sheldon
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Greetings,
I have been simulating a high dynamic range A/D Converter and had some questions/observations that I would like to share.
The following analysis was performed using the standard Cadence tools: ADE, Spectre, and Calculator.
1) Philosophical question: It seems like Spectre's Fourier Integral based analysis is the best method for simulating the dynamic performance of D/A Converters. Since the FFT samples the D/A Converter output, it is difficult to evaluate the effect of glitch impulse on the output spectrum. While the Fourier Integral evaluates the entire data set it is more accurate. Is this observation correct?
2) When simulating the dynamic response of a D/A Converter, SFDR, THD, SINAD, etc. the output response tends to suffer from the "picket fence" effect, that is, energy tends to be concentrated in the harmonics of the clock and input frequencies. This phenomena occurs even if the normal precautions are taken, that is, the input and clock frequencies are non-harmonically related. Is there a method to eliminate this phenomena from simulations?
3) For high dynamic range Data Converters, the interpolation error is an issue. Strobing the output helps somewhat, so does setting the maxstep to < 1/2 the FFT step size. Are there any other approaches that can be used to improve the accuracy of the FFT?
4) A question on the difference between Nyquist rate ADCs and Delta-Sigma ADCs. When using synchronuos, non-coherent sampling to simulate SFDR/THD of a Nyquist rate ADC, the FFT "Rectangular" window has more dynamic range than the "Hanning" window. However, for Delta-Sigma Modulators, the "Rectangular" window gives inconsistent results, that is, the SFDR/ THD are a function of the data window used for the FFT. while the "Hanning" gives consistent results.
Is this difference because Nyquist converters have are "memoryless" while Delta-Sigma Modulators have "memory"? Since the state of a Delta-Sigma Modulator is dependent on the previous state of the system there is effective spectral leakage DSM that does not occur for Nyquist-rate ADCs.
5) Some questions on the Fourier component from analogLib and Spectre Fourier analysis: a) When using Fourier Analysis, the fundamental frequency is defined relative to the last simulation time point. If the simulation time is selected such that simulation stop time - (1/fundamental frequency), then the initial time points will be ignored and start-up effects will be ignored. Is this correct? b) Is there a parameter equivalent to refnormharm the numerator of the Fourier Integral? When testing Data Converters, the reference tone is seldom at the fundamental freuquency. c) When simulating a Delta-Sigma Modulator, what is the appropriate type of interpolation to use? Since there are a large number of points and the curve is highly non-linear, that is a stream of 1s and 0s, is linear or quadratic interpolation better?
Looking forward to your comments.
Best Regards,
Sheldon
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