2. Theory of Operation
2.1 Background on weight measurements
Weight-scale and body composition measurements are two separate signal chains, so weight-scale application will be discussed
first. While there are multiple techniques for converting weight into an electrical signal, perhaps the most common is to use a
resistive load cell configured as a Wheatstone bridge.
Figure 1 Load Cell Bridge Sensor
Figure 1 shows a bridge configuration where one of the resistors in the bridge changes value as weight is applied. Depending on
the construction of the bridge, more resistors may change value as weight is applied (four resistors at most for a Wheatstone
bridge). Either way, an excitation voltage is applied across the top and bottom of the bridge. The output signal is measured as
the differential voltage across the middle nodes. The challenge in designing a weigh-scale is how to make accurate
measurements of the signal produced by the resistor bridge, because this signal is usually very small (mV magnitude).
Body composition is traditionally obtained by measuring the impedance across several points on the body and matching the
result in a table linking both the impedance measured and the body composition. Bio-impedance is the response of a living
organism to externally applied electric current. It is a measure of the opposition to the flow of that electric current through the
tissues, the opposite of the electrical conductivity.
Figure 2 Cell Bio-impedance model
The bio-impedance model of a cell is described in Figure 2, where Re is the extracellular resistance, Cm is the cell membrane
capacitor, Rm is the cell membrane resistor, Ri is the intracellular resistance. The bio-impedance of cell is decided by various
factors like cell size, shape, density, homogeneity etc.