1 Introduction

Op amps find extensive use in a wide variety of circuits, and their appropriate specification for a particular application requires knowledge of relevant data sheet parameters. Data sheet specifications are divided into two general categories: DC parameters and AC parameters. The DC parameters represent internal errors that occur as a result of mismatches between devices and components inside the op amp. These errors are always present from the time the power is turned on (for example, before, during, and after any input signal is applied), and they determine how precisely the output matches the ideal op amp model. Thus, the precision of the op amp is determined by the magnitude of the DC errors.

The objective of this report is to provide the information necessary for the designer to understand each parameter: what it is; what causes it; and how it is measured, trimmed, and specified.

Figure 1-1 presents an ideal op amp model together with a table of ideal parameters (see Understanding Basic Analog – Ideal Op Amps for more information on the ideal op amp). The general assumptions listed in the table simplify design analysis and provide a good first order approximation that is reasonable when the op amp limits are not being pushed. Most applications, however, use the op amp to the fullest extent for one or more parameters and require more detailed analysis. It is then that the non ideal, or real, op amp model must be used. Figure 1-2 shows this non ideal op amp model and uses the OPAx991 as an example for the op amp typical parameter values.