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ADS1298 Anti-Aliasing

Sebastian Loer
Intellectual 505 points
Other Parts Discussed in Thread: ADS1298, ADS1292

Hello,

i decided to add an external analog Anti-Aliasing Filter at each input of the ADS1298.

The frequency spectrum which interests me is up to 150 Hz.

I want to use the oversampling of the delta-sigma ADC and sample the signal with 8kSPS, to reduce the filter order.

But what characteristics must the filter has, to reach good results?

Is it enough to use a first order filter?

regards,

Sebastian

  • 0
    TI__Mastermind 43915 points

    Hi Sebastian,

    First order should be fine but keep in  mind, at the faster data rates, you have a higher passband within the digital filter. You may need to do some more post processing on the back end. Take a look at some of the examples here when I was showing software pace detection methods -

    Also, look at the ECG-FE schematic for either the ADS1298 or the ADS1292 as we included anti-aliasing filters on those.

    Regards,

    Tony Calabria

    PACE_detection_ECGFEBrd.pptx
  • 0 in reply to Tony Calabria
    Intellectual 505 points

    Hi Tony,

    thanks for the reply. Yes i know i have to do more post processing at the back end with higher data rates.

    But for example i sample with 500SPS i need an anti-aliasing filter with higher order and thats difficult in my PCB.

    In your presentation pace_detection you show the ECG signal sampled with 500SPS. Did you get these result only with a first order anti-aliasing filter with

    a cut-off frequency at 150Hz?

  • 0 in reply to Sebastian Loer
    TI__Mastermind 43915 points

    Hi Sebastian,

    I used the ECG-FE board with the default values shown in the User's Guide. The Anti-aliasing filter is well beyond 150Hz as I did not want any attenuation of the signal in the 100Hz frequency of interest.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Intellectual 505 points

    Hey Tony,

    thanks for your help.

    I place now a simple RC lowpass in front of the inputs with a high capacity and a low resistor (160Ohm, 1µF) because of the noise a high resistor would cause.

    But i dont keep in mind that i have to meaure the ECG signal with high impedance. In the shematic of your ECG-FE board i see you have an AA-Filter with 22.1kOhm and 47pF, right?!

  • 0 in reply to Sebastian Loer
    TI__Mastermind 43915 points

    Hi Sebastian,

    We have a two stage RC filter with 22.1k and 47pF as well as 10k and 47pF. You will want to make sure that your 3dB point from your RC filter is out far enough so that you are not attenuating the ECG signal of interest.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 130 points

    Hi Tony,

    just a quick question regarding the anti alaising filter. The -3db corner frequency is for the combination above 228 kHz.

    If you now for example have a datarate of 2000 SPS, the corner frequency should be lower than1 kHz to aviod alaising.

    Right?

    Heiko

     

  • 0 in reply to Heiko Glindemann
    Prodigy 130 points

    Would be really good to have an answer regarding this! :)

    Thanks a lot!

  • 0 in reply to Heiko Glindemann
    TI__Guru 66566 points

    Hi Heiko, 

    There are a few things to consider when designing your anti-aliasing filter.

    For starters, we do not want our front-end filter to cut into the bandwidth of interest. For ECG, that could mean 0.05Hz to 40 Hz, or as much as 150 Hz for diagnostic applications. 

    Now, the digital filter inside the ADS1298 gives you some additional anti-aliasing protection. Your -3 dB bandwidth is limited to Data Rate * 0.262. For DR = 2kSPS, that's only 524 Hz. This filter is a sinc-3 filter, which gives you great attenuation at multiples of fDR.

    However, the trouble starts when input frequencies get near the modulator clock, fMOD, which is either 256 kHz or 512 kHz depending if you use High-Resolution or Low-Power Mode. The sinc filter will not give you much attenuation at multiples of fMOD, so it is often advised to have at least -40dB of attenuation from you anti-aliasing filter by that time. I believe the second-order filter described above (which comes from the User's Guide) was designed with a 512 kHz modulator clock in mind.

    To summarize, I think 1 kHz filter should be fine. Your -3dB bandwidth would be limited to about 524Hz from the digital filter for DR = 2kSPS and the anti-aliasing filter should provide enough attenuation by the time frequencies reach fMOD.

     

    Best Regards,