Max Slew Rate

CIRCUIT

Schematic: Op_Amp_slew.asc
Op Amp Symbol: Opamp_2.asy
Op Amp Shematic: Opamp_2.asc
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SPEC IT

Intro	So, you've designed a signal bandwidth of 1MHz for your 200kHz sinewave - should be sufficient. But what happened? The output looks distorted! Why? You discover the op amp's output can only change so fast. How can you select a slew rate to pass the signal with it's shape preserved?
Definition	Max Slew Rate - the limit of how fast the ouput voltage can change versus time. SlewRate = dV / dt max
Design Goal	Pass a 10V 200kHz Sinewave Undistorted
Design Spec	Total Harmonic Distortion (THD) < 1%

DESIGN IT

• Circuit Design
  • Kcl = (R2+R1)/R1 = 1V/V
  • Choose: R2=10k
  • Select: R2=open
• Op Model Param
  • Slew Rate - 5 V/us Max
  • fu = 1MHz
  • Aol=100000 fu=1e6 Slew=5 Ro=1
• Circuit Test
  • Input: sinewave, 200kHz, Vstep = 1V and 10V
  • THD (Fourier) analysis: .four 200kHz v(vo)
  • Expected THD < 1%

TEST IT

• Small signal test:  Vstep = 1V
• Run a Transient simulation. (.TRAN)
• Plot v(vo):  check for clean sinewave
• Read THD:  View > SPICE Error Log - read THD below Fourier Components
• Did it meet spec?
  • If THD < 1%, then PASS, else FAIL

• Large signal test:  Vstep = 10V
• Rerun sim, visually check the waveform and the THD.
• What happened to the sinewave?
• You should see a triangle output as the 5V/us slew struggles to keep up with the sinewave.
• How high is the THD above 1%?
•  DESIGN ISSUE: The output THD fails to meet spec! 

SOLVE IT

• Incrementally increase Slew (by factors of 2).
• Rerun the SPICE sim.
• What's the minimum Slew needed for a clean sinewave and THD < 1%?

THEORY REFRESH

• What is the slew rate max for a sine or exponential waveform?
• For a Sinewave, the signal and derivative look like this

• Where is the max slew? At the zero crossing point, t=0

• For an Exponential (Low-Pass Filter output), the signal and derivative look like this.

• Notive that Tau is the time constant (or R*C product) of the Low-Pass filter.
• Where is the max slew? At the leading instant of the step, t=0.

 

• So what happened to our original design?
  • Example: Vpeak=10V, f=200kHz
  • Slew = 2*pi*Vpeak*f = 12.6 V/us
  • Aha! The 5 V/us was doomed form the start.

• Let's solve it
  • What Slew Rate is required for Vpeak=10V, f=200kHz?
  • We already calculated the minimum 12.6 V/us above.
  • Choose an op amp above this minimum.
• Retest the circuit with the new Slew Rate. Did your circuit meet spec?

Way to go! Select a real-world op amp that meets or exceeds your required Slew Max.

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