Very-Wide-Dynamic-Range Logarithmic Amplifier

April 14, 2007
TI Introduces a DC precision, high-speed (1 micro-second rise and fall time), precision logarithmic amplifier that computes the logarithm or log-ratio of an input current or voltage relative to a reference current or voltage. The LOG114 can handle eight decades of dynamic range and the completely tested log function allows use without external components. The small size of the LOG114 makes it a good fit for single-supply systems with a limited number of supply rails. Specifically designed to amplify photodiode signals from fiber optic cables in optical networking communication systems (ONET), the LOG114 can also be used for industrial and medical optical density/absorbance measurements and general signal compression for a very wide dynamic range of signals in front of an analog-to-digital converter (ADC). The QFN-16 package allows use in small plug-in modules and multi-channel systems. The high speed and eight decades of dynamic range (equivalent to 27 bits; 100pA to 10mA input) with high precision make it highly desirable in laser control systems. It has an internal 2.5V reference and two independent op amps that can be used for offsetting, amplitude scaling, threshold detection or other functions. In addition, the DC precision permits accurate instrumentation system measurements. The LOG114 operates on a single +5V or +/-5V dual supply over a temperature range of -5C to +75C. Excellent gain stability over temperature is achieved using a special thermal compensation technique on the integrated circuit. Low noise, wide bandwidth amplifiers assure accurate measurement even for low-level signals at relatively higher frequencies, while operating on a low quiescent current of 10mA. When used for general signal compression, the LOG114 complements TI’s ADCs, such as the 12-bit ADS7816, ADS7822 and ADS7844, and the 16-bit ADS8320 and ADS8344. In addition to the LOG114, which is a log transimpedance amplifier, other ICs from TI also have linear transimpedance amplifiers, such as OPA380/381 (resistor feedback transimpedance amplifiers), IVC102 (switch capacitor transimpedance amplifier), and DDC112 (direct digital converter transimpedance amplifier).