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ADS1299 and pre-amplifier

Other Parts Discussed in Thread: ADS1299, INA333, INA217, INA163, OPA2188, INA118

Hello,

I'm developing an EEG acquisition device and I was using the ADS1299, according to the data scheet the ADS1299 offer a max gain of 24 but i know EEG have low amplitude signals , so schould i use some pre-amplifier such AD620 or INA 333??

Thanks in advance

  • Hi,

      ADS1299 uses a very low noise 24bit delta sigma ADC. This allows us to use low gain in the front-end amplifier. The Gain of 24 gives a input referred noise of as low as 1uVpp which is low enough for most EEG application. The electrode offset (which can be as high as 300mV) limits using a very large gain.  The DC offset must be removed in the first stage of gain if you plan to use a large gain in the amplifier.

     

    With regards,

    amit

     

  • Amit thank you,

    but I have a big problem to convert the 24 bit Delata SEGMA (binary to nimbre reel) to design the EEG signal with software on the PC.
    I read that the gain of an EEG signal with traditional methods (with Amplification instrimentation) is almost gain 1000 ,why I don't  know how ADS1299 with a gain of 24 gets an EEG signal?

    thank you

  • Hi Amit,

    I am designing an EEG system with ADS1299 now. I want a pre-amplifier stage too. I do not understand clearly your explanation here: 

    The electrode offset (which can be as high as 300mV) limits using a very large gain.  The DC offset must be removed in the first stage of gain if you plan to use a large gain in the amplifier.

    Can you talk about the details?

    If I use a 3.0V battery, as you know, the input range of INA333 will be 0.1V to 2.9V. 0.1V is much bigger than an EEG signal, so I should set the reference pin of INA333 to 1.5V, the output will be shifted to about 1.5V too. This output of INA333 is good for ADS1299? Is it necessary to add HP filter and LP filter in the design?

    Thank you.

    Hao

  • Hi Hao,

        The signal coming out of a typical electrode has the desired EEG signal which is small but it also has a DC component because of electrode potential. This DC signal can be much larger than the desired EEG signal.

    If you AC couple the electrode input to the analog front end design then the DC component can be removed. This will essentially be a high pass filter at the input. In this scenario you can use a large gain in the pre-amplifier without saturating its output.

    The large gain is essential if the ADC you are using has lot of noise, so you need to gain up the signal to get decent SNR. In case of ADS1299  the ADC is 24 bit and is low enough noise so that internal gain of 24 should suffice in most cases.

    If you need to further improve the performance of ADS1299 you can use a low noise preamplifier. The noise on this should be 5nV/sqrt(Hz) or lower to get any performance benefits.

     

    With regards,

    amit

     

     

     

     

     

  • Hi.

    Thank you for your reply. 

    According to your explanation, INA333 is not good for this design, because of the high noise level (50nV/sqrt(Hz)). I just find 3 TI instrumentation amplifiers whose noise is below 10nV/sqrt(Hz), can you recommend some single-supply and micro-power amplifiers for an EEG design? 

    Thank you!

    Hao

  • Hello Hao, 

    Almost always, power and noise performance tend to be trade-offs of one another.

    For best noise-performance, you might consider the INA163 or INA217 (~1nV/rt(Hz) each). However, the typical quiescent current for these devices is close to 10mA, more than the ADS1299 itself (~7.6mA).

    Two lower-power solutions that still give you some noise-performance benefits would be the INA118 (350uA) and the OPA2188 (dual op-amp, 475uA per amplifier). Noise at 100Hz is between 9nV/rt(Hz) and 11nV/rt(Hz).

    Can you tell us what gain and bandwidth is ultimately needed?

    Regards,

  • Hi,

    The current of INA118 is much bigger than INA333. 

    For the pre-amplifier, the gain is about 10.  I also put one HP fliter (fc=0.5Hz) and one LP filter (fc=150Hz) in this stage.

    Thanks.

    Hao