Operational amplifiers, or op amps, are two-port integrated circuits (ICs) that apply precise gain on the external input signal and provide an amplified output as: input × closed-loop gain. Precision op amps behave close to ideal when operated at low to moderate frequencies and moderate DC gains. However, even under these conditions, op-amp performance is influenced by other factors that can impact accuracy and limit performance. Most common among these limitations are input referred errors that predominate in high-DC gain applications.
In this article I will discuss the effects of input-referred errors on op amps. These errors include input bias current, input offset current, input offset voltage, CMRR, PSRR, and finite input impedance. In reality, all these errors will occur at the same time. I will also explain why a designer should be wary that the op-amp performance specifications described in the EC Table of a data sheet are only guaranteed for the conditions defined at the top of that table, unless otherwise noted as a specific characteristic. In reality, the effects of these DC errors change when the supply voltage, common-mode voltage range, and other conditions change.
Errors Caused by Input Bias and Input Offset Currents
We are all familiar with potential dangers around us, and we engineers tend to forget that there are also dangerous traps to avoid when designing. Let’s see how this affects op amps (Figure 1A and 1B).
Figure 1A. A roadside danger sign, warning of an automotive skid hazard under certain conditions (rain and snow); Figure 1B on the right is an op-amp “alert sign,” constructed from data sheet parameters and the specifications, warning that the signal must be contained between the power and ground rails.
We start with two basic equations:
IB = (I BP + I BN)/2 ….. (Eq. 1)
IOS = I BP - I BN ….. (Eq. 2)
IB is average input bias current flowing