SLOA059B October   2022  – March 2023 OPA2991 , TLC2654 , TLC4502 , TLE2021 , TLV2721

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2Input Offset Voltage Defined
  5. 3Cause of VOS
  6. 4VOS and Temperature Drift in the Major Device Types
    1. 4.1 Bipolar
    2. 4.2 JFET
    3. 4.3 CMOS
  7. 5Manufacturer Measurement, Trim, and Specification of VOS
    1. 5.1 Measurement
    2. 5.2 Trim
    3. 5.3 Specifications
  8. 6Impact of VOS on Circuit Design and Methods of Correction
    1. 6.1 AC Coupling
    2. 6.2 DC Feedback
    3. 6.3 Internal Calibration
  9. 7Summary
  10. 8References
  11. 9Revision History

6 Impact of VOS on Circuit Design and Methods of Correction

Table 6-1 Range of Input Offset Voltage and Drift Per Device Process Technology †Typical specifications

Note: Devices listed are commercial, ranges valid for all temperature ranges

Device TypeProcessYearVOS†at 25 °C (μV)ΔVOS/ΔT† (μV/°C)Technology
OPA397CMOS2021200.18e-trimTM
OPA2387CMOS20200.350.003Zero Drift
OPA210Bipolar201850.1Super Beta
OPA2325CMOS2016402Zero Crossover

Figure 6-1 shows an inverting op amp circuit with VOS included. Superposition is used to find the closed-loop gain of the circuit (ACL) in Equation 13 (see Understanding Basic Analog - Circuit Equations).

Equation 13. V O U T = V I N - R F R G ± V O S 1 + R F R G
GUID-20220329-SS0I-NGLQ-K1W9-WC2SSQSLTD2Q-low.svg Figure 6-1 Inverting Op Amp Circuit With VOS Included

VOS is always multiplied by the non-inverting gain of the op amp and added to (or subtracted from) the signal gain of the circuit, which is –(RF/RG) in this example. In large-gain DC-coupled circuits, VOS can be significant and may need to be reduced through offset adjustment techniques, although an op amp with very low offset may not require adjustment. Normally, adjustment of VOS is used only when the DC accuracy is necessary in order to reduce distortion.

Adding the effects of temperature drift to Equation 13 gives Equation 14. This allows a fairly accurate calculation of the worst-case change in the output due to VOS. However, resistor values also change with temperature and will affect the gain of VOS. Equation 14 focuses only on the effects of drift and does not include the errors from the other DC and AC sources as shown in Figure 1-2 for the nonideal op amp, so it is not completely accurate.

Equation 14. V OUT = V IN - R F R G + V OS 1 + R F R G + ∆ V OS ∆ T T

The first step is to determine the maximum allowable DC error in the system. An error budget analysis must be performed to determine all the DC error sources in the system, and the maximum contribution the design allows for each section. If the op amp or other device fails to meet the specification for VOS, they must be compensated to remove or reduce the offset.

Methods for reducing the effects of VOS include circuit modifications such as AC-coupling and DC feedback. In some applications, the solution is to use devices that have some form of internal calibration, such as chopper stabilization, an auto-zero loop, or offset trim. These methods are briefly described in the following sections.