Op amp DC error characteristics and the effect on high-precision applications
This article discusses the DC limitations of operational amplifiers and their effects, including input bias currents, input offset voltage, CMRR, PSRR, and input impedance. The article will provide a reader with a better understanding of how these limitations can create accuracy issues in high-precision applications.
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 we 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. We 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 Currents1
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 = (IBP + IBN)/2 ….. (Eq. 1)
IOS = IBP - IBN ….. (Eq. 2)
IB is average input bias current flowing into input pins;
IBP is input bias current flowing into the positive input;
IBN is input bias current flowing into the negative input;
IOS is the input offset current.
Input bias and input offset currents are two of the most critical characteristics in many precision amplifier applications; they affect the output with resistive and capacitive feedback. Many of the inverting, noninverting, summing, and differential amplifiers reduce to Figures 2A and 2B once their active inputs are set to zero. For this analysis, we set all input signals as zero to assess the effect of input currents on the output accuracy. We will analyze resistive feedback (Figure 2A) and capacitive feedback (Figure 2B) circuits separately.
Figure 2A.Operational amplifier with resistive feedback. Figure 2B. Operational amplifier with capacitive feedback. Example devices are the MAX9620 and MAX4238 op amps.
Applying the superposition theorem on Figure 2A yields:
VOUT = (1 + RF/RG) x [(RF//RG) x IBN – RP x IBP] …… (Eq. 3)
The following inferences can be made from Equation 3:
- Without any input signal, the circuit yields a finite output voltage. This unwanted output error is also called output DC noise.
- Output voltage is produced by amplifying the input error or input DC noise by (1 + RF//RG).
- Input DC noise has two components: voltage drop as IBP flows through RP, and voltage drop because IBN flows through a combination of RF//RG.