Circuit Note CN-0272 Devices Connected/Referenced AD8065 High Performance, 145 MHz FASTFET Op Amp Circuits from the Lab reference circuits are engineered and Precision, Selectable Gain, Fully Differential AD8475 tested for quick and easy system integration to help solve todays Funnel Amp analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0272. AD9629-20 12-Bit, 20 MSPS, 1.8 V ADC Ultralow Noise, LDO XFET 2.5 V Voltage ADR441 Reference with Current Sink and Source 2 MHz Bandwidth PIN Photodiode Preamp with Dark Current Compensation Other suitable applications for this circuit are as an analog opto- EVALUATION AND DESIGN SUPPORT isolator. It can also be adapted to applications that require larger Circuit Evaluation Boards bandwidth and less resolution such as adaptive speed control CN-0272 Circuit Evaluation Board (EVAL-CN0272-SDPZ) systems. System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files This circuit note discusses the design steps needed to optimize the Schematics, Layout Files, Bill of Materials circuit shown in Figure 1 for a specific bandwidth including stability calculations, noise analysis, and component selection CIRCUIT FUNCTION AND BENEFITS considerations. The circuit shown in Figure 1 is a high speed photodiode signal conditioning circuit with dark current compensation. The system converts current from a high speed silicon PIN photodiode and drives the inputs of a 20 MSPS analog-to-digital converter (ADC). This combination of parts offers spectral sensitivity from 400 nm to 1050 nm with 49 nA of photocurrent sensitivity, a dynamic range of 91 dB, and a bandwidth of 2 MHz. The signal conditioning circuitry of the system consumes only 40 mA of current from the 5 V supplies making this configuration suitable for portable high speed, high resolution light intensity applications, such as pulse oximetry. +5V ADR441 +5V +2.5V V V IN OUT C 3.3pF F GND 1k R 24. 9k +1.8V F TP1 +5V TP3 INP 2. 5k OUT 33 AVDD VIN AD8065 VOCM AD8475 22pF AD9629-20 R F 5V 2. 5k 24. 9k 0.1F +2.5V VIN+ SFH 2701 0.1F INN +OUT 33 GND VCM TP2 V BIAS 5V 1k +0.9V 5V Figure 1. Photodiode Preamp System with Dark Current Compensation (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. B Circuits from the Lab circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 20122013 Analog Devices, Inc. All rights reserved. 10599-001CN-0272 Circuit Note C F R F C = C + C + C IN S D M 11 I R = 10 PHOTO SH C M C S C D C M V O V B 0.1F R F Figure 2. Wideband Photodiode Preamp Equivalent Circuit for AC and Noise Analysis Where IPHOTO is the output current of the photodiode, and the CIRCUIT DESCRIPTION parallel combination of RF and CF sets the signal bandwidth. Ideally, Component Selection all of the output current from the photodiode passes through R , F Photodiodes are high impedance sensors used to detect the but all op amps have input bias currents that introduce errors on intensity of light. Photodiodes do not have internal gain but can the output. Op amps with picoamps of input bias current and low operate at much higher light levels than other light detectors. input offset voltages are preferred to minimize the error. The Photodiodes operate either with zero bias (photovoltaic mode) AD8065 has only 2 pA of input bias current and only 400 V of or with a reverse bias (photoconductive mode). The most precise input offset voltage. linear operation is obtained in the photovoltaic mode, while The circuit is designed to have a full scale output of 5 V with a higher switching speeds are realizable when the diode is operated maximum photodiode current of 200 A. This determines the in the photoconductive mode at the expense of linearity. Under value of the feedback resistor to be these reverse bias conditions, a small amount of current called R = 5 V/200 A = 24.9 k F dark current flows even when there is no illumination. The dark current error can be cancelled using a second photodiode of the The stable bandwidth attainable with this preamp is a function of same type in the noninverting input of the op amp as is shown RF, the gain bandwidth product of the amplifier (65 MHz), and the in Figure 1. total capacitance at the amplifiers summing junction, CIN. For this circuit, the SFH 2701 diode (OSRAM Opto Semiconductors Three factors influence the response time of a photodiode GmbH) has a maximum capacitance of C = 5 pF. The AD8065 D The charge collection time of the carriers in the depleted common-mode input capacitance is C = 2.1 pF, and the M region of the photodiode differential-mode input capacitance is C = 4.5 pF. Therefore, D The charge collection time of the carriers in the undepleted the total input capacitance is C = 11.6 pF. IN region of the photodiode It can be shown that the signal bandwidth resulting in a 45 The RC time constant of the diode circuit combination phase margin, f(45), is defined by Because the junction capacitance is dependent on the diffused area f 65 MHz CR f = = of the photodiode and the applied reverse bias, faster rise times are (45) 2 R C 2 24.9 k 11.6 pF F IN obtained with smaller diffused area photodiodes and larger applied = 5.7 MHz reverse biases. The junction capacitance for the SFH 2701 PIN photodiode used in the CN-0272 circuit note is 3 pF typical, 5 pF Because the maximum attainable bandwidth is larger than the maximum for 0 V bias. The typical capacitance is 2 pF for 1 V intended bandwidth, the AD8065 is a good candidate for the reverse bias and 1.7 pF for 5 V reverse bias. Measurements in application, which is largely due to its large ratio of f to C . CR IN the circuit were all taken with a reverse bias of 5 V. Figure 2 shows an I/V converter with an electrical model of a photodiode. The basic transfer function is I R PHOTO F VOUT = 1 + sC R F F Rev. B Page 2 of 8 10599-002