Max Slew Rate
CIRCUIT
Schematic: Op_Amp_slew.asc
Op Amp Symbol: Opamp_2.asy
Op Amp Shematic: Opamp_2.asc
Right Click on filename, select "Save link as...",
Back to Design Series
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 (LowPass Filter output), the signal and derivative look like
this.
 Notive that Tau is the time constant (or R*C product) of the
LowPass 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 realworld op amp that meets or exceeds your required
Slew Max.
Back to Design Series...