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Original question:

ADS1198: ECG Input from Simulator

ADS1298: ECG wave problem

Somnath Kavathekar
Prodigy 70 points
Part Number: ADS1298

Hello! 

My 12 lead ECG board is designed exactly similar to "ADS1298ECG-FE".

The problem is :

When input from ECG simulator is given, all the waves shift like some offset is introduced and the offset may not have fixed values every time.

In Test Mode, output is as desired without any noise or offset.

Input from simulator:

schematic is as below:

Input circuit for all channels is as below:

R15, R16 and C12 are not populated.

AVDD = 3V, AVSS= AGND= Ground, DVDD= 3.3V

Internal clock  is used.

Configuration is as below:

WriteADSReg(0x41, 0x46, 3) ; //CONFIG1

WriteADSReg(0x42, 0x12, 3) ; //CONFIG2

WriteADSReg(0x43, 0xCC,3) ; //CONFIG3

CHNSET from Addr 0x45 through 0x4C = 0x10

WriteADSReg(0x4D, 0x07, 3) ; //RLDSENSP

WriteADSReg(0x4E, 0x07, 3) ; //RLDSENSN

WriteADSReg(0x54,0, 3) ;// GPIO

WriteADSReg(0x57, 0x02, 3) ; // CONFIG4

WriteADSReg(0x58, 0x0b, 3) ; //WCT1

WriteADSReg(0x59, 0xd4, 3) ; //WCT2

All  other registers left with default values.(i.eCode does not write anything in other registers)

Please Guide .

Regards,

Manisha

  • 0
    TI__Guru 51051 points

    HI Manisha,

    Thanks for your post and welcome to the forum! 

    None of your pictures came through properly, can you re-post them please? 

  • 0 in reply to Alexander Smith
    Prodigy 70 points

    Hello Alex,

    Thank you for immediate response. Will attach the pictures and resend.

    ADS1298_ECGwaveproblem_screenshots.docx

  • 0 in reply to Somnath Kavathekar
    TI__Guru 51051 points
    Hello Manisha,

    I can't find R15, R16 and C12 on the schematic since it is blurry.

    When you say offset do you mean that the voltage level is changing or that the measurement stops like in the second picture?

    If you mean that the voltage level is changing:
    Do you have waveform captures showing the issue? What is the magnitude of the offset?
    Is the simulator GND and the ADS1298 AVSS tied together?
    Is there noise or an offset on AVSS?

    If you mean that the measurement stops like in the second picture (between the red and green waveforms):
    What is the communication to the device during this time?
    Is something resetting the device or stopping the measurement in your code?
  • 0 in reply to Alexander Smith
    Prodigy 70 points

    Hello Alex,

    If you refer to the "Wave forms with test input" from the attachment,  the square wave is correctly displayed as expected.

    but if you look at the "ECG Wave forms with Normal Electrode input from Simulator", ECG waves of e.g. LeadII,  LeadIII,  AVR are shifted from their respective reference levels.  AVL is shifted from its reference level so much that it is not visible.

    If test waves are getting displayed properly but ECG waves are not means some noise is introduced from electrode inputs.

    Hope this explains what the problem is.

    Measurement is not getting stopped or device is not getting reset. Board is continuously communicating with the PC side software.

    One more observation: If I power the simulator from ac adapter, only noise appears .  If simulator is powered from battery then the output as in attachment can be seen.

    (My board is battery powered)

    Regards,

    Manisha

  • 0 in reply to Somnath Kavathekar
    TI__Guru 51051 points
    Hi Manisha,

    I see now. Without a labeled axis it was difficult for me to identify.

    Have you probed DVDD while this is occurring to see if that is causing the shift?
    What else is on the board besides the battery and ADS1298? Anything that is switching on and off?

    How are you connecting the AC adapter? Is there circuitry in place is properly clean and regulate the signal?
  • 0 in reply to Alexander Smith
    Prodigy 70 points

    Hello Alex,

    DVDD is  stable.

    Other than Battery and ADS there are battery charger circuit, power supply regulators and micro controller circuitry.

    AC adapter is required by Simulator. Need to check the adapter signal.

    Will check and let you know.

    Thank you for the support so far!

    Regards,

    Manisha

  • 0 in reply to Somnath Kavathekar
    TI__Guru 51051 points
    You're welcome, Manisha!

    I would recommend probing various points of your digital supply/communication. Check to see if when the reported values have a "shift" does the SPI line see the shift as well?

    This may help you narrow down the root cause of the issue.
  • 0 in reply to Alexander Smith
    Prodigy 70 points

    ADS1298_ECGWaveformScreenshots.docxHello ,

    I found one capacitor near battery power  input was de soldered during initial debugging. After soldering that and touch up to other components in the input circuit clear and steady wave forms were observed. Please check the attachment.

    Please note that the Simulator was battery powered.

    But the problem of noise in the wave forms  with simulator input when powered from adapter remains the same.

    Also it was observed when the sine wave of 1mV amplitude was given input,  the output wave amplitude  was 0.6mV .

    When inputs shorted, noise of 500/600uV is observed.

    How to take care of this problem?

    Regards,

    Manisha

  • 0 in reply to Somnath Kavathekar
    TI__Guru 51051 points
    Hi Manisha,

    Glad to hear that the initial issue is fixed!

    The noise is most likely common-mode noise from the power mains. The noise from power connections or lights can couple onto the signal path and show up as noise on the output. This is typically fixed with an RLD connection. This app note may help: www.ti.com/.../sbaa188.pdf

    What is the frequency of the noise? Is it 50Hz?
    How much noise is coming from the simulator output without being connected to the board?
    What is the gain of the ADS1298 PGA?

    What is the sampling and input frequency?
    If your normalized frequency (fin/fDR) is large , then you will see some attenuation. Please see figure 52./53. for details.
  • 0 in reply to Alexander Smith
    Prodigy 70 points

    Hello Alex,

    RLD is implemented. PGA is 1 and sampling is 250 sps.

    Will check the other parameters as suggested by you.

    Regards,

    Manisha