SLOA332A July   2023  – September 2024 LMV821-N , LMV831 , OPA2991 , OPA345 , OPA376 , OPA376-Q1 , OPA377 , OPA377-Q1 , OPA4991 , OPA991 , TL074 , TLV376 , TLV9001 , TLV9002 , TS321

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1 Slew Rate Definition
    1. 1.1 Virtual Ground and Slew Rate
  5. 2 Bipolar Op Amp Slew Rate Example
  6. 3 CMOS Op Amp Slew Rate Example
    1. 3.1 Slew Boost Example 1
    2. 3.2 Slew Boost Example 2
    3. 3.3 Slew Boost Summary
  7. 4 Four Methods to Determine Boost or No Boost Using the Data Sheet
    1. 4.1 Method 1: Compare Slew Rate Versus Gain Bandwidth
    2. 4.2 Method 2: Compare Quiescent Current Versus Similar SR Devices
    3. 4.3 Method 3: Evaluate Large Signal Response
    4. 4.4 Method 4: Evaluate Small Signal Response
  8. 5 Slew Rate Dependencies on Circuit Signal Levels and Op Amp Gain Set by Feedback Network
  9. 6 How Much Output Slew Rate is Needed to Support a Sine Wave or Other Non-step Inputs
  10. 7 Stability Also Plays a Role in Observed Slew Rate
  11. 8 Summary
  12. 9 References
  13. 10Revision History

1.1 Virtual Ground and Slew Rate

Op amp circuits are easy to create and understand due to the ‘virtual ground’ mental concept. In closed loop circuits, the large gain of the op amp drives the input voltages (IN+ and IN-) to be the same. Input voltage difference (VID) is assumed to be zero. This assumption makes op amp circuit math simple. In reality, there are three effects that degrade this virtual ground concept. The first effect is DC offset voltage (VOS). The second effect is small signal gain. VID for a small signal is simply VOUT divided by AOL (open-loop gain). The third effect is slew rate generation. This application note focuses solely discussing on slew rate.

VID must be non-zero to generate slew rate. The greater the VID, the greater the slew rate. At some point, increasing VID no longer increases slew rate. The data sheet value is the slew rate where a larger VID has no effect.