60 dB Range (100 nA to 100 µA)
Low Cost Logarithmic Converter
ADL5306
FEATURES
Optimized for fiber optic photodiode interfacing
FUNCTIONAL BLOCK DIAGRAM
VPOS +5V
NC
Measures current over 3 decades
I
PD
0.2 log
10
(
)
VOUT
Law conformance 0.1 dB from 100 nA to 100 μA
Single- or dual-supply operation (3 V to 5.5 V total)
Full log-ratio capabilities
VREF
1nA
BIAS
2.5V
GENERATOR
80kΩ
R
20kΩ
COMM
REF
200kΩ
Temperature stable
0.5V
Nominal slope of 10 mV/dB (200 mV/decade)
Nominal intercept of 1 nA (set by external resistor)
Optional adjustment of slope and intercept
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
SCAL
BFIN
IREF
V
14.2kΩ
BIAS
V
BE2
1kΩ
Q2
Q1
I
LOG
TEMPERATURE
COMPENSATION
1nF
VLOG
451Ω
V
BE1
I
PD INPT
VSUM
1nF
6.69kΩ
COMM
APPLICATIONS
1kΩ
Low cost optical power measurement
Wide range baseband logarithmic compression
Measurement of current and voltage ratios
Optical absorbance measurement
0.5V
1nF
VNEG
COMM
03727-0-001
Figure 1. Functional Block Diagram
The logarithmic intercept (reference current) is nominally
GENERAL DESCRIPTION
positioned at 1 nA by using the externally generated, 100 µA IREF
current provided by a 200 kΩ resistor connected between VREF, at
2.5 V, and IREF, at 0.5 V. The intercept can be adjusted over a
narrow range by varying this resistor. The part can also operate in a
log-ratio mode, with limited accuracy, where the numerator and
denominator currents are applied to INPT and IREF, respectively.
The ADL5306∗ is a low cost microminiature logarithmic converter
optimized for determining optical power in fiber optic systems. The
ADL5306 is derived from the AD8304 and AD8305 translinear
logarithmic converters. This family of devices provides wide
measurement dynamic range in a versatile and easy-to-use form. A
single-supply voltage between 3 V and 5.5 V is adequate; dual
supplies may optionally be used. Low quiescent current (5 mA
typical) permits use in battery-operated applications.
A buffer amplifier is provided to drive substantial loads, raise the
basic 10 mV/dB slope, serve as a precision comparator (threshold
detector), or implement 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.
IPD, the 100 nA to 100 µA input current applied to the INPT pin, is
the collector current of an optimally scaled NPN transistor that
converts this current to a voltage (VBE) with a precise logarithmic
relationship. A second converter is used to handle the reference
current, IREF, applied to 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 logarithmic front end’s output is available at VLOG.
A fundamental aspect of translinear logarithmic converters is that
small-signal bandwidth falls as current level diminishes, and low
frequency noise-spectral density increases. At the 100 nA level, the
ADL5306’s bandwidth is about 100 kHz; it increases in proportion
to IPD up to a maximum of about 10 MHz. The increase in noise
level at low currents can be addressed by using a buffer amplifier to
realize low-pass filters of up to three poles.
.
The basic logarithmic slope at this output is 200 mV/decade
(10 mV/dB) nominal; a 60 dB range corresponds to a 600 mV
output change. When this voltage (or the buffer output) is applied
to an ADC that permits an external reference voltage to be
employed, the ADL5306’s 2.5 V voltage reference output at VREF
can be used to improve scaling accuracy.
The ADL5306 is available in a 16-lead LFCSP package and is
specified for operation from–40°C to +85°C.
∗Protected by US Patents 4,604,532 and 5,519,308; other patents pending.
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. 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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