ADS1298: ADS1298 ECG FE Low Pass Filter
Part Number: ADS1298
Hello TI support team,
For my projeckt i am using the ADS1298ECG-FE to get ECG signals from my body. So far i've got results but i have also a few questions related on the look of my ecg. i am trying to plot the raw data from ADC by Excel. Firstly the test signal looks right with amplitide and frequency when the date rate is set 500SPS. Secondly i got these ECG signals, which seem to look right, but my questions are:
1/ why do LEAD I and LEAD II have amplitudes of more than 15 mV instead of 1 mV (as required)?
2/ why are the SQRT peaks so hard to see? do we actually need some extra filter to plot these better?
Hi Van Tu,
This is probably due to your gain settings. What is requiring the signal to only be 1mV? Higher signal amplitude means a higher signal to noise ratio, I do not see a problem here.
I'm not sure what you mean be SQRT peaks, do you mean the undershoot right before the positive peak? You may want to increase your sample rate if you want more precise measurements here.
Alex SmithApplications Engineer | Precision Delta-Sigma Converters
Check out our helpful resources: BIO-FAQ: Common Questions for TI's ADS129x Family of Bio-Potential ADCs
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In reply to Alexander Smith:
Thank you for your quick answer.
Your answer help me to clear things a bit now. As i known, as an ecg its amplitude should be 1mV - 3mV, well maybe a little bit more. But in my case it is about 18 mV on the R-peak, which is weird for me. You meant that it is because of the more higher amplitude the more higher signal to noise ratio (SNR). i know that this ratio can be deal in Sigma-Delta ADC, right? But can you pls explain me that more specific that why i do have 18 mV, what is the roll of signal to noise ratio here? I am not a master at this area.
Yes, i mean the PQRST-Peaks of an normal ecg. But as you said, those could be seen by increasing the sample rate.
In reply to Van Tu Hoang:
Happy to help!
What is your gain setting? I've known ECG signals to be as high as 5mV, but I agree that 18mV is a little high. So there's probably either gain somewhere in the signal chain (you could probe the input pins to check magnitude after your ECG cables, the internal PGA gain is enabled (default is 6) or something is incorrect in the code to voltage calculation. Check your LSB and code to voltage conversion equations using the BIOFAQ, perhaps your LSB size is 10x what it should be?
Yes. The amount of noise in the system will be static, meaning that it is at a certain level and will not change. If your ECG signal is small, then the noise will have a greater impact on the measurement. If your ECG signal is large, then the same noise level will have less of an impact on the measurement. So having a larger ECG signal is actually a good thing.
My gain setting is 6. I dont think there is something incorrect in the code to voltage calculation because i got the Test Signal right with this calculation.
Oh right, your explaination is really helpful.
To prove it, i will post my register settings and test signal, also the way that i calculated the voltage from hex codes, so that you can check it for me. Thank you very much.
Lets say we have a hex code: 3d3c, which is 15676 in dezimal. gain is 6, and Vref = 2,4 V, so
Looks good to me! Please let me know if you have additional questions!
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