BESCHREIBUNG DES PRODUKTS

The AD8304 is a monolithic logarithmic detector optimized for the measurement of low frequency signal power in fiber optic systems. It uses an advanced translinear technique to provide an exceptionally large dynamic range in a versatile and easily used form. Its wide measurement range and accuracy are achieved using proprietary design techniques and precise laser trimming. In most applications only a single positive supply, VP, of 5 V will be required, but 3.0 V to 5.5 V can be used, and certain applications benefit from the added use of a negative supply, VN. When using low supply voltages, the log slope is readily altered to fit the available span. The low quiescent current and chip disable features facilitate use in battery-operated applications.

The input current, IPD, flows in the collector of an optimally scaled NPN transistor, connected in a feedback path around a low offset JFET amplifier. The current-summing input node operates at a constant voltage, independent of current, with a default value of 0.5 V; this may be adjusted over a wide range, including ground or below, using an optional negative supply.

An adaptive biasing scheme is provided for reducing the dark current at very low light input levels. The voltage at Pin VPDB applies approximately 0.1 V across the diode for IPD = 100 pA, rising linearly with current to 2.0 V of net bias at IPD = 10 mA. The input pin INPT is flanked by the guard pins VSUM that track the voltage at the summing node to minimize leakage.

The default value of the logarithmic slope at the output VLOG is accurately scaled to 10 mV/dB (200 mV/decade). The resistance at this output is laser-trimmed to 5 kΩ, allowing the slope to be lowered by shunting it with an external resistance; the addition of a capacitor at this pin provides a simple low-pass filter. The intermediate voltage VLOG is buffered in an output stage that can swing to within about 100 mV of ground (or VN) and the positive supply, VP, and provides a peak current drive capacity of ±20 mA. The slope can be increased using the buffer and a pair of external feedback resistors. An accurate voltage reference of 2 V is also provided to facilitate the repositioning of the intercept.

Many operational modes are possible. For example, low-pass filters of up to three poles may be implemented, to reduce the output noise at low input currents. The buffer may also serve as a comparator, with or without hysteresis, using the 2 V reference, for example, in alarm applications. The incremental bandwidth of a translinear logarithmic amplifier inherently diminishes for small input currents. At the 1 nA level, the AD8304’s bandwidth is about 2 kHz, but this increases in proportion to IPD up to a maximum value of 10 MHz.

The AD8304 is available in a 14-lead TSSOP package and specified for operation from –40°C to +85°C.

 

FEATURES

Optimized for Fiber Optic Photodiode Interfacing

Eight Full Decades of Range

Law Conformance 0.1 dB from 1 nA to 1 mA

Single-Supply Operation (3.0 V– 5.5 V)

Complete and Temperature Stable

Accurate Laser-Trimmed Scaling:

Logarithmic Slope of 10 mV/dB (at VLOG Pin)

Basic Logarithmic Intercept at 100 pA

Easy Adjustment of Slope and Intercept

Output Bandwidth of 10 MHz, 15 V/s Slew Rate

1-, 2-, or 3-Pole Low-Pass Filtering at Output

Miniature 14-Lead Package (TSSOP)

Low Power: ~4.5 mA Quiescent Current (Enabled)

 

ANWENDUNGEN

High Accuracy Optical Power Measurement

Wide Range Baseband Log Compression

Versatile Detector for APC Loops

 

ANWENDUNGEN

The AD8304 incorporates features that improve its usefulness in both fiber optic supervisory applications and in more general ones. To aid in the exploration of these possibilities, a SPICE macromodel is provided and a versatile evaluation board is available.

The macromodel is shown in generalized schematic form (and thus is independent of variations in SPICE programs). Q1, QM, and Q2 (here made equal in size) correspond to the identical transistors in Figure 1. The model parameters for these transistors are not critical; the default model provided in SPICE libraries will be satisfactory. However, the AD8304 employs compensation techniques to reduce errors caused by junction resistances (notably, RB and RE) at high input currents. Therefore, it is advisable to set these to zero. While this will not model the AD8304 precisely, it is safer than using possibly high default values for these parameters. The low current model parameters may also need consideration. Note that no attempt is made to capture either dynamic behavior or the effects of temperature in this simple macromodel; scaling is correct for 27°C.