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Error in analysis using ADS1298 ECGFE

IIT Kharagpur
Prodigy 120 points
Other Parts Discussed in Thread: ADS1298

Hi,

I am having some problems analysing the data from ADS1298 ECGFE.
I have provided sine waves having an amplitude 200mV p-p and a frequency of 10Hz and have set my PGA gain settings to 6 for all the channels.
Now,giving this input to just the channel 1 and on 'unchecking' the input referred data box in the scope tab,I am getting,as expected,an output of 1.2Vp-p.

1.On analysing the FFT of my data,I find that in the  range of 0-20 Hz,I am getting SNR(dBFS) of  -3.51.Should'nt I be expecting a better SNR?Can you tell me whether it is as expected?
 It also says that the fundamental dB is -109.06.Can you please let me know what exactly does the fundamental dB value indicate??
 I am also not sure whether I am getting the correct THD(dBc) values.

2. When I used the device with an arb signal from the function generator with amplitude of 47.6mVp-p and 10 Hz frequency with PGA of 6 and again unchecking the input referred data box,I found,in the output waveform that  my signal   is attenuated.Since I'll be performing experiments with eeg signals,I would be working with very low amplitude signals which needs to be largely amplified.Can you guide me as to where might be the problem.


Thanking you

Regards,
Shobhit

  • 0
    TI__Mastermind 23235 points

    Please see email that was previously sent to you discussing this question -

    Posting email to address this question -

    Shobhit –

     From the FFT plot that you sent “screenshot.jpg”, it looks like you have some power supply issues since the FFT is showing a significant contribution at 50Hz vs your signal at 10Hz.  This is very likely messing up some of the calculations since the program tries to determine the fundamental component based on the magnitudes of the FFT bins.  It assumes the largest signals is the fundamental.  I don’t know how you are powering the EVM (perhaps external supply) or where the input signal is coming from, but you might look into this before trusting the measurement results.

     The “shapshot_arb.jpg” shows the oscilloscope of your input signal may be OK data.  It looks like you might have at least one signal up at ~2.4V (railed?) and another (or more) around 0.1V…see read trace at top of screen and white trace near bottom.  It looks off because you are showing all signals at the same time.  If you using the “Plot” control near the bottom right of the screen, you should be able to look at each signal individually.  When you do, the voltage scale will scale to the individual signal instead of scaling to accommodate all the input signals.  Try this and it may help you determine why the signal looks distorted.

     Greg

  • 0 in reply to Greg Hupp
    Prodigy 120 points

    Hi Greg,

    Can you please look into the follow up question to the above reply.

    Thanks.

  • 0 in reply to IIT Kharagpur
    TI__Guru**** 181521 points

    Hi Kharagpur,

    I believe we resolved this in another post - the ARB signal frequency was not actually 10Hz if I recall correctly.