A micropower logarithmic A/D with offset and temperature compensation

Logarithmic circuits are useful in many applications that require nonlinear signal compression, such as in speech recognition front-ends (SRFEs) and cochlear implants or bionic ears (BEs). A logarithmic current-input analog-to-digital converter (A/D) with temperature compensation and automatic offset calibration is presented in this paper. It employs a diode to compute the logarithm, a wide linear range transconductor to perform voltage-to-current conversion, and a dual-slope auto- zeroing topology with 60 dB of dynamic range for sampling the envelope of speech signals. The temperature dependence of the logarithm inherent in a diode implementation is automatically cancelled in our circuit topology. Experimental results from a 1.5-μm 3-V BiCMOS process show that the converter achieves a temperature stability lower than 150 ppm/°C from 12°C to 42°C, and consumes only 3 μW of power when sampling at 300 Hz. At this level of power consumption, we show that the design is thermal-noise limited to 8 bits of precision. This level of precision is more than adequate for deaf patients and for speech recognition front-ends. The power consumption is almost two orders of magnitude lower than state-of-the-art DSP implementations, and the use of a local feedback topology achieves a 2.5-bit improvement over conventional dual-slope designs.