sheldon
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Kapileshwar,
The general guideline discussed elsewhere, you can look for the post but I will walk you through the setup again for you since there is some new technology to automate the measurement is now available.
Basic simulation setup, useful for THD and SNDR
1) Using 8192 samples gives you an FFT fundamental frequency of about 15Hz
2) Since the input frequency should not be a harmonic of the sampling frequency, select the third FFT harmonic 46.875 as the input frequency.
3) Since the circuit is an ADC, we will assume that the digital outputs settle quickly to the final value of (0, 1) or (0, Vdd). So we will not need to set skipstart, skipstop, strobeperiod. These are required when using the fourier integral for to analyze a DAC or when using the FFT to analyze a continuous waveform. Caution: You will need to select the start and stop time for the FFT so that the sample occurs when the signal stable. Or be careful not to sample when the digital levels are changing.
4) One additional comment, you have not provided any information about your circuit. You will need the circuit to reach steady-state before starting to the FFT. For example, if the op amps have dynamic common mode feedback, the common mode feedback level needs to reach steady state.
5) You will need to use the cosine2 window when performing the FFT.
6) Some additional comments: a) Use the ViVA calculator in IC615, it has a new tool for spectral analysis of data converters. It makes the setup much easier. b) If you do not have IC615, but have IC61 ViVA, then you can use the spectrum Meas function for the measurement. c) If you don't have either of those, then let me know and I can walk you through the measurement
Advanced Measurement
Strictly speaking the setup discussed above does not get you SNR, it gets you THD/SNDR(SINAD). In "Understanding Delta-Sigma Data Converters", Schreier & Temes recommend oversampling by 10x to get the noise floor. Using the setup above all the tones contain distortion because the cosine2 window function is used. I increased the sampling rate 32x so that the ratio of distortion samples to noise samples is ~1:10.
Using an FFT fundamental frequency of 0.48828125Hz, 266144 samples, should give you enough noise samples to measure SNR. The input frequency is 49.3164012 Hz for this setup.
Also, for the SNR measurement you will need access to IC615 VIVA spectrum toolbox. It automates the calculation, it in this case it is quite involved.
Hope this helps! Let me know if you have any other questions.
Best Regards,
Sheldon
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