spectrum analysis in Virtuoso ADE

This post shows how to analyze spectrum features, which is useful when running ADC/DAC simulations.

Here is an example of an 8-bit DAC simulation.

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1. Plot the output analog signal (use the differential signal if the outputs are differential)
2. In the Viva window, click Measurements and select Spectrum.

• Select Calculate Stop Time (make sure the simulation time is enough)
• Set sample count and frequency according to simulation settings
• Click S to get the Start/End frequency
• Click Plot to get results

1. In the output window at the bottom right, select one item and right-click to send it to ADE. Then you can edit the equation using variables so that you don’t need to modify the equation after changing the variables.

The format of ENOB is as follows:

spectrumMeasurement(analog_signal t t_start t_stop number_of_samples 0 half_of_sample_fre) 0 “Rectangular” 0.0 0 number_of_harmonics “enob”)

Note

通常，对AD/DA进行频谱分析，只看在奈奎斯特带宽内（直流至fs/2）的信号和谐波。

比如，这里采样频率fs=500MHz，频谱上输出频率只到250MHz。这个时候可以得到正确的动态性能，如ENOB， SFDR等。

但是实际上，高频处存在镜像频率（image frequency），f_img = fs - fo。这里存在一个500M-38.6M = 461.4MHz的混叠频率。 因此，后级需要一个LPF滤除杂散频率成分。