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ADS1298 Gain

Sebastian Loer
Intellectual 505 points
Other Parts Discussed in Thread: ADS1298

Hello,

i use the ADS1298 to realize a ECG circuitry.

Do i really not need any gain before the inputs of the ADS1298?

Because the maximum gain of the chip is 12, and the ECG-signal is about 1µV - 1mV.

So i think it would be necessary to amplify the signal before???

regards

  • 0
    TI__Mastermind 43915 points

    Hi Sebastian,

    You are right that the ECG amplitude is on the order of a few mVs max. Gain is really going to depend on what you want to do/analyze. If do just standard R-wave detection, you probably can get by in a gain of 1 and just look for the R peaks to determine heard rate. If wanting to do some more detailed analysis on the ECG waveform, you may want to introduce some gain to give you more resolution to look at details of the ECG.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Intellectual 505 points

    Hi Tony,

    thanks for your reply. I only want to display the ECG waveform and look for the R-peaks.

    So for this application the gain of the ADS1298 is enough, i think.

  • 0 in reply to Sebastian Loer
    Prodigy 115 points

    From what I have seen so far, if your signal is from an electrode connected to the chest or somewhere near the heart, the ADS signal is quite good. Although it's not a high numerical value, the high resolution will do the trick for you. Check the document - Analog Front-End Design for ECG Systems Using Delta-Sigma ADCs for an idea of how high gain + low resolution ADC can be replaced with low gain + high resolution ADC systems. All you have to do is directly feed the electrode output to the chip.

    But I do see some problems with baseline drift. I don't know how to solve that...

  • 0 in reply to Govind M
    Intellectual 505 points

    Yes there are some problems with baseline drift, i think.

    Maybe a highpass between the electrode and the input of the chip is a solution or i maybe can remove

    the baseline drift with digital filtering.

  • 0 in reply to Sebastian Loer
    TI__Mastermind 43915 points

    Govand, Sebastian,

    We have used both AC coupling methods on the front end and digital filtering on the back end to help with baseline drift. If doing it in the analog domain, you will need to bias the ADC inputs on the ADC side of the AC coupling capacitor to keep the input signal within the range of the ADC inputs.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Intellectual 505 points

    One more question concerning the gain of the ECG signal:

    I measure the signal not close to the heart, but at the right and left hand.

    Do yout think the gain+adc resolution is enough yet?!

    regards

    Sebastian

  • 0 in reply to Sebastian Loer
    TI__Mastermind 43915 points

    Hi Sebastian,

    The answer really depends on what you are trying to do. For simple R wave detection or simply QRS analysis, you would probably be fine with the ADS1298/1292 using the hands. It is hard for me to comment in detail as we do not connect these devices to patients and stick with strictly simulators. You may need to do some sort of post processing in the digital domain to help remove any baseline drift and muscle movement signals.

    Regards,

    Tony Calabria

  • 0 in reply to Sebastian Loer
    Prodigy 115 points

    Hi Sebastian,

    Generally, if you want to get a good signal that can be used for detailed diagnosis, you need to measure at the chest with wet electrodes. If you measure at the fingers or so, the signal is quite weak and corrupted, so it will be difficult to pick out things like the ST segment accurately etc but you can notice arrhythmias and so on.

    Govind

  • 0 in reply to Govind M
    Intellectual 505 points

    Okay thank you both for your answers.

    I only want to detect the RR-intervalls and to display the ECG signal.

    So i try it with connecting the electrodes directly to the inputs of the chip and do some digital signal processing.

    regards,

    Sebastian