Quantization of the response signal differences for the electrical bioimpedance measurement

Solutions for practical implementation of the electrical bioimpedance acquisition typically need to be optimized and adapted for the task at hand. One of the most challenging measurement tasks is impedance plethysmography, the test that measures tiny changes in electrical conductivity, related to changes in biological volumes. These changes depend on the underlying biological processes, measurement locations, and added random artefacts. Changes in the acquired impedance due to the cardiac activity can easily be 0.1% and lower. In modern devices, the digitalization of the data acquisition results is almost mandatory, and it places heavy demand on the used analog-to-digital converters. Only few levels are left from 16-bit converter for the direct digitalization of the impedance plethysmography signal, given that the useful signal is only around thousandth of the full impedance scale, and that some margin must be left. Several solutions have been proposed, such as a direct compensation of the carrier of the response signal in order to emphasize tiny variations of it, or in the case of analog preprocessing capacitive coupling helps to separate static component of the impedance. A novel solution for the direct digitalization of the response signal is proposed. Differences between adjacent analog samples taken synchronously to the carrier are quantized instead of the full value. Essentially derivative of the response signal is digitized in this case. Number of the required quantizing levels is reduced significantly. Solution is easily embeddable and customizable.