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Op amp DC limits, their impact on precision apps

Posted: 13 Jan 2014     Print Version  Bookmark and Share

Keywords:Operational amplifiers  op amps  CMRR  PSRR  data-sheet 

Operational amplifiers, or op amps, are two-port integrated circuits (ICs) that apply accurate gain on the external input signal and provide an amplified output as: input x 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 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 1).

Figure 1: A) A roadside danger sign, warning of an automotive skid hazard under certain conditions (rain and snow). B) 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)

Where:
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 analyse resistive feedback (figure 2A) and capacitive feedback (figure 2B) circuits separately.

Figure 2: A) Operational amplifier with resistive feedback. B) 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)

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