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ADS1298R respiration increase current I_Resp

Other Parts Discussed in Thread: ADS1298R, ADS1296R

Hi at all,

I have a question about the ADS1298R Evaluation Board.

For the measurement of the respiration there is a circuit on the board (see picture 1). And for limitation of the current I_Resp there are two resistors on the board (R96 and R97, both 40.2k), see also Equation in picture 2.

My question is: up to what value can I limit the resistors (R96 and R97) to increase the current through the body? What is the maximum current of the internal current source of the ADS? I found no limitation / value in the datasheet (SBAS459J).

Thanks a lot,

Constantin

  • Knows nobody an answer?

  • Hi Constantin,

    Thanks for your question.

    The output of RESP_MODP and RESP_MODN is actually a square wave voltage output with magnitude of VREFP - VREFN. The current flowing through the respiration circuit can be scaled up by decreasing the series 40.2k resistors. These values were chosen for the EVM to limit the current through a patient to a safe limit.

    Best Regards,

  • Hi Ryan,

    Thank you for your reply.

    Actually I know that the "measurement signal" for respiration is a square wave.

    My question was, what is the (possible) max. current that can generate the ADS1298R? In the datasheet I found only the typ. value of 29uA.

    For example, if I choose for VREFP=4V, up to what value can I decrease the resistors (R96, R97) on the evaluation board to not destroy the source in the ADS1298R? Is it possible to choose a value of 1k for both resistors?

    Best Regards,
    Constantin
  • Hi Constantin,

    I understand your question now. It looks like we do not give a spec for the output current of the internal reference amplifier. I know that I have done some rough bench measurements myself in the past, and it looks like you can only get a few hundred uA before the reference voltage collapses. However, since it is not characterized for every device, I cannot promise you a max limit.

    Even though the current is an AC current of either 32kHz or 64kHz, there are still limits you must keep in mind per IEC standards. The current limits will be a little higher for AC current than for DC current, but I'm not sure what they are. I would consult the IEC standards for AC current first, then design your respiration circuit accordingly.

    Best Regards,
  • Hi Ryan,

    thank you for the detailed answer.

    Best Regards,
    Constantin
  • Hi Ryan,
    My customer also have some questions about ADS1296R resp. One question is: If we can use a smaller resistor to get a larger currrent, then, we can get a better SNR or resolution for the signal, right?
    Regards,
    Haroad
  • I have a followup question to this. 

    When you say "safe limit" to the patient, what is the safe limit here. How much current can be passed through the patients body? 

  • Hello Mouli - the limit is set by IEC standards and depends on the frequency of the signal. Please consult the applicable IEC standards.

    Regards,