How to make opamp simulation of slew rate and fft

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alvays

Joined: 07 Jul 2004
Posts: 65
Helped: 3

15 Mar 2005 14:48 slew rate simulation

I'm not definitely aware of how to make the slew-rate simulation and fourier analysis.
1)Which method we should choose for slew-rate simulation, open loop or closed loop? If closed loop, should we connect it be voltage follower? And which point we should choose for the result: maximum derivative of output or the slope from 1% to 90% of output? Why?
2)How do we choose the fundamental frequency of fft analysis? What decide it? And should we connect the opamp to be closed loop while fft analysis? What's the usual spec for THD? Why?

Thank you all

snoop835

Joined: 07 Feb 2005
Posts: 104
Helped: 3
Location: Penang, Malaysia

17 Mar 2005 7:40 slew make

Hi ya!

To simulation for slew rate, you will need to configure your opamp as noninverting unity gain buffer (closed-loop). Your input stimulus should a pulse waveform connected to the +input pin. Then run transient analyis and observe the output waveform vs input waveform. Your slew rate should be 10% from the rising edge to the 10% before it settle, expressed in V/us. The reason we use square pulse input is because easier to see the how fast the output response when input is applied, this is what slew rate is all about.

To perform fft analysis, you will need to run it together with transient analysis. Your input stimulus is best using a sinewave with the frequency set to your fundamental frequency. The amplitude of the sinewave should be just below your full scale input voltage range. For example, if your input voltage range is from 0V to 1V (single supply opamp), then you should set the sinewave amplitude to 0.5V.

The SPICE syntax are:

***(HSPICE)*******
.tran 0.1ns 2000ns
.fft V(Vout) NP=4096

see HSpice manual for more detail.

The fft plot should display a graph of frequency bins vs power(dB).
From the graph, you will see the highest peak point is your fundamental frequency. To determine the dynamic range, you measure the difference between peak of fundamental frequency to the peak of second highest peak (second harmonic).

alvays

Joined: 07 Jul 2004
Posts: 65
Helped: 3

18 Mar 2005 2:43 op amp slew rate 1.5

Thank you very much, snoop.
Another quetion, if i make fourier analysis, how can i choose the fundamental frequency? is it the working frequency of the circuits? and how to define the THD spec usually?

muldersun

Joined: 03 Dec 2003
Posts: 41
Helped: 2

18 Mar 2005 2:47 slew rate frequency operational amplifier

Can you explain why to choose the base frequency at 1kHz?

snoop835

Joined: 07 Feb 2005
Posts: 104
Helped: 3
Location: Penang, Malaysia

18 Mar 2005 4:44 opam simulation

The fundamental frequency is the working frequency for your circuit. I'm not so sure about how to analyse the Total harmonic Distortion for an opamp.

I have simulated the opamp performance in terms of gain, phase margin, phase response, settling time, slew rate and input voltage range. Normally for frequency response analysis, I used 1kHz input frequency. This frequency is choosed for a bandwidth of approximately 1.5kHz. I supposed that if your opamp bandwidth is say, 1MHz. Then you can choose frequency below the bandwidth for maximum gain.