3. Almost DC Circuits

There are two types of circuits operating at almost DC. They are power supplies and sensor networks, in particular human body sensors for medical instrumentations. The former requires very low output impedance for transient uses while the latter needs narrow bandwidth. All electronic circuits need to draw energy from constant DC power sources for proper operations. Power supplies perform two basic functions of DC–DC conversion and voltage regulation. Voltage regulator is to reduce ripple in the supply output, but consumes power due to the voltage drop in the pass transistor. In recent years, switching DC–DC converters are commonly used for high efficiency together with low-dropout (LDO) regulators. Almost DC sensing circuits are for high-impedance instrumentations such as voltmeters, which suffer classic problems such as DC wander and motion artifact due to capacitive coupling. Based on the basic energy conservation law for charge and flux storage devices, the power efficiencies of DC–DC converter and LDO are redefined, and the transient distortion of LDO outputs is addressed using their output impedances. The DC wandering and motion artifact problems in almost-DC body sensor circuits are also clearly defined.