AD8305_15 Datasheet, PDF (1/24 Pages) Analog Devices – 100 dB Range Logarithmic Converter

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
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.
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.
1 _{Protected by U.S. Patent No. 5,519,308.}
Rev. B
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100 dB Range (10 nA to 1 mA)
Logarithmic Converter
AD8305
FUNCTIONAL BLOCK DIAGRAM
VRDZ
V_P
VPOS
I_PD
0.20 log_{10 1nA}
VOUT
VREF
200kΩ
0.5V
IREF
V_BIAS
I_PD
INPT
20kΩ
80kΩ
2.5V
BIAS
GENERATOR
COMM
Q2
Q1
SCAL
V_BE2
14.2kΩ
–
TEMPERATURE
I_LOG
BFIN
451Ω
+ COMPENSATION
VLOG
V_BE1
6.69kΩ
VSUM 0.5V
COMM
VNEG
Figure 1.
COMM
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.
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