SLOA011B January   2018  – July 2021 LF347 , LF353 , LM348 , MC1458 , TL022 , TL061 , TL062 , TL071 , TL072 , UA741

 

  1. 1Introduction
    1. 1.1 Amplifier Basics
    2. 1.2 Ideal Op Amp Model
  2. 2Non-Inverting Amplifier
    1. 2.1 Closed Loop Concepts and Simplifications
  3. 3Inverting Amplifier
    1. 3.1 Closed Loop Concepts and Simplifications
  4. 4Simplified Op Amp Circuit Diagram
    1. 4.1 Input Stage
    2. 4.2 Second Stage
    3. 4.3 Output Stage
  5. 5Op Amp Specifications
    1. 5.1  Absolute Maximum Ratings and Recommended Operating Condition
    2. 5.2  Input Offset Voltage
    3. 5.3  Input Current
    4. 5.4  Input Common Mode Voltage Range
    5. 5.5  Differential Input Voltage Range
    6. 5.6  Maximum Output Voltage Swing
    7. 5.7  Large Signal Differential Voltage Amplification
    8. 5.8  Input Parasitic Elements
      1. 5.8.1 Input Capacitance
      2. 5.8.2 Input Resistance
    9. 5.9  Output Impedance
    10. 5.10 Common-Mode Rejection Ratio
    11. 5.11 Supply Voltage Rejection Ratio
    12. 5.12 Supply Current
    13. 5.13 Slew Rate at Unity Gain
    14. 5.14 Equivalent Input Noise
    15. 5.15 Total Harmonic Distortion Plus Noise
    16. 5.16 Unity-Gain Bandwidth and Phase Margin
    17. 5.17 Settling Time
  6. 6References
  7. 7Glossary
  8. 8Revision History

5.8.2 Input Resistance

Two parameters for input resistance, ri and rid, are defined in Texas Instruments’ data book, Amplifiers, Comparators, and Special Functions, pg. 1-39. Input resistance, ri, is "the resistance between the input terminals and either input grounded." Differential input resistance, rid, is "the small-signal resistance between two ungrounded input terminals."

To relate ri to Figure 5-6, if you ground Vp, ri = Rd || Rn. Depending on the type of input, values usually run on the order of 107Ω to 1012Ω.

To relate rid to Figure 5-6, with both input terminals floating, rid = Rd || (Rn + Rp). Depending on the type of input, values usually run on the order of 107Ω to 1012Ω.

Sometimes common-mode input resistance, ric, is specified. To relate ric to Figure 5-6, if you short Vp to Vn, ric = Rp || Rn, the input resistance a common mode source would see to ground.