This chapter considers some essential aspects of design in the analog domain of signal processing. In general, an output signal from a sensor or another stage of a circuit needs to be carefully analyzed and treated before the signal is passed to analog-to-digital conversion (ADC) stages. All signals people come across in real-world systems are band limited, and many sensor output signals are baseband type, where a low range of frequencies from DC to a finite maximum value are output by the sensor. In a typical circuit situation, such as a sensor output being amplified by an amplifier stage, noise sources can be represented by the circuit. In most designs, to save hardware resources people attempt to switch and multiplex signals toward an ADC or a processor. In this process, analog switches and multiplexers are used as common building blocks.