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Original question:

ADS1298: input capacitor value of the CLK pin

ADS1298: The performance difference between the internal reference and an external reference

Wang Zeyu
Prodigy 40 points
Part Number: ADS1298
Other Parts Discussed in Thread: OPA350, REF5025

Hi, Team, hope everything goes well with you all~

My question is about the performance differences between the internal reference and an external reference when deploying the ADS129x series-based system, ADS1298 for instance.

Currently, optimal system performance is crucial for our application scenarios. After checking the related description in the ADS1298 datasheet, we found there are not too many details about the reference voltage, such as noise level and stability, etc., and this datasheet just proposes an example based on REF5025 and OPA350 solution. My question is if an external reference will perform better than the internal reference in terms of the collected signal quality, such as ECG measurement? In addition, is there any guidance document when using an external reference?

Meanwhile, if a multi-channel system, which means more than one chip will be used, is designed and how to determine how many reference chips will be needed given the driven capability and noise issue?

Looking forward to your reply, and thanks ahead for the help from our community!

Zeyu

  • 0
    TI__Expert 4134 points

    Hello Zeyu, 

    Please give me 24 hours to dig through some resources and get you a comprehensive answer.

    Kind regards,
    Nick 

  • 0 in reply to Nick Zahabizadeh
    Prodigy 40 points

    Many thanks, Nick! Really looking forward to your input!

  • 0 in reply to Wang Zeyu
    TI__Expert 4134 points

    Hello Zeyu, 

    There's a couple pros and cons to each configuration. An external reference could be more accurate than than the internal, but it would also require extra design from your side to make sure the layout doesn't inadvertently add noise to your system. Using the internal reference is faster for design time and the noise performance is usually fine for most ECG applications of our customers. However, using the internal reference limits you to only two output voltages: 2.4 and 4V. 

    Like I mentioned, the performance using the internal reference is sufficient for most ECG applications, so most of our customers opt for that route. 

    In multi-AFE designs, one external reference should be able to deliver fine performance for all the AFEs cascaded together. 

    Kind regards,
    Nick

  • 0 in reply to Nick Zahabizadeh
    Prodigy 40 points

    Hi, Nick, thanks for your reply, I really appreciate them, but I still felt confused about those questions. My question is:

    As you mentioned that an external reference with extra designs could be more accurate than an internal one, could you give me some suggestions or detailed guidelines for an external reference design, such as layout suggestions or filtering strategy? Although the performance is sufficient when an internal reference is used for ECG monitoring in most cases, we want to evaluate the optimal performance, i.e., when approximating the theoretical or optimal performance of the chip.

    Zeyu

  • 0 in reply to Wang Zeyu
    TI__Expert 4134 points

    Hello Zeyu, 

    Our recommendation for filtering is listed in the datasheet in figure 32. In terms of physical layout, that would be dependent on your application and the physical characteristics of your PCB, thus we cannot provide definitive guidance on this medium.  

    Kind regards,
    Nick