120 dB Range (3 nA to 3 mA)

Dual Logarithmic Converter

ADL5310

Rev. A

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FEATURES

2 independent channels optimized for photodiode

interfacing

6-decade input dynamic range

Law conformance 0.3 dB from 3 nA to 3 mA

Temperature-stable logarithmic outputs

Nominal slope 10 mV/dB (200 mV/dec), externally scalable

Intercepts may be independently set by external resistors

User-configurable output buffer amplifiers

Single- or dual-supply operation

Space-efficient, 24-lead 4 mm × 4 mm LFCSP

Low power: < 10 mA quiescent current

APPLICATIONS

Gain and absorbance measurements

Multichannel power monitoring

General-purpose baseband log compression

PRODUCT DESCRIPTION

input current over a wide dynamic range to a linear-in-dB

output voltage. It is optimized to determine the optical power

in wide-ranging optical communication system applications,

including control circuitry for lasers, optical switches, atten-

uators, and amplifiers, as well as system monitoring. The device

is equivalent to a dual AD8305 with enhanced dynamic range

(120 dB). While the ADL5310 contains two independent signal

channels with individually configurable transfer function

constants (slope and intercept), internal bias circuitry is shared

between channels for improved power consumption and

channel matching. Dual converters in a single, compact LFCSP

package yield space-efficient solutions for measuring gain or

attenuation across optical elements. Only a single supply is

required; optional dual-supply operation offers added flexibility.

The ADL5310 employs an optimized translinear structure that

use the accurate logarithmic relationship between a bipolar

transistor’s base emitter voltage and collector current, with

appropriate scaling by precision currents to compensate for the

inherent temperature dependence. Input and reference current

pins sink current ranging from 3 nA to 3 mA (limited to ±60 dB

between input and reference) into a fixed voltage defined by the

VSUM potential. The VSUM potential is internally set to

500 mV but may be externally grounded for dual-supply opera-

tion, and for additional applications requiring voltage inputs.

FUNCTIONAL BLOCK DIAGRAM

TEMPERATURE

COMPENSATION

REFERENCE

GENERATOR

451

Ω

14.2k

Ω

80k

Ω

20k

Ω

6.69k

Ω

COMM

COMM

VREF

VREF

VRDZ

VNEG

VSUM

INP2

IRF2

2.5V

0.5V

OUT2

SCL2

BIN2

LOG2

TEMPERATURE

COMPENSATION

451

Ω

14.2k

Ω

6.69k

Ω

COMM

VNEG

VSUM

INP1

IRF1

OUT1

SCL1

BIN1

LOG1

665k

Ω

665k

Ω

Figure 1.

The logarithmic slope is set to 10 mV/dB (200 mV/decade)

nominal and can be modified using external resistors and the

independent buffer amplifiers. The logarithmic intercepts for

each channel are defined by the individual reference currents,

which are set to 3 μA nominal for maximum input range by

connecting 665 kΩ resistors between the 2.5 V VREF pins and

the IRF1 and IRF2 inputs. Tying VRDZ to VREF effectively sets

the x-intercept four decades below the reference current—

typically 300 pA for a 3 µA reference.

The use of individually optimized reference currents may

be valuable when using the ADL5310 for gain or absorbance

measurements where each channel input has a different current-

range requirement. The reference current inputs

are also fully functional dynamic inputs, allowing log ratio

operation with the reference input current as the denominator.

The ADL5310 is specified for operation from –40°C to +85°C.