Datasheet ADA4817-1, ADA4817-2 (Analog Devices) - 19

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Data Sheet ADA4817-1/ADA4817-2 APPLICATIONS INFORMATION LOW DISTORTION PINOUT
The stable bandwidth attainable with this preamp is a function of RF, the gain bandwidth product of the amplifier, and the total The ADA4817-1/ADA4817-2 feature a new low distortion capacitance at the summing junction of the amplifier, including the pinout from Analog Devices. The new pinout provides two photodiode capacitance (CS) and the amplifier input capacitance. advantages over the traditional pinout. The first advantage is RF and the total capacitance produce a pole in the loop improved second harmonic distortion performance, which is transmission of the amplifier that can result in peaking and accomplished by the physical separation of the noninverting instability. Adding CF creates a zero in the loop transmission input pin and the negative power supply pin. The second that compensates for the effect of the pole and reduces the advantage is the simplification of the layout due to the dedicated signal bandwidth. It can be shown that the signal bandwidth feedback pin and easy routing of the gain set resistor back to obtained with a 45° phase margin (f(45)) is defined by the inverting input pin. This allows a compact layout, which helps to minimize parasitics and increase stability. fCR f  (14) (45) The designer does not need to use the dedicated feedback pin to 2  R  (C  C  C ) F S M D provide feedback for the ADA4817-1/ADA4817-2. The output where: pin of the ADA4817-1/ADA4817-2 can still be used to provide fCR is the amplifier crossover frequency. feedback to the inverting input of the ADA4817-1/ADA4817-2. RF is the feedback resistor.
WIDEBAND PHOTODIODE PREAMP
CS is the source capacitance including the photodiode and the The wide bandwidth and low noise of the ADA4817-1/ board parasitic. ADA4817-2 make it an ideal choice for transimpedance amplifiers, CM is the common-mode capacitance of the amplifier. such as those used for signal conditioning with high speed photo- CD is the differential capacitance of the amplifier. diodes. Figure 50 shows an I/V converter with an electrical The CF value that produces f(45) is shown to be model of a photodiode. The basic transfer function is C  C  C S M D I  R C  (15) F PHOTO F V  (13) 2  R  f F CR OUT 1  sC R F F The frequency response shows less peaking if bigger CF values where: are used. IPHOTO is the output current of the photodiode. The parallel combination of R The preamplifier output noise over frequency is shown in F and CF sets the signal bandwidth. Figure 51.
CF f 1 1 = 2 RF (CF + CS + CM + CD) RF f 1 ) 2 = 2 RFCF Hz f V/ f CR 3 = (C I R C (n F + CS + CM + CD)/CF PHOTO SH = 1011Ω M E IS CS O R C F NOISE D E N C V M OUT G VEN (CF + CS + CM + CD)/CF f A 3 T L f
8
2 V
04
B VO
6- 75
f1
07 Figure 50. Wideband Photodiode Preamp
VEN NOISE DUE TO AMPLIFIER
43 0 6-
FREQUENCY (Hz)
75 07 Figure 51. Photodiode Voltage Noise Contributions Rev. C | Page 19 of 25 Document Outline FEATURES APPLICATIONS CONNECTION DIAGRAMS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ±5 V OPERATION 5 V OPERATION ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE MAXIMUM SAFE POWER DISSIPATION ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TEST CIRCUITS THEORY OF OPERATION CLOSED-LOOP FREQUENCY RESPONSE NONINVERTING CLOSED-LOOP FREQUENCY RESPONSE INVERTING CLOSED-LOOP FREQUENCY RESPONSE WIDEBAND OPERATION DRIVING CAPACITIVE LOADS THERMAL CONSIDERATIONS POWER-DOWN OPERATION CAPACITIVE FEEDBACK HIGHER FREQUENCY ATTENUATION LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS SIGNAL ROUTING POWER SUPPLY BYPASSING GROUNDING EXPOSED PADDLE LEAKAGE CURRENTS INPUT CAPACITANCE INPUT-TO-INPUT/OUTPUT COUPLING APPLICATIONS INFORMATION LOW DISTORTION PINOUT WIDEBAND PHOTODIODE PREAMP HIGH SPEED JFET INPUT INSTRUMENTATION AMPLIFIER ACTIVE LOW-PASS FILTER (LPF) OUTLINE DIMENSIONS ORDERING GUIDE