60 dB Range (100 nA to 100 µA)

Low Cost Logarithmic Converter

ADL5306

Rev. 0

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FEATURES

Optimized for fiber optic photodiode interfacing

Measures current over 3 decades

Law conformance 0.1 dB from 100 nA to 100 μA

Single- or dual-supply operation (3 V to ±5.5 V total)

Full log-ratio capabilities

Temperature stable

Nominal slope of 10 mV/dB (200 mV/decade)

Nominal intercept of 1 nA (set by external resistor)

Optional adjustment of slope and intercept

Rapid response time for a given current level

Miniature 16-lead chip scale package (LFCSP 3 mm × 3 mm)

Low power: ~5 mA quiescent current

APPLICATIONS

Low cost optical power measurement

Wide range baseband logarithmic compression

Measurement of current and voltage ratios

Optical absorbance measurement

GENERAL DESCRIPTION

The ADL5306

∗ is a low cost microminiature logarithmic converter

optimized for determining optical power in fiber optic systems. The

ADL5306 is derived from the AD8304 and AD8305 translinear

logarithmic converters. This family of devices provides wide

measurement dynamic range in a versatile and easy-to-use form. A

single-supply voltage between 3 V and 5.5 V is adequate; dual

supplies may optionally be used. Low quiescent current (5 mA

typical) permits use in battery-operated applications.

the collector current of an optimally scaled NPN transistor that

relationship. A second converter is used to handle the reference

above ground (0.5 V). This is generally acceptable for photodiode

applications where the anode does not need to be grounded.

The logarithmic front end’s output is available at VLOG.

The basic logarithmic slope at this output is 200 mV/decade

(10 mV/dB) nominal; a 60 dB range corresponds to a 600 mV

output change. When this voltage (or the buffer output) is applied

to an ADC that permits an external reference voltage to be

employed, the ADL5306’s 2.5 V voltage reference output at VREF

can be used to improve scaling accuracy.

FUNCTIONAL BLOCK DIAGRAM

TEMPERATURE

COMPENSATION

BIAS

GENERATOR

451

Ω

14.2k

Ω

80k

Ω

20k

Ω

200k

Ω

6.69k

Ω

COMM

COMM

COMM

VNEG

VSUM

VPOS

INPT

VREF

NC

IREF

0.5V

+5V

2.5V

0.5V

VOUT

SCAL

BFIN

VLOG

03727-0-001

( )

1nA

1k

Ω

1k

Ω

1nF

1nF

1nF

Q2

Q1

Figure 1. Functional Block Diagram

The logarithmic intercept (reference current) is nominally

current provided by a 200 kΩ resistor connected between VREF, at

2.5 V, and IREF, at 0.5 V. The intercept can be adjusted over a

narrow range by varying this resistor. The part can also operate in a

log-ratio mode, with limited accuracy, where the numerator and

denominator currents are applied to INPT and IREF, respectively.

A buffer amplifier is provided to drive substantial loads, raise the

basic 10 mV/dB slope, serve as a precision comparator (threshold

detector), or implement low-pass filters. Its rail-to-rail output stage

can swing to within 100 mV of the positive and negative supply

rails, and its peak current-sourcing capacity is 25 mA.

A fundamental aspect of translinear logarithmic converters is that

small-signal bandwidth falls as current level diminishes, and low

frequency noise-spectral density increases. At the 100 nA level, the

ADL5306’s bandwidth is about 100 kHz; it increases in proportion

level at low currents can be addressed by using a buffer amplifier to

realize low-pass filters of up to three poles.

The ADL5306 is available in a 16-lead LFCSP package and is

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

∗

Protected by US Patents 4,604,532 and 5,519,308; other patents pending.