100 dB Range (10 nA to 1 mA)

Logarithmic Converter

AD8305

Rev. B

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FEATURES

Optimized for fiber optic photodiode interfacing

Measures current over 5 decades

Law conformance 0.1 dB from 10 nA to 1 mA

Single- or dual-supply operation (3 V to 12 V total)

Full log-ratio capabilities

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

Nominal intercept of 1 nA (set by external resistor)

Optional adjustment of slope and intercept

Complete and temperature stable

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

Optical power measurement

Wide range baseband logarithmic compression

Measurement of current and voltage ratios

Optical absorbance measurement

FUNCTIONAL BLOCK DIAGRAM

BIAS

GENERATOR

VLOG

COMM

VNEG

VSUM

IREF

0.5V

80k

Ω

0.5V

TEMPERATURE

COMPENSATION

6.69k

Ω

Q1

COMM

20k

Ω

451

Ω

VREF

VRDZ

14.2k

Ω

Q2

INPT

COMM

2.5V

VPOS

BFIN

SCAL

VOUT

200k

Ω

1nA

–

+

Figure 1.

GENERAL DESCRIPTION

The AD83051 is an inexpensive microminiature logarithmic converter

optimized for determining optical power in fiber optic systems. It uses

an advanced implementation of a classic translinear (junction based)

technique to provide a large dynamic range in a versatile and easily

used form. A single-supply voltage of between 3 V and 12 V is

adequate; dual supplies may optionally be used. The low quiescent

current (typically 5 mA) permits use in battery-operated applications.

collector current of an optimally scaled NPN transistor, which converts

(0.5 V). This is generally acceptable for photodiode applications where

the anode does not need to be grounded. Similarly, this bias voltage is

front end is available at Pin VLOG.

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

(10 mV/dB). Thus, a 100 dB range corresponds to an output change of

1 V. When this voltage (or the buffer output) is applied to an ADC that

permits an external reference voltage to be employed, the AD8305

voltage reference output of 2.5 V at Pin VREF can be used to improve

the scaling accuracy. Suitable ADCs include the AD7810 (serial 10-bit),

AD7823 (serial 8-bit), and AD7813 (parallel, 8-bit or 10-bit). Other

values of the logarithmic slope can be provided using a simple external

resistor network.

The logarithmic intercept (also known as the reference current) is

nominally positioned at 1 nA by the use of the externally generated

between VREF, at 2.5 V, and the reference input, IREF, at 0.5 V. The

intercept can be adjusted over a wide range by varying this resistor.

The AD8305 can also operate in a log ratio mode, with the numerator

current applied to INPT and the denominator current applied to IREF.

A buffer amplifier is provided for driving a substantial load, for use in

raising the basic slope of 10 mV/dB to higher values, as a precision

comparator (threshold detector), or in implementing 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.

It is a fundamental aspect of translinear logarithmic converters that the

small signal bandwidth falls as the current level diminishes, and the

low frequency noise-spectral density increases. At the 10 nA level, the

bandwidth of the AD8305 is about 50 kHz and increases in proportion

amplifier, the increase in noise level at low currents can be addressed by

using it to realize lowpass filters of up to three poles.

The AD8305 is available in a 16-lead LFCSP package and is specified

for operation from −40°C to +85°C.