Texas Instruments ADS131E08, User Manual, Page: 26 / 47

f
f
AIN
SAMPLE
=
N
N
WINDOW
TOTAL
ADS131E08 ANALYSIS TOOLS
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5.3 FFT Tool
The FFT subtab displays the data in the frequency domain by performing an FFT on the eight channels.
Details of the FFT, including SNR, THD, and more, can be viewed using the FFT Analysis button located
at the bottom of the display.
Figure 27 shows the FFT display.
Figure 27. Analysis → FFT Graph of Normal Electrode Configuration
5.3.1 Coherent Frequency Calculator
The red box labeled 1 in Figure 27 shows the Coherent Frequency Calculator. Coherent sampling in an
FFT is defined in Equation 3 :
where:
• f
AIN
= input frequency
• f
SAMPLE
= ADS131E08 sampling frequency
• N
WINDOW
= number of odd integer cycles during a given sampling period
• N
TOTAL
= number of data points (in power of 2) that is used to create the FFT. (3)
If the conditions for coherent sampling can be met, the FFT results for a periodic signal are optimized. The
ideal A
IN
frequency is a value that is calculated based on the sampling rate so that the coherent sampling
criteria can be met.
26 Performance Demonstration Kit for the ADS131E08 SBAU200 – June 2012
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