1 Features
• Advantages:
– Tiny for high density systems
– Precision on one supply
– Fast over eight decades
– Fully-tested function
• Two scaling amplifiers
• Wide input dynamic range: eight decades, 100pA
to 10mA
• 2.5V reference
• Stable over temperature
• Low quiescent current: 10mA
• Dual or single supply: ±5V, 5V
• Package: Small QFN-16 (4mm × 4mm)
• Specified temperature range: −5°C to 75°C

2 Applications
• Onet erbium-doped fiber optic amplifiers (EDFA)
• Laser optical density measurement
• Photodiode signal compression amplifiers
• Log, log-ratio function
• Analog signal compression in front of analog-todigital
converters (ADC)
• Absorbance measurement

3 Description
The LOG114 is specifically designed for measuring
low-level and wide dynamic range currents in
communications, lasers, medical, and industrial
systems. The device computes the logarithm or
log-ratio of an input current or voltage relative
to a reference current or voltage (logarithmic
transimpedance amplifier).
High precision is ensured over a wide dynamic range
of input signals on either bipolar (±5V) or single
(5V) supply. Special temperature drift compensation
circuitry is included on-chip. In log-ratio applications,
the signal current may be from a high impedance
source such as a photodiode or resistor in series
with a low impedance voltage source. The reference
current is provided by a resistor in series with a
precision internal voltage reference, photo diode, or
active current source.
The output signal at VLOGOUT has a scale factor
of 0.375V/decade of input current, which limits the
output so that it fits within a 5V or 10V range. The
output can be scaled and offset with one of the
available additional amplifiers, so it matches a wide
variety of ADC input ranges. Stable DC performance
allows accurate measurement of low-level signals
over a wide temperature range. The LOG114 is
specified over a −5°C to 75°C temperature range and
can operate from −40°C to 85°C.