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CCS/ADS1299EEGFE-PDK: Physiological signal input problem

user5121970
Intellectual 425 points
Part Number: ADS1299EEGFE-PDK
Other Parts Discussed in Thread: ADS1299, ALLIGATOR

Tool/software: Code Composer Studio

I use a signal generator to generate a 10HZ SIN wave. As shown in Figure 1 below, the waveform is very good, but when I convert the signal into an ECG signal for physiological signals, the waveform becomes as shown in Figure 2. I suspect that the amplitude of the waveform is too large , Causing the upper and lower halves of the waveform to be cut off, but I have adjusted the gain of the channel to 1. Is there any way to solve this problem, and the possible causes of this problem?

  • 0
    TI__Guru 55342 points

    Hi,

    Sorry I didn't have time to get to this today. I'll get back to you on Monday! 

  • 0 in reply to Alexander Smith
    Intellectual 425 points

    I provide some information about the above, I hope it can help you find my problem.
    Register:
    Config1: 0x96
    Config2: 0xC0
    Config3: 0xE8
    Channel1: 0x08
    MISC1: 0x00

    The figure below is my hardware wiring diagram. The red frame is jumper, the green frame is connected to the left hand, the blue frame is connected to the right hand, and the yellow frame is connected to the right foot as the reference potential.

    If you use a signal generator to input sin waves, change the wiring diagram to CH1+ to connect to the positive pole of the signal generator, CH1- not to connect, and TP12 to connect to the negative pole of the signal generator. I let the signal generator input SIN wave, square wave, triangle wave, and all can be output completely, but the human body physiological signal ECG does not know why it is wrong.

  • 0 in reply to user5121970
    TI__Guru 55342 points

    Hi,

    What are you using for ECG signals? Is it an ECG simulator of some sort? Can you confirm what this signal looks like when not connected to the ADS1299 using an oscilloscope? Is it meant to be sinusoidal as shown or more like a traditional ECG waveform?

    This problem is typically seen when clipping as you mentioned (input signal amplitude is too large) :https://e2e.ti.com/support/data-converters/f/73/t/379380?tisearch=e2e-quicksearch&keymatch=ads1298%20analog%20inputs

    Or when the sample rate is too low compared to the frequency of the input signal. Try increasing the sampling rate of the converter, or reducing the frequency of the input signal to see if that helps. 

  • 0 in reply to Alexander Smith
    Intellectual 425 points

    I use human physiological signals, from a real person to directly connect the alligator clip and electrode patch to CHANNEL1 of ADS1299.
    I tried to increase the sampling frequency, but the signal did not change.
    I think it’s not that the input signal amplitude is too large. Human physiological signal amplitude doesn’t seem to be that big. I think it might be a wiring problem. Maybe it’s a wrong connection. For example, the reference electrode shouldn’t be all on tp12, it might have to be connected somewhere. , This is my current speculation.

  • 0 in reply to user5121970
    TI__Guru 55342 points

    Hi,

    It looks like you have SRB2 closed on channel 1. This means that the positive input of channel 1 will be connected to the SRB2 pin. The SRB2 pin is commonly used as a reference for EEG measurements, with respect to the reference electrode. I would recommend changing Ch1 to 0x00h. However, since SRB2 does not seem to be driven to any potential from your description, I do not believe it explains this behavior. 

    Are you able to see the physiological signals on an oscilloscope? Do they look correct or similar to what the ADS1299 is outputting?