ADL5308 Logarithmic Transimpedance Amplifier

ADI's 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. The reverse PD bias is kept small at a low diode current 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 and 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.

The ADL5308-EVALZ evaluation board allows the evaluation of the ADL5308 logarithmic converter IC. The ADL5308 is a monolithic logarithmic transimpedance amplifier optimized for low-frequency measurement and wide dynamic range signal power in fiber optic systems. It produces an output voltage proportional to the logarithm of the ratio between the input current at pin INP and an internal reference current. The reference current can also be supplied externally through the IREF interface. The logarithmic slope and intercept are both factory-trimmed to a nominal value of 200 mV/dec and 10 pA, respectively. Both can be adjusted through the I²C interface.