6 Detailed Description

6.1 Overview

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 designed for 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 can 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 the output signal fits within a 5V or 10V range. The output can be digitized directly, or scaled and offset with

one of the available additional amplifiers, to match 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.

6.2 Functional Block Diagram

I1

I2

Logarithmic

Amplifier (A1)

VCM IN

Logarithmic

Amplifier (A2)

VCM

Difference

Amplifier (A3)

Auxiliary

Amplifier (A4)

COM

VLOGOUT

VO4

+IN4

–IN4

Voltage

Reference

VREF

Auxiliary

Amplifier (A5)

+IN5

VO5

–IN5

6.3 Feature Description

6.3.1 Logarithmic and Difference Amplifier

The LOG114 uses two matched logarithmic amplifiers (A1 and A2 with logging diodes in the feedback loops) to

generate the outputs log (I1) and log (I2), respectively. The gain of 6.25 differential amplifier (A3) subtracts the

output of A2 from the output of A1, resulting in [log(I1) − log(I2)], or log(I1/I2). The symmetrical design of the A1

and A2 logarithmic amps allows I1 and I2 to be used interchangeably, and provides good bandwidth and phase

characteristics with frequency.

6.3.2 COM Voltage Range

The voltage on the COM pin is used to bias the differential amplifier, A3, within its linear range. This voltage can

provide an asymmetrical offset of the VLOGOUT voltage.

6.3.3 VCM IN

The VCM IN pin is used to bias the A1 and A2 amplifiers into their common-mode input voltage range, (V−) + 1.5V

to (V+) − 1.5V.

6.3.4 Auxiliary Operational Amplifier

The LOG114 features two additional wide bandwidth amplifiers, A4 and A5. These amplifiers are for use to

support functions such as single-ended to differential conversion, or single-ended gain, scaling, offsetting,

threshold detection, filtering, or other functions.

LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

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