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Several questions of ADS1299

Eric Tian
Prodigy 135 points
Other Parts Discussed in Thread: ADS1299, REF5025, OPA376

I'm trying to build a portable EEG system using ADS1299, the interested frequency range is 0.5~100Hz and I have some questions:

Q1 : If I set the gain to 24 and VREF=4.5V, when do I need a pre-amplifier for the EEG signals? Is there a suggested input Vpp threshold for whether to use the pre-amplifier or not?

Q2: Since the max gain of ADS1299 is 24, and the internal reference is 4.5V, in most cases there will be much ''room'' left for the output range. If I want to get a better resolution, should I use pre-amplifier or use a lower reference? which is better? Or neither is necessary, just amplifying the raw EEG by 24 is enough for most applications?

Q3:Fig.31 of ADS1299 datasheet depicts a typical reference drive circuitry which I don't quite understand. Are the TRIM pin of REF5025 and a 22uF capacitor used to get a more accurate output from REF5025? How do they manage to do that? What are the other components used for? Are they used to filter out the AC and noise of the output of REF5025? Sorry my analog knowledge is not so good.

Q4:Could anyone give me the password of ftp://ftp.ti.com/pub/ADC/ADS1299/ADS1299EVM.zip?

Any reply/comment is appreciated, thanks!

Regards,

Eric

  • 0
    TI__Guru 57336 points

    Hi Eric, 

    In bio-potential applications like EEG, you will hardly ever utilize the entire full-scale range of the ADC. With gain = 24 and Vref = 4.5V, your LSB size for the ADC is already (2*4.5)/24 / 2^24 = 22.35nV. Reducing the reference voltage or increasing the input signal gain would further increase your resolution, but it is probably not necessary at that point as long as you can still resolve the smallest expected signal amplitude.

    If you decide that you do need an external gain stage, you will want an amplifier with very low leakage current and noise that is lower than the ADS1299. Something like the OPA376 may be a good fit.

    As you expected, the extra capacitors placed around the REF5025 are for filtering the reference output. The TRIM/NR (noise-reduction) pin serves two purposes: to trim the output voltage or to reduce the noise of the internal bandgap voltage supply. A 1uF from this pin to ground creates a low-pass filter around 10-20Hz with the internal resistors. Higher capacitance further filters the bandgap for a cleaner output, but increases start-up time.

    The 22uF output load capacitor is used to stabilize the reference voltage. For more information, please consult the REF50xx datasheet.

    I will follow up with an email shortly with the password. 

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 135 points
    Hi Ryan,

    Thank you! Your answers are really helpful.

    Regards,

    Eric
  • 0 in reply to Ryan Andrews
    Prodigy 135 points
    Hi Ryan,
    Could you please send the password of ftp.ti.com/.../ADS1299EVM.zip to yitian2@cityu.edu.hk? Thanks.
    Regards,
    Eric