Figure 3 Bio-impedance measurement model
To measure the bio-impedance, a high-frequency AC current is injected into the tissue through the drive electrodes (refer
Figure 3). The AC current causes a potential voltage difference of the two receive electrodes (refer Figure 3). This potential
voltage difference is related to the resistivity of the tissue between the voltage-sensing or receive electrodes. The equivalent
resistance is defined as the ratio of the voltage difference between the two receiver electrodes and the current that flows
through the tissue.
2.2 Hardware overview and circuit description
2.2.1 Weight-Scale Analog Front-End
Figure 4 Weight - Scale Front-End
2.2.1.1 Overview and Gain Calculation
The weight-scale front-end has two stages of gain, with an offset correction DAC in the second gain stage. The reason for using
INA and PGA is to fit different input voltage from the sensor bridge. Though not shown in the diagram, an anti-aliasing network
is required in front of the INA to filter out electromagnetic interference (EMI) signals or any other anticipated interference
signals. A simple RC network should be sufficient, combined with of the attenuation provided by the on-chip decimation filter.
