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AD8305:  100 dB-range (10nA-1mA) Logarithmic Converter

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The AD8305 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.

The input current, IPD, of 10 nA to 1 mA applied to the INPT pin is the collector current of an optimally scaled NPN transistor, which converts this current to a voltage (VBE) with a precise logarithmic relationship. A second such converter is used to handle the reference current (IREF) applied to pin IREF. These input nodes are biased slightly above ground (0.5 V). This is generally acceptable for photodiode applications where the anode does not need to be grounded. Similarly, this bias voltage is easily accounted for in generating IREF. The output of the logarithmic 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 current, IREF, of 10 μA, provided by a 200 kΩ resistor connected 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 to IPD up to a maximum value of about 15 MHz. Using the buffer 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.

  • Optical power measurement
  • Wide range baseband logarithmic compression
  • Measurement of current and voltage ratios
  • Optical absorbance measurement


  • Optimized for Fiber-Optic Photodiode Interfacing
  • Measures Current Over Five Decades
    Law Conformance 0.1dB from 10nA to 1mA
  • 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

Functional Block Diagram for AD8305


Наименование Тип контента Тип файлов
AD8305: Dual 12-Bit, High Bandwidth, Multiplying DAC with 4-Quadrant Resistors and Serial Interface Data Sheet  (Rev B, 04/2010) (pdf, 1301 kB) Технические описания PDF
AN-643: Closed-Loop Control Circuit Implementation of the ADuC832 MicroConverter®IC and the AD8305 Logarithmic Converter in a Digital Variable Optical Attenuator  (pdf, 395 kB) Статьи по применению PDF
CN-0057: Interfacing the ADL5317 High Side Current Mirror to a Translinear Logarithmic Amplifier in an Avalanche Photodiode Power Detector Application  (pdf, 107 kB) Circuit Note PDF
CN-0056: Interfacing the ADL5315 High Side Current Mirror to a Translinear Logarithmic Amplifier in a Photodiode Power Detector Application  (pdf, 107 kB) Circuit Note PDF
Log Amps and Directional Couplers Enable VSWR Detection  (pdf, 2223 kB)
by Anil Rachakonda and Larry Hawkins, Analog Devices, Inc. (RF Design, 1/1/2007)
Технические статьи PDF
Design a Logamp RF Pulse Detector  (pdf, 338 kB)
by Eamon Nash
(Microwaves & RF, January 1, 2000)
Технические статьи PDF
Logarithmic Processing Applied to Network Power Monitoring  (pdf, 593 kB)
by Eric Newman, Analog Devices, Inc.
Технические статьи PDF
Detecting Fast RF Bursts Using Log Amps
(Analog Dialogue, Vol. 36, No. 5, September-October, 2002)
Журнал Analog Dialogue HTML
Optical and High Speed Networking  (pdf, 2236 kB)
Analog Devices’ optical and high speed networking ICs solve a depth and breadth of challenges faced by today’s designers of datacom and telecom systems, optical modules, and subsystems. Analog Devices products address a wide range of networking applications from O/E/O conversion, clock recovery, and backplane transmission to monitoring and control of optical power, power management, and clock generation and distribution.
Обзор PDF
RAQs index Rarely Asked Questions HTML
Словарь терминов Глоссарий HTML

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AD8305-EVALZ Статус: Prodn Evaluation Board $ 100,19 Да -

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