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ADS1298: Cannot get ECG signal

JxJxJ
Prodigy 40 points
Part Number: ADS1298

Hi everyone

I'm a student and i have a project using a custom board provided by my professor with the ADS1298 device to measure 12 leads, first i want to get the lead I, but i can't get the signal from the electrodes, i got connected the 2 electrodes in the ch2 but when i put the electrodes in my arms or my chest the converted value it's only a constant, when i disconnect them i got noise, the registers configuration it's :

CONFIG1 0x46

CONFIG3 0x41

ch1 0x81

ch2 0x60

ch3-8 0x81

the other ones by default

i've tried to connect a signal generator and i can get te signal.

Can anyone help me please?

  • 0
    TI__Guru 60866 points
    Hello,

    Thanks for your post!

    There could be a number of problems that prevent you from seeing a real ECG signal. One explanation might be the electrode-to-skin impedance. If this is too high, the signal may be severely attenuated. Or, if the impedance of two electrodes is very different, the mismatch could create a large differential offset voltage. This offset voltage, followed by the gain of the internal PGA, may saturate the ADC inputs. What is the constant value that you are seeing at the ADC output?

    I would try using wet electrodes (i.e. with some sort of gel) to reduce the contact impedance and reduce the gain of the PGA to avoid saturation.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 40 points
    Hello Ryan,

    Thanks for your reply,

    I'll try with wet electrodes, the constant value it's "8388607" and sometimes "8388608",
    I have another 2 questions first about the RLD , it's necessary to connect it to the leg? and if it is necessary, the configuration of the chn2 (which it's the channel for my 2 electrodes) should have 0x00 or 0x02 in the mux configuration?
    And the other question, it's correct configure only the CONF1,CONF3,CHnSET registers for one lead?

    Regards.
  • 0 in reply to JxJxJ
    TI__Guru 60866 points
    Hello,

    8388607 represents positive full-scale (0x7FFFFF). 8388608 represents negative full-scale (0x800000). These codes indicate that the differential output voltage of the PGA is greater than the ADC reference voltage (VREFP - VREFN).

    Please share the complete register settings in a table and I will review them all for you. It's difficult to understand if everything is correct with only some of the settings.

    Can you also share a schematic? I would like to see which electrodes you are measuring and how they are connected to the ADS1298.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Hello, 

    This is schematic for the electrodes input:

    This is for the ADS:

    and this is for de supply:

    I have this register settings:

    CONFIG1 0x46
    CONFIG2 0x10
    CONFIG3 0x41
    LOFF Default
    CH1Set 0x81
    CH2Set 0x60
    CH3-8Set 0x81
    RLD_SENSP Default
    RLD_SENSN Default
    LOFF_SENSP Default
    LOFF_SENSN Default
    LOFF_STATP Default
    LOFF_STATN Default
    PACE Default
    RESP Default
    CONFIG4 Default
    WCT1-2 Default

    Regards.

  • 0 in reply to Ryan Andrews
    Prodigy 40 points
    I found an error with the board, the VREFN and VREFP was inverted, i've already corrected the error and now i'm able to get the ECG signal correctly, thanks for you time Ryan.