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ADS1298ECGFE-PDK: Voltage pins and ADC concern using ADS1298 for a differential EEG signal

Roshan Sivakumar
Prodigy 60 points
Part Number: ADS1298ECGFE-PDK
Other Parts Discussed in Thread: ADS1298, INA333

Before asked as to why i'm using ADS1298 instead of 99 for EEG, it's cuz the latter is out of stock and i'm working with what I have at the moment.

I understand that the noise floor is high so, i'm using a differential instrumentation amplifier (ina333) - electrode and a reference electrode. Even though I said differential, it's how the electrodes are recorded by through the instru. amp the input to the ads1298 is single ended.

My battery is 3.3V and i have a 1.65 voltage reference, the ina333 REF pin is tied to the 1.65V as i'm using only a single supply.

Concern 1

In the ads12988 datasheet and through other questions on here I understand that if AVDD is 3.3v and AVSS (the analog ground) = 0V and my mid supply voltage is 1.65V. Where do I connect the mid supply voltage on the chip?

What about Vref, there is VREFP and VREFN on the chip, how to select the value for those? Is it just set to mid supply? It seems like the VRef value dictates the input voltage range of the ADC, can anyone please shed more light on this?

Can I leave the Dgnd and Dvdd unconnected if i'm supplying the digital side straight from my battery, I don't see the need to use the ads1298 as the digital supply, just an additional step, any problems that might arise?

Concern 2

It looks like the RLD on the ads1298, bias's the body to mid supply rather than doing it based on the common mode, would anything happen if I used the ina333's common mode for the RLD with an external opamp to inject this voltage instead of the RLD pins on the chip. Hence leaving the pins unconnected (RLDINV, REF, IN, OUT)

Concern 3

Do i even need a ina333, although many sources highlighted the need. The output of ina333 will be centered around 1.65 with the signal amplified n amount, that supposedly solves the concern of the noise floor right? But I can see how there should be another way to just center the input around a reference on the chip itself. Once the signal is centered around dc level, is any gain even required? How to calculate gain need on ADS1298 and ina333 (if needed).

I'm concerned that using a high enough gain for the pre-amplifier, if needed, defeats the purpose of a 24bit ADC, just unsure how to calculate as such.

Any help would be much appreciated

  • 0
    TI__Mastermind 19815 points

    Hi,

    The app engineer was out for business travel, shall get back to you around 7/28.

  • 0 in reply to ChienChun Yang
    TI__Mastermind 19815 points

    Hi,

    1. ADS1298 doesn't support for 1.65V reference voltage. If you have supply around 3.3V,  why not use internal 2.4V ref?

    DVDD and DGND are supply for the digital signals voltage used inside the ADS, they have to be supplied by the voltages suggested in datasheet.

    2. Customer could build their own external RLD driving circuit if they want. Just need to do experiments, tests, data collection/analysis and V&V.

    3. ADS1298 has built in RLD Amp, so don't know why you need to use/build external RLD Amp? 

    Customer could build their own external RLD driving circuit if they want. Just need to do experiments, tests, data collection/analysis and V&V.

    ---------------------

    This statemen is not clear to me, Could you explain a bit more about this 

    "I'm concerned that using a high enough gain for the pre-amplifier, if needed, defeats the purpose of a 24bit ADC, just unsure how to calculate as such."

    Thanks,

  • 0 in reply to ChienChun Yang
    Prodigy 60 points

    Sorry about the late response.

    1. As I had mentioned, I am using INA333 prior to chip to amplify the signal. In order to amplify the INA333 properly - I would have to do that at half supply. The end application will be a wearable hence my limitations on the supply voltage being <=3.7V. How would that work out - having 1.65V reference for INA333 and internal reference. Since the output of ina is centered around 1.65. I don't see how this would work. Any suggestions on what adjustments must be made when using INA333 prior to ADS1298 for further amplification.

    I've always understood that reference needs to be half supply why does 2.4V work for 3.3V supply?

    3. Again, since I have instrumentation amplifiers at the input, wouldn't it make the most sense using the common mode voltages to feed into the RLD rather than using the output which feeds into ADS1298?

  • 0 in reply to Roshan Sivakumar
    TI__Mastermind 19815 points

    Hi,

    I will get back to you around or before 8/17.

  • 0 in reply to ChienChun Yang
    Prodigy 60 points

    Can another app engineer potentially get to this before?

  • 0 in reply to Roshan Sivakumar
    TI__Mastermind 19815 points

    Hi,

    I don't quite understand your request or question for 1.

    And, why do you need additional INA before the ADS1298's ECG inputs pins?

    3. I also don't quite understand your concern or question for question 3? Are you saying you want to create your own RLD drive and use that RLD drive?

    If yes, customers can have/go to that options. Just need to do tests&experiments&data collection/analysis and V&V.

    Thanks,

  • 0 in reply to ChienChun Yang
    Prodigy 60 points

    "Before asked as to why i'm using ADS1298 instead of 99 for EEG, it's cuz the latter is out of stock and i'm working with what I have at the moment.

    I understand that the noise floor is high so, i'm using a differential instrumentation amplifier (ina333)"

    Please refer the start of my question. I mention that i'm using EEG with ADS1298. Since i'm using two TI products : INA and ADS, I am requesting assistance from the support team here to help with using these two products together as I am facing issues due to discrepancies throughout this forum. 

    Please help with understanding how to use INA with ADS product

  • 0 in reply to Roshan Sivakumar
    TI__Mastermind 19815 points

    Hi,

    I don't understand what you mean by this -

    "

    I would have to do that at half supply. The end application will be a wearable hence my limitations on the supply voltage being <=3.7V. How would that work out - having 1.65V reference for INA333 and internal reference. Since the output of ina is centered around 1.65.

    "

    It seems the INA333 already takes/converts the differential to single end output.

    So, maybe you can try/start with setting INA333 as shown in INA333 datasheet page 14 Figure 32. Basic Connections.

    In that way, the Vout from INA333 is already the differential measurement of EEG signal with gain.

    Then, connect INA333's Vout to ADS1298's + input terminal and connect GND to ADS1298's - input terminal.

    Thanks

  • 0 in reply to ChienChun Yang
    Prodigy 60 points

    "It seems the INA333 already takes/converts the differential to single end output"

    Yes, that is right, I have obtained that already as per one of my previous questions. See (e2e.ti.com/.../ina333-ina333-output-is-not-amplified-and-is-odd-on-an-oscilloscope-for-an-eog-circuit)

    Where the app engineer mentions that it will not amplify correctly if the REF pin is not at mid-supply!

    "Then, connect INA333's Vout to ADS1298's + input terminal and connect GND to ADS1298's - input terminal"

    Which is what I too thought about doing although I am using a single supply, I can't connect ground to INxN since the output range of the PGA is limited to within 300 mV of the supply rails right? Hence, what should I do?

  • 0 in reply to Roshan Sivakumar
    TI__Mastermind 19815 points

    Hi,

    Why can't you connect the ground to INxN?

    How much are you planning to amplify from the INA333 and what are the expected signal range output from your INA333?

    Thanks