Datasheet ADL5306 (Analog Devices)

link to page 3 60 dB Range (100 nA to 100 µA) Low Cost Logarithmic Converter ADL5306
FEATURES FUNCTIONAL BLOCK DIAGRAM Optimized for fiber optic photodiode interfacing VPOS +5V Measures current over 3 decades NC Law conformance 0.1 dB from 100 nA to 100 μA 0.2 log VREF 10 ( ) IPD 1nA VOUT Single- or dual-supply operation (3 V to ±5.5 V total) 2.5V BIAS GENERATOR Full log-ratio capabilities 80k
Ω
RREF 20k
Ω
Temperature stable 200k
Ω
COMM Nominal slope of 10 mV/dB (200 mV/decade) 0.5V Nominal intercept of 1 nA (set by external resistor) IREF SCAL VBIAS 14.2k
Ω
Optional adjustment of slope and intercept BFIN VBE2 1k
Ω
Rapid response time for a given current level Q2 I TEMPERATURE LOG Miniature 16-lead chip scale package (LFCSP 3 mm × 3 mm) 1nF COMPENSATION Q1 451
Ω
VLOG Low power: ~5 mA quiescent current VBE1 I PD INPT 6.69k
Ω
APPLICATIONS 1k
Ω
COMM VSUM Low cost optical power measurement 0.5V 1nF Wide range baseband logarithmic compression 1nF VNEG COMM 03727-0-001 Measurement of current and voltage ratios Optical absorbance measurement
Figure 1. Functional Block Diagram
GENERAL DESCRIPTION
The logarithmic intercept (reference current) is nominally positioned at 1 nA by using the externally generated, 100 µA IREF The ADL5306∗ is a low cost microminiature logarithmic converter current provided by a 200 kΩ resistor connected between VREF, at optimized for determining optical power in fiber optic systems. The 2.5 V, and IREF, at 0.5 V. The intercept can be adjusted over a ADL5306 is derived from the AD8304 and AD8305 translinear narrow range by varying this resistor. The part can also operate in a logarithmic converters. This family of devices provides wide log-ratio mode, with limited accuracy, where the numerator and measurement dynamic range in a versatile and easy-to-use form. A denominator currents are applied to INPT and IREF, respectively. single-supply voltage between 3 V and 5.5 V is adequate; dual supplies may optionally be used. Low quiescent current (5 mA A buffer amplifier is provided to drive substantial loads, raise the typical) permits use in battery-operated applications. basic 10 mV/dB slope, serve as a precision comparator (threshold detector), or implement low-pass filters. Its rail-to-rail output stage IPD, the 100 nA to 100 µA input current applied to the INPT pin, is can swing to within 100 mV of the positive and negative supply the collector current of an optimally scaled NPN transistor that rails, and its peak current-sourcing capacity is 25 mA. converts this current to a voltage (VBE) with a precise logarithmic relationship. A second converter is used to handle the reference A fundamental aspect of translinear logarithmic converters is that current, IREF, applied to IREF. These input nodes are biased slightly small-signal bandwidth falls as current level diminishes, and low above ground (0.5 V). This is generally acceptable for photodiode frequency noise-spectral density increases. At the 100 nA level, the applications where the anode does not need to be grounded. ADL5306’s bandwidth is about 100 kHz; it increases in proportion Similarly, this bias voltage is easily accounted for in generating IREF. to IPD up to a maximum of about 10 MHz. The increase in noise The logarithmic front end’s output is available at VLOG. 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 The ADL5306 is available in a 16-lead LFCSP package and is output change. When this voltage (or the buffer output) is applied specified for operation from–40°C to +85°C. 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. Protected by US Patent 5,519,308.
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Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION AND PIN FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS GENERAL STRUCTURE THEORY MANAGING INTERCEPT AND SLOPE RESPONSE TIME AND NOISE CONSIDERATIONS APPLICATIONS USING A NEGATIVE SUPPLY CHARACTERIZATION METHODS EVALUATION BOARD OUTLINE DIMENSIONS ORDERING GUIDE