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

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.

Applications 
  • Optical power measurement
  • Wide range baseband logarithmic compression
  • Measurement of current and voltage ratios
  • Optical absorbance measurement
Features and Benefits
  • 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
  • Optical
    Amplifiers
    Data Sheets
    Documentnote
    AD8305: Dual 12-Bit, High Bandwidth, Multiplying DAC with 4-Quadrant Resistors and Serial Interface Data Sheet (Rev. B)PDF 1301 kB
    Application Notes
    Documentnote
    AN-643: Closed-Loop Control Circuit Implementation of the ADuC832 MicroConverter®IC and the AD8305 Logarithmic Converter in a Digital Variable Optical Attenuator (Rev. 0)PDF 395 kB
    Order Information
    Part NumberPackagePacking QtyTemp RangePrice 100-499Price 1000+RoHS
    AD8305ACP-R2 Obsolete16 ld LFCSP (3x3mm, 1.3mm exposed pad)REEL 25000N
    AD8305ACP-REEL7 Obsolete16 ld LFCSP (3x3mm, 1.3mm exposed pad)REEL 150000N
    AD8305ACPZ-R2 Production16 ld LFCSP (3x3mm, 1.3mm exposed pad)REEL 250-40 to 85C00Y
    AD8305ACPZ-RL7 Production16 ld LFCSP (3x3mm, 1.3mm exposed pad)REEL 1500-40 to 85C07.9Y
    Evaluation Boards
    Part NumberDescriptionPriceRoHS
    AD8305-EVALZEvaluation Board100.19Y
    Reference Materials
    AD8305: Dual 12-Bit, High Bandwidth, Multiplying DAC with 4-Quadrant Resistors and Serial Interface Data Sheet (Rev. B) ad8305
    AN-643: Closed-Loop Control Circuit Implementation of the ADuC832 MicroConverter®IC and the AD8305 Logarithmic Converter in a Di aduc832
    CN-0057: Interfacing the ADL5317 High Side Current Mirror to a Translinear... ad8304
    CN-0056: Interfacing the ADL5315 High Side Current Mirror to a Translinear... ad8304
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