Ac Testing-Fft Capture - Analog Devices EVAL-AD7625FMCZ User Manual

UG-745
AC TESTING—FFT CAPTURE
Figure 22 shows the FFT tab. This tests the traditional ac
characteristics of the converter and displays a FFT of the results.
As in the histogram test, raw data is captured and passed to the
PC where the FFT is performed to display the signal-to-noise
ratio (SNR), signal-to-noise-and-distortion ratio (SINAD), total
harmonic distortion (THD), and spurious-free dynamic range
(SFDR).
To perform an ac test, apply a sinusoidal signal to the evaluation
board at the SMA inputs, J1 and J2. A very low distortion, better
than 130 dB input signal source (such as audio precision) is
required to allow true evaluation of the device. One possibility
is to filter the input signal from the ac source. There is no
suggested band-pass filter, but carefully consider the choices.
Furthermore, if using a low frequency band-pass filter when the
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EVAL-AD7625FMCZ/EVAL-AD7626FMCZ User Guide
full-scale input range is more than a few volts peak-to-peak, it is
recommended to use the on-board amplifiers to amplify the signal,
thus preventing the filter from distorting the input signal.
Figure 22 displays the results of the captured data.
FFT Analysis shows the input signal information (Label 1 in
Figure 22), as well as the performance data, including SNR,
dynamic range, THD, SINAD, and noise performance (Label 3
in Figure 22).
Show Harmonic Content displays the fundamental frequency
and amplitude in addition to the second to fifth harmonics
(Label 2 in Figure 22).
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Figure 22. FFT Capture Tab
Rev. A | Page 14 of 28
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