Analog Devices

The ADL5303 is a monolithic logarithmic detector optimized for the measurement of low frequency signal power in fiber optic systems and offers a large dynamic range in a versatile and easily used form. Wide measurement range and accuracy are achieved using proprietary design and precise laser trimming. The ADL5303 requires only a single positive supply, VPS, of 5 V. When using low supply voltages, the log slope can be altered to fit the available span. Low quiescent current and chip disable facilitate use in battery-operated applications.The input current, IPD, flows in the collector of an optimally scaled NPN transistor, connected in a feedback path around a low offset JFET amplifier. The current summing input node operates at a constant voltage, independent of current, with a default value of 0.5 V; this may be adjusted over a wide range. An adaptive biasing scheme is provided for reducing photo-diode dark current at very low light input levels. The VPDB pin applies approximately 0.1 V reverse bias across the photodiode for IPD= 100 pA, rising linearly to 2.0 V of reverse bias at IPD= 10 mA to improve response time at higher power levels. The input pin INPT is flanked by the VSUM guard pins that track the voltage at the summing node. Connecting the exposed pad of the device to the VSUM pins provides a continuous guard to minimize leakage into the INPT pin.The default value of the logarithmic slope at the VLOG output is set by an internal 5 kΩ resistor. Logarithmic slope can be lowered with an external shunt resistor or increased using the buffer and a pair of external feedback resistors. The addition of a capacitor at the VLOG pin provides a simple low-pass filter. The intermediate voltage, VLOG, is buffered in an output stage that can swing to within about 100 mV of ground and the positive supply, VPS, and provides a peak current drive capacity of ±20 mA. An on-board 2 V reference is provided to facilitate the repositioning of the intercept. The incremental bandwidth of a translinear logarithmic amplifier inherently diminishes for small input currents. At IPD=1 nA, the bandwidth of the ADL5303 is approximately 2 kHz increasing in proportion to IPDup to a maximum value of 10 MHz.APPLICATIONSHigh accuracy optical power measurementWide range baseband log compressionVersatile detector for APC loops

The ADL5304 is a high speed logarithmic converter with fast response and low noise over a 200 dB (1 pA to 10 mA) measurement range. The ADL5304 provides a nominal logarithmic slope of 10 mV/dB (200 mV/decade); other values are easily configured. Logarithmic intercept can be programmed over a wide range with the internal 100 nA current source or externally for log ratio applications. The default intercept value of 3.162 fA places the midpoint of the measurement range of 100 nA at VLOG= 1.5 V.A single positive supply of 5 V is all that is required for operation over a specified 1 pA to 3 mA input range. Dual-supply operation extends the specified input current range to 10 mA.The ADL5304 accepts two current inputs to the logarithmic argument. The numerator input, INUM, flows in the collector of an NPN transistor, connected in a feedback path around a low offset JFET amplifier. The denominator current, IDEN, is treated in the same way, which allows for log ratio operation. The input summing nodes (INUM and IDEN) operate at a constant default voltage of 1.5 V. The VSM1 to VSM4 pins flank the INUM and IDEN inputs to provide a guard voltage to minimize leakage currents.Adaptive photodiode biasing is provided for optical measurements. A monitor current 1.1 times INUMis output at the IMON pin, and an external resistor, RMNTR, at 10 times the photodiode series resistance (RS) applies a voltage across the photodiode that 1st order keeps the internal PD junction at 0 V to minimize dark current.The VLOG output is buffered and can be rescaled through internal gain setting resistors. The internal ILOGvaries from −400 μA to +400 μA as INUMchanges over 10 decades from 1 pA to 10 mA. This corresponds to 0.5 V to 2.5 V at the VLOG pin in the default configuration shown in Figure 1.Accurate 1.5 V (Pin 1P5V) and 2.0 V (Pin 2VLT) reference outputs allow precise repositioning of the intercept using external resistors.The ADL5304 is available in a 32-lead, 5 mm × 5 mm LFCSP and specified for operation from −40°C to +85°C.ApplicationsHigh accuracy optical power measurementWide range baseband log compressionVersatile detector for high speed APC loops

The ADL5306 is a low cost micro-miniature logarithmic converter optimized for the determination of optical power in fiber-optic systems. It uses an advanced implementation of a classic translinear (junction-based) technique to provide a large dynamic range in a versatile and easily-used form. A single supply voltage of between 3 V and 5.5 V is adequate; dual supplies may optionally be used. The low quiescent current (typically 5 mA) permits use in battery-operating applicationsThe input currentIPDof 100 nA to 100 µA applied to theINPTpin is the collector current of an optimally-scaled NPN transistor, which converts this current to a voltage (its VBE) with a precise logarithmic relationship. A second such converter is used to handle the reference current, IREF, applied to pinIREF. These input nodes are biased slightly above ground (0.5 V). This is generally acceptable for photodiode applications where the anode does not need to be grounded. Similarly, this bias voltage is easily accounted for in generating IREF. The output of the logarithmic front-end is available at pinVLOG.

The ADL5308 is a logarithmic transimpedance amplifier optimized for wide dynamic range signal level monitoring in fiber optic systems.It produces an accurate, temperature-compensated output voltage proportional to the logarithm of the ratio between the input current at pin INP, and a reference current. The reference current can either be generated internally or externally provided through the IREF interface (LOG-ratio detection). The logarithmic slope and intercept are both accurately trimmed to a nominal value of 200 mV/decade and 10 pA respectively.The low-impedance VLOG logarithmic output has enough drive capability to drive a wide range of analog-to-digital converters (ADCs) and other circuits and its gain can be adjusted by adding a resistor-divider driving the FB pin.A built-in fast comparator provides a compact solution to compare the logarithmic output to an external reference level, either programmed through I2C or supplied through the CREF interface. The comparator has adjustable hysteresis and an optional output latch function using the HYST pin.Adaptive photodiode (PD) biasing is supported through the PDB pin. At a low diode current, the reverse PD bias is kept small to minimize the dark current. At higher input currents, the bias voltage scales linearly with the current to avoid nonlinearity due to PD saturation. The starting bias level as well as the scale factor at higher currents are configurable through I2C.The ADL5308 is specified for operation from −40°C to +105°C ambient temperature and is offered in a small 2 mm × 3 mm, 14-terminal LGA package.ApplicationsOptical power monitoringErbium-doped fiber amplifiers (EDFA)

The ADL5309 contains dual monolithic logarithmic transimpedance amplifiers that are optimized for measuring low-frequency and wide dynamic range optical signal power in fiber optic systems.The device produces highly accurate, temperature compensated output voltages that are proportional to the logarithm of the ratio between the input current at the INP1 and INP2 pins and the internally generated reference currents. The logarithmic slope and intercept are both accurately trimmed to a nominal value of 200 mV/decade and 10 pA, respectively. The low-impedance OUT1 and OUT2 logarithmic outputs have the capability to drive a wide range of analog-to-digital converters (ADCs) and other circuits.A digital representation of the OUT1 and OUT2 outputs is available through the inter-IC bus (I2C) interface. A 14-bit successive approximation register (SAR) ADC, controlled through the I2C interface, samples the OUT1 and OUT2 channel outputs along with a built-in digital temperature sensor.Adaptive photodiode biasing is supported through the PDB1 and PDB2 interfaces. At low diode currents, the photodiode reverse bias is kept small to minimize the dark current. At higher input currents, the bias voltage scales linearly with the current to avoid nonlinearity due to photodiode saturation. Both the starting bias level and the scale factor at higher currents are configurable through I2C.The ADL5309 is specified for operation from −40°C to +105°C ambient temperature and is offered in a small 2.040 mm × 1.640 mm, 20-lead wafer level chip scale package (WLCSP).APPLICATIONSAutomatic test equipmentOptical power monitoringMachine automationOptical modules

The ADL5310 is a low-cost dual logarithmic amplifier that converts input current over a wide dynamic range to a linear-in-dB output voltage. It is optimized for the determination of optical power in wide-ranging optical communication system applications including control circuitry for lasers, optical switches, attenuators, and amplifiers, as well as general system monitoring. The device is equivalent to a dual AD8305 with enhanced dynamic range (120 dB). While the ADL5310 contains two independent signal channels with individually-configurable transfer function constants (slope and intercept), internal bias circuitry is shared between channels for improved power consumption and channel matching. Dual converters in a single compact LFCSP package yield space-efficient solutions for measuring gain or attenuation across optical elements. Only a single supply is required, while optional dual supply operation offers added flexibility.The ADL5310 employs an optimized translinear structure, which utilizes the accurate logarithmic relationship between a bipolar transistor's base-emitter voltage and collector current, with appropriate scaling by precision currents to compensate the inherent temperature dependence. Input and reference current pins sink current ranging from 3 nA to 3 mA (limited to ±60 dB between input and reference) into a fixed voltage defined by the VSUM potential, which is internally set to 500 mV but may be externally-grounded for dual-supply operation and additional applications requiring voltage inputs.

The ADL5315 is a wide input current range precision high-side current mirror featuring a stable and user-adjustable input voltage. It is optimized for use with PIN photodiodes, but its flexibility and wide operating range make it suitable for a broad array of additional applications. The part provides a compact solution for the common and often challenging problem of interfacing an optical power monitoring device (such as a log amp) to the cathode-side of a PIN photodiode. The ADL5315 mirrors the current at the cathode at a 1:1 ratio over a six-decade range, producing an output exactly proportional to the reference current monitored. Competing optical current monitors have much higher mirror ratios over narrower dynamic ranges, rendering proportional outputs at very low input currents impossible, while discrete solutions are more tedious to implement and may require significantly more board space.