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ADS1299: Understand ADS1299 gain

Norbert Toth
Expert 1145 points
Part Number: ADS1299

Hi Folks,

first of all, I forget to collect forum links about this questions, but seeking on the net, I have found several pages and questions:

why developer is not necessary to use external amplifier at the input of ADS1299???

As I calculate it, (ECG signals around 1 mV, EEG signals 10uV, fetal ECG should be same as EEG, and so on...) using the maximal gain, the input range is around 200 mV. It is far less then the signals in interest.

  1. So if I want all of the 24 bits for data representation, why I cannot use external amplifier stage?
  2. I using it, should I use the SoC BIAS amplifier output as DRL, or should I use an own?
  3. What should be the reference of the ADS1299? Connecting the SRB1 to GND, BIAS etc?

Regards,

Norbert

  • 0
    TI__Guru 52772 points

    Hi Norbert,

    This is typically not a good idea since the input referred noise of the pre-amplifier is multiplied by both gain stages. The end result is that your measurement takes advantage of a larger dynamic range, but the extra noise introduced into the measurement actually causes your signal to noise ratio to decrease when compared to the measurement without the pre-amplifier. 

  • 0 in reply to Alexander Smith
    Expert 1145 points

    Hi Alex,

    you are absolutely right. This is what I also read out from the datasheet. On the other hand, without external amplifier, the resolution is somewhere 5-16 bits depending on the internal PGAs, but not 24 bits.

    This 5-16 bits is very tiny in the smaller signals, like EEG or fetal ECG.

    Regards,

    Norbert

  • 0 in reply to Norbert Toth
    Expert 1145 points

    Hi All,

    Is here anybody who can help me how to design a pre-amplifier stage before an ADS1299? Preferably using INA series amplifier but other solutions are also welcomed.

    Regards,

    Norbert

  • 0 in reply to Norbert Toth
    TI__Guru 52772 points

    Hi Norbert,

    Like we already talked about, this is not a good idea due to the extra noise that will be introduced into the measurement. What is really important for these measurements is the signal to noise ratio, not the amount of dynamic range that is used. This is against the normal convention since most ADC's have much higher noise than the ADS1299.