A continuous-time noise-shaping modulator for logarithmic A/D conversion

This paper presents a practical structure that implements a logarithmic noise shaping modulator. The circuit may be used to replace the combination of a logarithmic amplifier followed by an analog-to-digital converter. The output of the modulator is a low resolution data stream similar to that of a sigma delta modulator. However, this data stream conveys information on the logarithm of the input signal instead of the signal itself. Provided that the logarithm of the input signal has a low-pass spectrum, a quantized sequence can be extracted from this data stream by a decimation filter. The paper presents both theoretical developments and practical results on a prototype. This circuit may be of interest in the measurement of magnitudes represented in logarithmic units (dB)