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ADS1198 Pacemaker detect logic

Santoshkumar Gurral
Intellectual 270 points
Other Parts Discussed in Thread: ADS1198

Hi,

ADS1198 IC paceout pins (TESTP_PACE_OUT1 & TESTN_PACE_OUT2), i have connected to a hardware pace detection circuit

(Differentiator, window comparator, SR latch) and SR latch output is connected to a GPIO pin of a  Micro controller (STMicroelectronics)

1) when a pace pulse event occurs, SR latch output changes it state form LOW to High which is seen by MCU, now how do i plot this event along with ECG signal on my device display(my ECG device has a display to show ECG waveform), because what if a pacemaker pulse event occurs before the AFE ADCs sample the ECG signal, packetize them then send over SPI interface and MCU plots it on the Display ?


How to integrate a pace maker pulse detect event along with the ECG waveform on a display?
What is the logic?

Santosh

  • 0
    TI__Mastermind 38745 points
    Hello Santosh,

    Your concerns about that topology is valid. It might be difficult to determine with precision the moment with respect to the ECG waveform that the pacemaker pulse occurred. The digital filter has some phase delay associated with it, as well as the delay for the shift register to be loaded with the new data and the delay from when new data is available and your program actually goes in and collects it. In addition, the location of the pulse in time will have a finite resolution determined by how fast you clock the latch.

    What is your goal with this topology? Is is just to detect that a pulse has occurred or do you need to know with some precision when it happened in relation to the ECG waveform? If you just need to detect it, this topology will be sufficient. If you need to determine precisely when it happened, you can probably sample the ADC fast enough to see it. The only drawback of sampling the ADS1198 fast enough to capture a pacemaker pulse is that you have to sample ALL the channels that fast, which may be undesirable when trying to achieve optimal noise performance.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Intellectual 270 points

    Hi Brian,


    My topology is not to just detect the pacemaker event but also to integrate (show a pulse along with ecg wave) it with the ECG waveform to show it on the display.

     
    My sampling frequency is 2Ksps and maximum can go up to 4Ksps, is it possible to plot pacemaker pulse with this sampling frequency or do i need to still increase the sampling rate ?

    santosh

  • 0 in reply to Santoshkumar Gurral
    TI__Mastermind 38745 points
    Hey Santosh,

    My gut reaction is that you would be sampling too slow in those cases to catch the fastest pulses. I'm not totally sure though, and would encourage further study.

    If you have a source that can produce signals similar to the pacemaker signals that your application would be identifying, produce it somehow and analyze it in the frequency domain (perhaps model it in MATLAB). From signal theory, we know that a thin pulse is likely to have a wide frequency bandwidth since the Fourier transform of a delta function is unity. Imagine sampling at either 2 kSPS or 4 kSPS and keep in mind that the ADS1198 has a digital filter that will tend to attenuate signals pretty significantly at f_dr/2. Determine the minimum acceptable bandwidth necessary to capture the pulse and correlate it with the ADS1198 data rates. I can't guarantee that you will be able to use the data rates you are limited to, but it might be worth studying.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 30 points
    Hi Brian,

    page 47 of ads1198 datasheet show
    TEST_PACE_OUT1. is set in pace Bits [4:3],
    but the page 62 show PACE[4:3] set the TEST_PACE_OUT2 。
    can you tell me which one is right?
  • 0 in reply to zach shaw
    TI__Mastermind 38745 points
    Hey Zach,

    Thank you for catching the error. You can trust the detailed register descriptions to have the correct assignments (page 47 is correct). This will be corrected in the next revision of the datasheet.

    Regards,
    Brian Pisani