ADL5306

Optimized for Fiber-Optic Photodiode Interfacing Measures Current Over Three Decades Law Conformance 0.3 dB from 100 nA to 100 A Single or Dual Supply Operation (±3 V to ±5.5 V total) Full Log-Ratio Capabilities Temperature Stable 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 pin Chip Scale Package (LFCSP 3x3 mm) Low Power: ~5mA Quiescent Current

Low Cost Optical Power Measurement Wide Range Baseband Logarithmic Compression Measurement of Current- and Voltage-Ratios Optical Absorbance Measurement

The ADL5306 is a low cost micro-miniature logarithmic converter optimized for the determination of optical power in fiber-optic systems. The ADL5306 is a derivative of the popular AD8304 and AD8305 translinear logarithmic converters. The family of devices provide wide measurement dynamic range in a versatile and easily-used form. A single supply voltage of between 3 V and 5.5 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 ADL5306 is 100 nA to 100 A, which is applied to the INPT pin is the collector current of an optimally-scaled NPN transistor, which converts this current to a voltage (its 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 60-dB range corresponds to an output change of 600mV. When this voltage (or the buffer output) is applied to an ADC that permits an external reference voltage to be employed, the ADL5306's 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 the AD7813 (parallel, 8/10 bits). Other values of the logarithmic slope can be provided using a simple external resistor network.

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 100 A, provided by a 200 k resistor connected between VREF, at 2.5 V, and the reference input IREF, at 0.5 V. The intercept can be adjusted over a narrow range by varying this resistor. The ADL5306 can also operate in a log-ratio mode, with limited accuracy, where 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 100 nA level, the bandwidth of the ADL5306 is about 100 kHz, and increases in proportion to IPD up to a maximum value of about 10 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 ADL5306 is available in a 16 pin LFCSP package and specified for operation from −40°C to +85°C.

US Patents 4,604,532, 5,519,308 and others pending.