GENERAL DESCRIPTION
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).

FEATURES
► 188 dB electrical dynamic range and 94 dB optical dynamic
range
► Dual-channel current inputs for direct optical gain measurements
► Accurately trimmed logarithmic response:
► Logarithmic slope 200 mV/decade
► Logarithmic conformance error ±0.4 dB over temperature
► Offset trimmed at 70°C to reduce dark current
► Integrated LDO: 26 dB PSRR at 60 Hz and 100 pA input current
(VCC = 3.0 V)
► I2C adjustable:
► Photodiode bias voltage
► Trade-off between noise and response time at low input currents
► Integrated 14-bit ADC
► Requires minimal external components
► 2.040 mm × 1.640 mm, 20-lead WLCSP

APPLICATIONS
► Automatic test equipment
► Optical power monitoring
► Machine automation
► Optical modules