Datasheet ADA4530-1 (Analog Devices) - 45

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Data Sheet ADA4530-1 PHOTODIODE INTERFACE
The low input bias current and low input offset voltage makes More importantly, the shunt resistance decreases by half for the ADA4530-1 an excellent choice for signal conditioning every 10°C increase in temperature. An error current is created photodiodes at extremely low illumination levels. Figure 124 because the amplifier offset voltage is applied across this shunt shows the ADA4530-1 configured in a transimpedance ampli- resistance, resulting in an RTI error (IVOS_RTI) equal to fier interfacing with a photodiode operating in photovoltaic IVOS_RTI = VOS/RSHUNT mode (photodiode is zero biased). A photodiode produces an output current proportional to the il umination level. The It is equivalent to think that the shunt resistance increases the amplifier converts the signal current, I DC noise gain (NG), which multiplies the offset voltage to the PD, into an output voltage with the following equation: output. The RTO error due to VOS is equal to V VOS_RTO = VOS × Noise Gain OUT = IPD × RF
C
V
F
OS_RTO = VOS × (1 + RF/RSHUNT) The amplifier input resistance and insulation resistance appear
RF
in paral el with the photodiode shunt resistance. These addi-
IPD
tional resistances reduce the effective shunt resistance, but they
VOUT PHOTODIODE
323 are much larger than the photodiode shunt resistance and can 13405- usually be ignored. Figure 124. Transimpedance Amplifier with Photodiode
AC ERROR ANALYSIS
Figure 125 replaces the photodiode with an equivalent circuit Photodiode TIA circuits typical y require external compensation to model. IPD is the photo current generated by incident light and give satisfactory dynamic performance. The large feedback is proportional to the light level. The shunt capacitance (CSHUNT) resistor (RF) interacts with the large photodiode capacitance models the depletion capacitance of the diode. This capacitance (CSHUNT) to create a low frequency pole in the feedback network. depends on the area of the photodiode and the voltage bias. The Photodiode shunt capacitance, amplifier input capacitance, and shunt resistance (RSHUNT) represents the voltage vs. current slope trace capacitance are lumped into a single element, CSHUNT. The of the exponential diode curve near zero bias voltage. phase shift due to this pole must be recovered prior to the
CF
crossover frequency for the feedback loop to be stable. The usual method to recover this phase shift is to create a zero in the
RF PHOTODIODE
feedback factor with the addition of the feedback capacitor (CF).
CSHUNT V
The classic way of analyzing this circuit is by examining the
OUT IPD RSHUNT
noise gain vs. frequency (see Figure 126). At low frequencies, the noise gain is determined by the ratio of the feedback to the 324 shunt resistance. 13405- Figure 125. Transimpedance Amplifier with Photodiode Model RF = 1 NG 1 +
DC ERROR ANALYSIS
S R HUNT Al of the errors described in the High Impedance Measurements The troublesome low frequency pole (which is a zero in the section related to TIA circuits are applicable to photodiode noise gain) occurs at Frequency f1. From this frequency onward, interfaces. the noise gain increases. If there is no feedback capacitor in the circuit, the noise gain fol ows the dotted line until it intersects The inverting input bias current, IB−, sums directly with the with the amplifier open-loop gain curve. If these curves photodiode current for a referred to input (RTI) error equal to intersect at the 20 dB/decade slopes shown in Figure 126, the IB−. This current flows through the feedback resistor, creating a circuit is unstable. referred to output (RTO) error, VIB_TRO, equal to The addition of CF adds a zero to the feedback factor (which is a VIB_RTO = IB− × RF pole in the noise gain) at Frequency f2. Beyond Frequency f2, the The amplifier offset voltage, VOS, is a major error source in noise gain is determined by the ratio of the shunt capacitance to photodiode interface circuits because of the relatively low shunt the feedback capacitance. resistance of large area photodiodes. Typical values are in the C range of 1 GΩ to 100 GΩ at 25°C. SHUNT NG = 1 + 2 CF Rev. B | Page 45 of 52 Document Outline FEATURES APPLICATIONS PIN CONNECTION DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS 5 V NOMINAL ELECTRICAL CHARACTERISTICS 10 V NOMINAL ELECTRICAL CHARACTERISTICS 15 V NOMINAL ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS MAIN AMPLIFIER, DC PERFORMANCE MAIN AMPLIFIER, AC PERFORMANCE GUARD AMPLIFIER THEORY OF OPERATION ESD STRUCTURE INPUT STAGE GAIN STAGE OUTPUT STAGE GUARD BUFFER APPLICATIONS INFORMATION INPUT PROTECTION SINGLE-SUPPLY AND RAIL-TO-RAIL OUTPUT CAPACITIVE LOAD STABILITY EMI REJECTION RATIO HIGH IMPEDANCE MEASUREMENTS INPUT BIAS CURRENT INPUT RESISTANCE INPUT OFFSET VOLTAGE INSULATION RESISTANCE GUARDING DIELECTRIC RELAXATION HUMIDITY EFFECTS CONTAMINATION CLEANING AND HANDLING SOLDER PASTE SELECTION CURRENT NOISE CONSIDERATIONS LAYOUT GUIDELINES PHYSICAL IMPLEMENTATION OF GUARDING TECHNIQUES GUARD RING GUARD PLANE VIA FENCE CABLES AND CONNECTORS ELECTROSTATIC INTERFERANCE PHOTODIODE INTERFACE DC ERROR ANALYSIS AC ERROR ANALYSIS NOISE ANALYSIS DESIGN RECOMMENDATIONS DESIGN EXAMPLE POWER SUPPLY RECOMMENDATIONS POWER SUPPLY CONSIDERATIONS LONG-TERM DRIFT TEMPERATURE HYSTERESIS OUTLINE DIMENSIONS ORDERING GUIDE