Most portable systems have one battery， thus the popularity of portable equipment results in increased single-supply applications. Split- or dual-supply op amp circuit design is straightforward because op amp inputs and outputs are referenced to the normally grounded center tap of the supplies. In the majority of split-supply applications， signal sources driving the op amp inputs are referenced to ground; thus with one input of the op amp referenced to ground， as shown in Figure 1， common-mode voltage and voltage bias problems are negligible. When signal sources are referenced to ground， singlesupply op amp circuits exhibit a large input common-mode voltage （Figure 2）。 The input voltage is not referenced to the midpoint of the supplies like it would be in a splitsupply application; rather， it is referenced to the lowerpower supply rail. This circuit malfunctions when the input voltage is positive because the output voltage should go negative; this is hard to do with a positive supply. It operates marginally with small negative input voltages because most op amps cannot function when the inputs are connected to the supply rails. The constant requirement to account for input references makes it difficult to design single-supply op ampcircuits. This application note develops an orderly procedure for designing single-supply op amp circuits that leads to a working design every time. Application Note SLAA068， entitled， “Understanding Basic Analog—Ideal Op Amps，” develops the ideal op amp equations. The ideal op amp assumptions used to write ideal op amp equations are shown in Table 1 for your reference.