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ADS1298: portable ECG

Gerald
Genius 9575 points
Part Number: ADS1298
Other Parts Discussed in Thread: , LM27761

Hi Experts,

Our customer has some doubts about ADS1298. Can we ask you some help?

In particular, he would like to know what is the current at the input that he should use (he want to realize a portable ECG). ECG is for Electrocardiogram.

He can't find anything on the datasheet, only the maximum current (10mA).
Is it the current at the power pin or at each channel?
Do you also recommend a particular LDO or Voltage switching regulator to use for the layout?

Also, he would like to know how to choose the right power supply. What’s the difference between +-2.5 V or +-1.5 V? He want to use a bipolar supply Configuration. What’s the impact on VREF and what the best way to choose VREF?

His application will be portable, so he will use batteries

Please advise. Thank you.

Best regards,
Gerald

  • +1
    TI__Mastermind 19815 points

    Hi,

    for 

    "what is the current at the input that he should us."

    Yes, he is referring to the datasheet page 12 correctly. typically the withdraw/consumption current should not exceed the stated 10mA or nor more than 15 to 20mA. 

    Please ask student to take a look of ADS1298 or ADS1298R EVM's user guide, which have the suggested power supply and LDO parts that are on the EVM, and they could use the same or the similar or quieter spec for the power supply.

    Regarding to unipolar or bipolar, please take a look of datasheet page 97.

    Regarding to reference voltage VREF, please take a look of datasheet page 33.  In general, it has to be very quiet and clean. Datasheet suggested an external reference part, customer can use that or find a part with similar or better spec.

    Thanks

  • 0 in reply to ChienChun Yang
    TI__Genius 9575 points

    Hi,

    First of all thank you for your kind reply.
    However, I wish I could ask you for more information.

    -Is the current you are referring to the one arriving at the power supply pin (Iavdd or Idvdd)? After I get to that pin, will it be split by the channels I choose to use?

    -If you want a bipolar type power supply, could LM27761 be fine (being careful that the switching frequency is a multiple of the sampling frequency)?

    -Regarding the START pin, on the datasheet I read that it must be kept in the LOW state if I want to control it from a microcontroller via a pull down resistor. Can you recommend me some resistance value that I can use (maybe also for the RESET and Power Down pin)?

    -For the value of the analog power supply, is there any value that you recommend to use for my application (ECG)?

    Thank you so much for your time.

    Best regards.

  • 0 in reply to Gerald
    TI__Mastermind 19815 points

    Hi,

    IAVDD indicates current going/consumed by the AVDD for analog power supply

    IDVDD indicates current going/consumed by the DVDD for digital power supply

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

    Power supply, please ask customer to check and compare the spec of the power supply suggested in the user guide. 

    Please refer to the user guide schematic for pull down resistor, typically 1k to 10k are fine.

    Please refer to the user guide schematic

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

    Thanks

  • 0 in reply to ChienChun Yang
    TI__Genius 9575 points

    Hi ChienChun,

    I received additional information from customer in regards to my previous comment. Please see below:

    "yesterday I sent an email response because I have other doubts about the device (ADS1298). However, the email I sent is not very clear. So, I send you the same email hoping that this time it will be more readable. I apologize for the inconvenience and thank you for your kind and availability.
    Here are my questions:

    -Is the current you are referring to the one arriving at the power supply pin (Iavdd or Idvdd)? After I get to that pin, will it be split by the channels I choose to use?

    -If you want a bipolar type power supply, could LM27761 be fine (being careful that the switching frequency is a multiple of the sampling frequency)?

    -Regarding the START pin, on the datasheet I read that it must be kept in the LOW state if I want to control it from a microcontroller via a pull down resistor. Can you recommend some resistance value that I can use (maybe also for the RESET and Power Down pin)?

    -For the value of the analog power supply, is there any value that you recommend to use for my application (ECG)?

    Thanks again for your support, I really appreciate it. Also, I apologize again for the inconvenience."

    Best regards,
    Gerald

  • 0 in reply to Gerald
    TI__Mastermind 19815 points

    Discussed in the WebEx Chat -Duplicated questions, Please see answers in the early replies.

    Thanks

  • 0 in reply to ChienChun Yang
    TI__Genius 9575 points

    Hi ChienChun,

    I received a response from our customer and he has a new question to ask. Please see below:

    "why are 338 kHz and 153 kHz cutoff frequencies chosen for the anti-aliasing filters in the user guide?

    I know that the only requirement is to cut the modulator frequency harmonics (in my case, HR mode, that would be 512 kHz), but why not choose lower frequency values to try and eliminate some noise?
    I also know that the sinc filter works very well to reduce noise, but I still don't understand why two such high frequencies have been chosen.
    I always thank you for your kind attention."

    Best regards,
    Gerald

  • 0 in reply to Gerald
    TI__Mastermind 19815 points

    Hi,

    Could customer identify which user guide are they referring to ?

    Is this one? 

    https://www.ti.com/lit/ug/sbau171d/sbau171d.pdf?ts=1683131006504&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FADS1298ECGFE-PDK

    And, which filters are they referring to if they could point out?

    Thanks

  • 0 in reply to ChienChun Yang
    TI__Genius 9575 points
    Hello ChienChun,
    Yes, that is the guide.
    The customer is talking about antialiasing filters (p. 52).
    Thank you and best regards,
    Gerald
  • 0 in reply to Gerald
    TI__Mastermind 19815 points

    Hi,

    I will need to discuss with other engineers before getting back to you.

    Thanks

  • 0 in reply to ChienChun Yang
    TI__Genius 9575 points

    Hi ChienChun,

    We understand and thank you for all the help.

    BRs,
    Gerald

  • 0 in reply to Gerald
    TI__Genius 9575 points

    Hi ChienChun,

    I would like to gently follow up on the status of our customer concern. We hope to hear again from you. Thank you.

    BRs,
    Gerald

  • +1 in reply to Gerald
    TI__Mastermind 19815 points

    Hi,

    Sorry for the delay.

    Discussed with other engineers, they don't have any particular reason to select that cut off frequency.

    It's optional for customers to change to the cut off frequency and do tests.

    Thanks