AD8305:  100 dB-range (10nA-1mA) Logarithmic Converter

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 translinear (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 input current, I_PD, of 10 nA to 1 mA applied to the INPT pin is the collector current of an optimally scaled NPN transistor, which converts this current to a voltage (V_BE) with a precise logarithmic relationship. A second such converter is used to handle the reference current (I_REF) applied to pin I_REF. 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 I_REF. 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 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 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 AD7813 (parallel, 8-bit or 10-bit). 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, I_REF, of 10 μA, provided by a 200 kΩ 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.

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 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 lowpass filters of up to three poles.

The AD8305 is available in a 16-lead LFCSP package and is specified for operation from −40°C to +85°C.

• Optical power measurement
• Wide range baseband logarithmic compression
• Measurement of current and voltage ratios
• Optical absorbance measurement