of the sum of the diode capacitance and the amplifier’s input capacitance (8 pF), which includes external parasitic For RS < 4 kΩ, en dominates and enTOTAL ≈ en. The current capacitance. Ct creates a pole in the frequency response that can noise of the ADA4001-2 is so low that its total density does lead to an unstable system. To ensure stability and optimize the not become a significant term unless RS is greater than bandwidth of the signal, a capacitor is placed in the feedback 100 MΩ, an impractical value for most applications. loop of the circuit shown in Figure 28. It creates a zero and The total equivalent rms noise over a specific bandwidth is yields a bandwidth whose corner frequency is 1/(2π(R2Cf)). expressed as The value of R2 can be determined by the ratio e  e BW nTOTAL nTOTAL V/ID where BW is the bandwidth in hertz. where: Note that the previous analysis is valid for frequencies larger V is the desired output voltage of the op amp. than 150 Hz and assumes flat noise above 10 kHz. For lower ID is the diode current. frequencies, flicker noise (1/f) must be considered. For example, if ID is 100 μA and a 10 V output voltage is desired,
I-V CONVERSION APPLICATIONS
R2 should be 100 kΩ. Rd (see Figure 28) is a junction resistance that drops typically by a factor of 2 for every 10°C increase in
Photodiode Circuits
temperature. Common applications for I-V conversion include photodiode A typical value for Rd is 1000 MΩ. Because Rd >> R2, the circuits where the amplifier is used to convert a current emitted circuit behavior is not impacted by the effect of the junction by a diode placed at the negative input terminal into an output resistance. The maximum signal bandwidth is voltage. The ADA4001-2 low input bias current, wide bandwidth, and ft f  MAX low noise makes it an excellent choice for various photodiode 2R Ct 2 applications, including fax machines, fiber optic controls, where ft is the unity gain frequency of the amplifier. motion sensors, and bar code readers. Cf can be calculated by The circuit shown in Figure 28 uses a silicon diode with zero bias voltage. This is known as a photovoltaic mode; Ct Cf  this configuration limits the overall noise and is suitable for 2R2 ft instrumentation applications. where ft is the unity gain frequency of the op amp, and it achieves a phase margin, φM, of approximately 45°. A higher phase margin can be obtained by increasing the value of Cf. Setting Cf to twice the previous value yields approximately φM = 65° and a maximal flat frequency response, but it reduces the maximum signal bandwidth by 50%. Rev. C | Page 10 of 12 Document Outline Features Applications Pin Configuration General Description Revision History Specifications Electrical Characteristics Absolute Maximum Ratings Thermal Resistance ESD Caution Typical Performance Characteristics Applications Information Total Noise Including Source Resistors I-V Conversion Applications Photodiode Circuits Input Bias Current Noise Considerations Outline Dimensions Ordering Guide