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

6 How Much Output Slew Rate is Needed to Support a Sine Wave or Other Non-step Inputs

The peak slew rate of an output sine wave of frequency (f) and a peak-to-peak voltage (VPP) is shown in Equation 3. If the op amp slew rate is not high enough, then the sine wave becomes triangular with a reduced amplitude. For any output waveform, the slew rate is simply the first derivative of the waveform. From these methods the required slew rate can be determined.

Equation 3. SR=VPP×π×f

For signals other than a step input, also consider gain bandwidth product (GBWP). The absolute minimum GBWP needed for an application is shown in Equation 4. For small signals on slew boost devices, it is recommended to use only GBWP for determining the sufficient op amp speed for a small input.

Equation 4. GBWP=fmax×Gnoise

With the required SR and GBWP determined, consider how much of the input signal can be used as VID to generate that slew rate and gain. Choosing an op amp with much greater SR and GBWP reduces the VID required and potentially reduces distortion. Higher SR and GBWP come with higher quiescent current. For low power applications there is a compromise between supply current and AC performance.