Principles and Applications of Ubiquitous Sensing

Chapter 3Conditioning Circuits

In producing an electrical signal that describes the condition of a measurand, the sensing system acts as a voltage or a current source. In some situations, the measurand itself acts as a voltage or a current source. An example is the human body, which produces action potentials (Kimura 2013; Pennisi 2005). Whenever a difference in the concentration of ions (or electrical charges) between two points exists, there will be a potential difference, which we measure in volts. If we set up an external circuit connecting these two points, charged particles (ions, electrons) begin to flow from the higher potential region to the lower potential region (see Figure 3.1). The amount of charge flow (current) depends on both the potential difference and the impedance of the external circuit.

Figure 3.1 Electrical potential difference and current: (a) a difference in the concentration of ions between two locations creates a potential difference between them; (b) when a load is connected between these two locations, a complete electrical circuit is established and the ion concentration difference forces a flow of charged particles. The amount of current that flows in the circuit depends both on the potential difference and the load resistance.

Figure 3.2 displays an electrocardiogram attached to a human body. Figure 3.3a displays the abstraction of these two ...