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ADS1299 Electrode Design (on Figure 59)

Jason Kim
Prodigy 175 points
Other Parts Discussed in Thread: ADS1299

Hello guys, 

 This might be a stupid question, but I just wonder ... 

 If you look at Figure 59 of ADS1299 EVM user manual,  each AINxN and AINxP has RC low-pass filter on the front. (low-pass w/ fc ~ 6700Hz)

Is that really low-pass filter or something else ?  Do you guys know what is the purpose of that ?

Thank you.

  • 0
    TI__Mastermind 38745 points
    Hey Jason,

    You're correct, those are RC low-pass filters. They are used for anti-aliasing. The digital filter will attenuate in-band noise consistent with its filter properties, but since it is digital its transfer function repeats at every multiple of the sampling frequency. That means that out of band noise will fold into the band of interest unless it is removed using an analog low-pass filter.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 175 points
    Hello, Brian.

    I really appreciate for your help.

    For the RC low-pass filters, with R = 4.9kohm, C = 4.7nF (Figure 59), I'm getting cut-off frequency fc = 6786 Hz.
    * fc= 1/(2*pi*R*C)

    If it is used for anti-aliasing, does it needs to be that high, then?

    Thank you Brian. !
  • 0 in reply to Jason Kim
    TI__Mastermind 38745 points
    Hi Jason,

    Since you can expect the nearest aliased noise to peak around the modulator frequency since the digital filter response is centered around integer multiples of the modulator frequency, the RC filter in this case was built to have ~40 dB of attenuation around the modulator frequency. The magnitude response for a simple RC filter is |H| = 1/sqrt(1+(2pi*fRC)^2). For this setup that corresponds to -43 dB gain. A cutoff of 6.8 kHz is actually relatively low given the datarates that this device is capable of putting out. The highest data rate is 16 kHz which gives you a Nyquist rate of 8 kHz. The RC filter will definitely attenuate frequencies that high. However, if your signal bandwidth isn't that large, then there's nothing to worry about. ECG signals can typically be captured in the tens of Hertz of bandwidth range.

    Regards,
    Brian Pisani