EVAL-AD7891-1CB Datasheet, PDF (9/12 Pages) Analog Devices – Evaluation Board for Single Supply, 12-Bit 454 kSPS ADC

EVAL-AD7891-1CB
Digital Spectrum Analyzer
The Digital Spectrum Analyzer performs a fast fourier transform
of the sampled data and displays the magnitude of the results on
its screen in decibels (dB). It also calculates the signal to noise
ratio of the sampled data, assuming the samples are those of a
sine wave. If the "blackman-harris" button near the top of the
main screen is on then the data is windowed by a blackman-
harris window before the fast fourier transform is performed.
Normally, the "blackman-harris" button needs to be to on in
order to obtain meaningful results from the spectrum analyzer.
If the evaluation board is used in a coherent sampling system,
ie. one where the conversion start signal applied to the AD7891-
1 is synchronized to an input sine wave, in such a way that an
integral number of periods of the sine wave are contained in the
sampled data, then the blackman-harris window can be turned
off.
The buttons to the right of the spectrum analyzer display
control the operation of the analyzer. These buttons are:
Avg When this button is on, the fourier transforms on each
new set of sampled data are averaged so that the noise
floor decreases relative to the signal contained in the
data.
Scale
When this button is on, the complete fourier transform
from 0 Hz up to fs/2 is displayed in the spectrum
analyzer window. When this button is off, the first 510
bins (less for transforms of less than 512 points) of the
fourier transform are displayed.
harms
When this button is on, the harmonics of the input
signal are included in the SNR calculation. When this
button is off, the harmonics are replaced by average
noise in the SNR calculation.
norm
When the "norm" button is on, the fourier transformed
data is normalized so that the zero dB level is set to the
level of the frequency bin with the largest magnitude.
The data is displayed on-screen with the largest
magnitude bin at the zero dB level. When the "norm"
button is off, the data is not normalized. Instead the
zero dB level is assigned to the level that the largest
magnitude input signal would have and the data is
displayed accordingly.
After an FFT has been performed and the frequency spectrum
is displayed in the Digital Spectrum Analyser window, the
mouse may be used to determine the frequency and dB level of
any point in the frequency spectrum. Note that this will only
work when one set of samples were taken, it will not work when
in the continuous sampling mode. Once the mouse is positioned
in the Digital Spectrum Analyser window the cordinates of the
mouse in terms of frequency and dBs will be displayed on the
right hand side of the window.
When a fourier transform of a windowed sine wave is per-
formed, the power contained in the fundamental frequency is
spread out over several frequency bins. To calculate the signal
to noise ratio of the signal, the power contained in those
frequency bins is summed to give the power of the fundamental.
Also, the dc component of the signal is spread over the first few
frequency bins. In the SNR calculation, the dc signal must be
replaced by the average noise.
The number of frequency bins that are summed to give the
power in the fundamental is controlled by the SigBins counter.
The range of valid values is 0 to 9. The number contained in
the SigBins counter is the number of frequency bins each side of
the central peak that are used to calculate the power in the
fundamental frequency. The counter is increased by clicking on
the SigBins "+" button and decreased by pressing on the SigBins
"-" button. As the counter is changed, the SNR is re-calculated
and displayed. For a blackman-harris window, the power in the
central peak plus the power in four bins each side of the central
peak is normally used for SNR calculations. In a coherent
sampled data system, all the power from the sine wave is
contained in one frequency bin. In such a case the SigBins
counter shoud be set to zero.
The number of low frequency bins that are replaced by average
noise in the SNR calculation, is controlled by the DcBins
counter. The range of valid values is 0 to 9. The counter is
increased by clicking on the DcBins "+" button and decreased
by pressing on the DcBins "-" button. As the counter is
changed, the SNR is re-calculated and displayed. For a
blackman-harris window, the first three to five bins are normally
replaced by average noise in the SNR calculation. In a coherent
sampled data system, all the dc power is contained in the first
frequency bin. In such a case the SigBins counter shoud be set
to one.
REV. A
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