GENERAL DESCRIPTION

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.

FEATURES

► Fast transient response (IPD stepped from 220 μA to 10 nA)

► Rise/fall time: <2.4 μs

► 2 dB Electrical (1 dB optical) settling time: <3.5 μs

► Flat Frequency Response/No Undershoot

► Bandwidth: 970 kHz at IINP = 10 nA

► Accurately trimmed logarithmic response

► Logarithmic slope: 200 mV/dec

► Logarithmic conformance error: ±0.2 dB at 25°C (IINP from 10

nA to 1 mA)

► Ratio input for direct optical gain measurements

► Comparator with adjustable hysteresis and latch enable

► I2C adjustable

► Adaptive photodiode bias

► Comparator reference level

► Minimal external components are required

► 34 dB PSRR at 20 kHz and IINP = 10 nA

► 2 mm × 3 mm, 14-terminal LGA package

APPLICATIONS

► Optical power monitoring

► Erbium-doped fiber amplifiers (EDFA)