EVAL-AD7450A/EVAL-AD7451
Evaluation Board User Guide
Rev. A | Page 18 of 28
GENERATING A FAST FOURIER TRANSFORM OF
AC CHARACTERISTICS
Figure 20 shows the
FFT
tab. This feature tests the traditional
ac characteristics of the converter and displays a fast Fourier
transform (FFT) of the results.
To perform an ac FFT test,
1.
Apply a sinusoidal signal with low distortion (better than
100 dB) to the evaluation board at the SMB Inputs J1/J2.
To attain the requisite low distortion, which is necessary
to allow true evaluation of the part, one option is to
a.
Filter the input signal from the ac source. Choose an
appropriate band-pass filter based on the sinusoidal
signal applied.
b.
If a low frequency band-pass filter is used when the
full-scale input range is more than a few volts peak-
to-peak, use the on-board amplifiers to amplify the
signal, thus preventing the filter from distorting the
input signal.
2.
Click the
FFT
tab from the main window.
3.
Click
Sample
.
As in the histogram test, raw data is captured and passed to the
PC where the FFT is performed displaying SNR, SINAD, THD,
and SFDR.
The
Spectrum Analysis
box displays the results of the cap-
tured data.
•
The area labeled 1 in Figure 20 shows the input signal
information.
•
The area labeled 2 in Figure 20 displays the fundamental
frequency and amplitude in addition to the second to fifth
harmonics.
•
The area labeled 3 in Figure 20 displays the performance
data, including the SNR, THD, and SINAD.
06231-
018
Figure 20. FFT Tab
