Fft Options - National Semiconductor ADC081500DEV Instruction Manual

samples, such as SNR, SINAD, THD, SFDR, and ENOB. These statistics are to be interpreted with the
following definitions (which are repeated in every National Semiconductor ADC datasheet):
Signal to Noise Ratio (SNR) is the ratio, expressed in dB, of the RMS value of the input signal to the
RMS value of the sum of all other spectral components below one-half the sampling frequency, not
including harmonics or DC.
Signal to Noise Plus Distortion (S/N+D or SINAD) Is the ratio, expressed in dB, of the RMS value of the
input signal to the RMS value of all of the other spectral components below half the clock frequency,
including harmonics but excluding DC.
Total Harmonic Distortion (THD) is the ratio, expressed in dBc, of the RMS total of the first five harmonic
levels at the output to the level of the fundamental at the output. THD is calculated as
+ +
2 2
L
f f
=
2 N
THD 20 log
2
f
1
where f is the RMS power of the fundamental (output) frequency and f through f are the RMS power in
1 2 N
the first N harmonic frequencies.
Spurious-Free Dynamic Range (SFDR) is the difference, expressed in dB, between the RMS values of
the input signal and the peak spurious signal, where a spurious signal is any signal present in the output
spectrum that is not present at the input.
Effective Number of Bits (ENOB, or Effective Bits) is another method of specifying Signal-to-Noise and
Distortion or SINAD. ENOB is defined as (SINAD - 1.76) / 6.02 and says that the converter is equivalent to
a perfect ADC of this (ENOB) number of bits.

FFT Options

FFT plots can be configured in many different ways. Clicking the "FFT Options" button at the top of the
plot will display a dialog showing the options for that particular plot. The software also maintains default
options for new FFT plots, which are editable. You can edit the default FFT options by choosing Default
FFT Options from the Settings menu. The options are:
Windowing: You may choose from one of five different window functions. The window function is applied
to the samples before computing the FFT to compensate for the fact that the sample set may not be an
integral number of wavelengths of the input signal. In general, Flat-Top will give the best results, but you
may find it easier to compare data with other systems when the windowing functions are the same.
dB Scale: You may select to represent power on the FFT in dBc (decibels relative to carrier), in which 0
dB is taken to be the fundamental (carrier) power, or dBFS (decibels relative to full-scale), in which 0 dB is
taken to the be power contained in a signal which uses the entire dynamic range of the ADC.
Harmonics: You may select the number of harmonics recognized (and labeled) by the software. You may
also select the number of FFT bins excluded around harmonics in, for example, SNR calculations. The
exclusion region around each harmonic will be shown in a different color than the rest of the data points.
IMD Calculations: The WaveVision software is capable of performing Intermodulation Distortion
calculations. When two fundamental frequencies within 3 dBFS are present in the waveform, The
software will normally perform IMD calculations. You may inhibit this behavior by deselecting the "Allow
IMD calculation" checkbox. When IMD calculation is enabled, you may also select whether the software
nd nd rd
will include only 2 order or both 2 and 3 order terms.
25 www.national.com