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What is the reference value used to calculate dBFS by the ADS1299ECG-FE analysis software's Fourier power spectrum?

Claire Christensen
Prodigy 20 points
Other Parts Discussed in Thread: ADS1299

Hello.

I am using the ADS1299ECG-FE development kit and associated software.  My question is a basic one regarding reference values for the analysis software's calculation of a Fourier power spectrum.  Specifically, I am looking at Figures 44-45 on pages 38-39 of the development kit instruction manual.  In Figure 45, the power spectrum associated with a 31.25Hz, 6uA (gain=12) signal is shown in dBFS.  My question is twofold:

1)  Does the analysis software use dBFS=10*log_10(P_obs/P_ref), where P_ref is the power of a full-scale signal, to calculate values for the power spectrum?

2)  If so, what is P_ref and does it change depending on gain?  In the case outlined above, is it the power we would observe for a 750mV peak-to-peak signal (since 750mV p2p is the full-scale voltage range at gain=12)?

If you could please be as specific as possible with numbers and equations in your reply, that would be great!

Thank you,

Claire

  • 0
    TI__Guru 66166 points

    Hi Claire,

    Thanks for using the TI E2E forums!

    As you mentioned, P_ref is the power of the full-scale signal, and that is what we are using to display the results in Figure 45.

    The full-scale range of the ADS1299 is 2*Vref. In dB, full-scale is represented as 0 dB, and your output signals are plotted with respect to full-scale. This does not change as you increase the gain of your PGA. Instead, the PGA gain dictates the allowable input voltage range at each of your input channels. In your example, with Vref = 4.5 V, the maximum differential input signal allowed is (2*Vref / 12) = 750 mV.

     

    Hope this clarifies it for you!

    Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hi Ryan.

    Sorry for the delayed response...  Thank you for your reply.  I wanted to confirm with you the actual value of P_ref.  Are you using the total area of the power spectrum for P_ref (which is independent of gain) or do you have a lookup table for the height of the maximal peak in the power spectrum at full-scale (this value does change with gain)?

    Thanks again for your help.

    Best,

    Claire