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

5 Slew Rate Dependencies on Circuit Signal Levels and Op Amp Gain Set by Feedback Network

Figure 3-2 has two schematics that produce a 1V output step. In the left circuit, the 1V is amplified by a noise gain of 1. In the right circuit, 10mV is amplified by a noise gain of 100. In these non-inverting circuits, signal gain and noise gain are equivalent.

TS321, TL074, TLV9001, OPA4991, OPA2991, OPA991, LMV831, OPA345, LMV821-N, OPA377-Q1, OPA376-Q1, OPA377, OPA376, TLV376, TLV9002 Two Schematics With The Same 1V Output StepFigure 5-1 Two Schematics With The Same 1V Output Step

Figure 5-1 is the resulting simulation waveform from the Figure 5-1 schematics. The unity gain (black curve) rises at 2V/µs, which is the same as the data sheet specification. The 100 gain (red curve) has a slower slew rate that continues to decrease as the voltage rises. The initial slew rate for a 10mV input step, following the Figure 3-1 output, is 14% of the natural slew maximum of 0.5V/µs, which is 70mV/µs. By the time the waveform rises half way, the VID has dropped to 5mV. Now, the SR is the Figure 3-1 output 7% (of 0.5V/µs), which is 35mV/µs.

TS321, TL074, TLV9001, OPA4991, OPA2991, OPA991, LMV831, OPA345, LMV821-N, OPA377-Q1, OPA376-Q1, OPA377, OPA376, TLV376, TLV9002 Two Waveforms With 1V StepFigure 5-2 Two Waveforms With 1V Step

Even if the TLV9002 device did not have slew boost, the unity gain slew rate (0.5V/µs natural) is much faster than a G=100 curve. Slew boost or not, a smaller input signal with higher gain must always have a slower observed slew rate than a larger input signal with lower gain.