Using the ADS1298ECG-FE Software
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22
SBAU171D–May 2010–Revised January 2016
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ADS1298ECG-FE/ADS1198ECG-FE
3.5.3 FFT Tab
The FFT tool allows the user to examine the channel-specific spectrum as well as typical figures of merit
such as SNR, THD, ENOB, and CMRR. Each feature is numbered below and described in detail in the
following subsections. Figure 22 illustrates an FFT plot for a normal electrode configuration.
Figure 22. FFT Graph of Normal Electrode Configuration
1 - Coherent Frequency Calculator
Coherent sampling in an FFT is defined as F
AIN
/F
SAMPLE
= N
WINDOW
/N
TOTAL
, where:
• F
AIN
is the input frequency
• F
SAMPLE
is the sampling frequency of the ADS1298
• N
WINDOW
is the number of odd integer cycles during a given sampling period
• N
TOTAL
is the number of data points (in powers of 2) that is used to create the FFT
If the conditions for coherent sampling can be met, the FFT results for a periodic signal will be
optimized. The Ideal A
IN
Frequency is a value that is calculated based on the sampling rate, such that
the coherent sampling criteria can be met.
2 - AC Analysis Parameters
This section of the tool allows the user to dictate the number of harmonics, DC leakage bins, harmonic
leakage bins, and fundamental leakage bins that are used in the creation of various histograms.
Pressing the Windowing button, illustrated in Figure 23, allows the user to evaluate the FFT graph
under a variety of different windows. Note that pressing the Reference button toggles between dBFS
(decibels, full-scale) and dBc (decibels to carrier).
Figure 23. AC Analysis Parameters: Windowing Options
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