100 dB Range (10 nA to 1 mA)
Logarithmic Converter
AD8305*
FEATURES
FUNCTIONAL BLOCK DIAGRAM
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
I
V
PD
P
0.20 log
10( )
1nA
VPOS
2.5V
VRDZ
VREF
VOUT
80k⍀
BIAS
GENERATOR
200k⍀
20k⍀
COMM
0.5V
SCAL
BFIN
IREF
V
14.2k⍀
BE2
V
I
BIAS
LOG
451⍀
Q2
Q1
–
+
TEMPERATURE
COMPENSATION
VLOG
I
PD
Low Power: ~5 mA Quiescent Current
V
BE1
6.69k⍀
COMM
INPT
APPLICATIONS
Optical Power Measurement
Wide Range Baseband Logarithmic Compression
Measurement of Current and Voltage Ratios
Optical Absorbance Measurement
0.5V
VSUM
COMM
VNEG
GENERAL DESCRIPTION
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 AD8305 is an inexpensive microminiature logarithmic
converter optimized for determining optical power in fiber optic
systems. It uses an advanced implementation of a classic trans-
linear (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 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 mA, provided by a 200 kW 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.
The input current, IPD, of 10 nA to 1 mA applied to the INPT
pin is the collector current of an optimally scaled NPN transis-
tor, 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 gen-
erally 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 loga-
rithmic front end is available at Pin VLOG.
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 dimin-
ishes, 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 low-
pass filters of up to three poles.
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’s voltage reference output of 2.5 V
at Pin VREF can be used to improve the scaling accuracy. Suit-
able ADCs include the AD7810 (serial 10-bit), AD7823 (serial
The AD8305 is available in a 16-lead LFCSP package and is
specified for operation from –40∞C to +85∞C.
*Protected by U.S. Patent No. 4,604,532 and 5,519,308; other patents pending.
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, norforanyinfringementsofpatentsorotherrightsofthirdpartiesthat
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
Fax: 781/326-8703
www.analog.com
© 2003 Analog Devices, Inc. All rights reserved.
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