SBOA590 November   2024 OPA186 , OPA206 , OPA328 , OPA391 , OPA928

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Input Offset Voltage (VOS) Definition
    1. 1.1 Input Offset Voltage Drift (dVOS/dT) Definition
    2. 1.2 VOS and VOS Temperature Drift Inside the Amplifier
    3. 1.3 Laser Trim to Adjust Performance
    4. 1.4 Package Trim (e-Trim™) to Adjust Performance
  5. 2Input bias current (IB) definition
    1. 2.1 Input Bias Current (IB) and IB Temperature Drift Inside the Amplifier
    2. 2.2 Derivation of IB Conversion to VOS
    3. 2.3 Internal Bias Current Cancelation
    4. 2.4 Super Beta Input Transistors
  6. 3Other Factors Influencing Offset
    1. 3.1 Finite Open Loop Gain (AOL)
    2. 3.2 Common Mode Rejection Ratio (CMRR)
    3. 3.3 Power Supply Rejection Ratio (PSRR)
    4. 3.4 AOL, CMRR, and PSRR Over Frequency
    5. 3.5 Electromagnetic Interference Ratio (EMIRR)
    6. 3.6 Mechanical Stress Induced Offset Shift
    7. 3.7 Parasitic Thermocouples
    8. 3.8 Flux Residue and Cleanliness
  7. 4Zero-drift Amplifiers to Minimize VOS and VOS Drift
  8. 5Calibration of VOS, IB, and Gain Error
  9. 6References
  10. 7Revision History

1 Input Offset Voltage (VOS) Definition

Input offset voltage can be modeled as an error voltage source in series with the non-inverting input of the op amp (see Figure 1-1). This offset voltage can range from microvolts to millivolts (see Table 1-1). For amplifiers, the term precision is generally used to describe amplifiers with input offset voltage less than 1mV.

OPA206 Offset Voltage Model Figure 1-1 Offset Voltage Model

Because the offset source is directly in series with the op amp input, the impact of offset voltage error is more significant for small input signal ranges. Figure 1-2, and Figure 1-3 illustrate the error introduced by a 1mV offset for an amplifier in a gain of 10 V/V and 100 V/V. Equation 1 through Equation 4 show the error calculation for gain of 10V/V and Equation 5 through Equation 8 show the calculation for 100V/V. The applied input signal drives the output to near full scale in both cases (4.5V on a 5V supply). Notice that the percentage error increased by a factor of 10 when the gain is increased by a factor of 10. Thus, offset voltage is often a greater concern in higher gain systems.

OPA206 Output Offset and Percent Error In Low Gain Figure 1-2 Output Offset and Percent Error In Low Gain
Equation 1. G = R F R G + 1 = 9 k Ω 1 k Ω + 1 = 10 V / V
Equation 2. V O U T = V I N + V O S G = 450 m V   +   1 m V ( 10 V / V ) = 4.51 V
Equation 3. V O U T ( I d e a l ) = V I N G = 450 m V   +   1 m V ( 10 V / V ) = 4.50 V
Equation 4. E r r o r ( % ) = 100 V O U T   -   V O U T ( I d e a l ) V O U T ( I d e a l ) = 100 4.51 V   -   4.50 V 4.50 V = 0.22 %
OPA206 Output Offset And Percent Error in High Gain Figure 1-3 Output Offset And Percent Error in High Gain
Equation 5. G = R F R G + 1 = 99 k Ω 1 k Ω + 1 = 100 V / V
Equation 6. V O U T = V I N + V O S G = 45 m V   +   1 m V ( 100 V / V ) = 4.60 V
Equation 7. V O U T ( I d e a l ) = V I N G = 45 m V   +   1 m V ( 100 V / V ) = 4.50 V
Equation 8. E r r o r ( % ) = 100 V O U T   -   V O U T ( I d e a l ) V O U T ( I d e a l ) = 100 4.60 V -   4.50 V 4.50 V = 2.22 %