LOG114 Single-Supply, High-Speed, Precision Logarithmic Amplifier

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-to-

digital 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.

Package Information

PART NUMBER

PACKAGE(1)

PACKAGE SIZE(2)

LOG114

RGV (VQFN, 16)

4mm × 4mm

(1)

For more information, see Section 10.

(2)

The package size (length × width) is a nominal value and

includes pins, where applicable.

+

–

+

–

+

–

I2

I1

V

CM IN

COM

+

–

V

O4

–IN

4

+IN

4

V+

V

REF GND

I

2

I

1

LOG114

V

LOGOUT

REF

V

REF

–

+

V

O5

+IN

5

–IN

5

V–

A.

Thermally dependent R1 and R3 provide temperature compensation.

B.

VLOGOUT = 0.375 × log(I1/I2).

C.

VO4 = 0.375 × K × log(I1/I2), K = 1 + R6/R5.

D.

Differential Amplifier (A3) Gain = 6.25

Functional Block Diagram

LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,

intellectual property matters and other important disclaimers. PRODUCTION DATA.