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ADS1298 for Evoked potential measurement

ramanjulu koneni39
Prodigy 70 points
Other Parts Discussed in Thread: ADS1298, ADS1299

Hello,

  • We want to measure Audio brain sterm responses (ABR) /Evoked potentials from electrodes for audiology application
  • We will have minimum three electrodes positive, negative and reference/ground electrodes
  • Before we take test, we measure impedance of each electrode(<5kOhm)
  • Currently, we have product based on C5515 DSP processor
  • We are planning to integrate ADS1298 to C5515 DSP
  • I didn't understand datasheet of ADS1298 for impedance measuring part. Please suggest how to do this on ADS1298?
  • Please share If you have any schematic design files

  • 0
    TI__Mastermind 38745 points

    Hey ramanjulu,

    The ADS1298 does not have an impedance measurement feature. You may be mistaking this with the "lead-off detection" feature which performs inaccurate impedance measurements as a means to indicate if the lead is connected to the patient.

    The "lead-off current sources" inject a current onto the patient. The voltage introduced by the current multiplied with the impedance of the connection to the patient is compared by integrated comparators in the ADS1298 to a threshold voltage. Both the lead-off current magnitude and the comparator threshold can be configured in the LOFF register.

    If the voltage is larger than the threshold, the comparator triggers indicating a lead is off. The comparator output is latched and stored in the LOFF_STAT registers. The contents of the register are also output within the first 24-bits of each conversion data output .

    The schematic for the ADS1298 EVM at the end of the user's guide shows how one might configure the ADS1298 for an ECG measurement system.

    Regards,

    Brian Pisani

  • 0 in reply to Brian Pisani
    Prodigy 70 points

    Hello Brian,

    Thanks for the quick reply

    From one of the post(Link given below), it says that, we can measure impedance Fdr/4. Is fdr is sampling rate?

    1. If we can measure, how to configure it. I want to run ADS1298 at 32KSPS. what should be the band frequency lead off 32KSPS/4?.
    2. how can we get these samples?
    3. Do we get along with channel data?
    4. Please let me know
    5. This application is not for ECG and Not for EEG, But, similar application ABR(Audio brain sterms responses(Evoked potentia;s)). We will have four electrodes two positive, negative and reference(Ground) electrode
    6. We will play 50us tone in the ear and we will measure response immediatly
    7. Our response will be exist only for 10ms after tone play. So, we need to read samples as fast as possible and as many samples as possible with in 10ms
    8. So, we are thinking to run ADS129X at higher sampling rates. Please suggest us, how to proceed on this
    9. I red in forums that, ADS1299 is lowest noise device compared to ADS129X family. If I run at 16KSPS, which is the best IC (Point of view of noise) compared to ADS1298?
    10. Please suggest us, how to proceed. If we get ADS1298/ADs1299, we can evaluate with C5515 EVM/EZDSP right?

    e2e.ti.com/.../238755

  • 0 in reply to ramanjulu koneni39
    TI__Mastermind 38745 points

    Hello Ramanjulu,

    Like it says in that post, it may be difficult to measure well since the current source is not very accurate. But you can use it as a benchmark.

    If you use the 32 kSPS sample rate, the lead-off signal will be 8 kHz. The process of measuring impedance using ac lead-off detection is described very well in that post you linked to. You collect data like normal, but to get the impedance information, you must band-pass filter it around 8 kHz then calculate the magnitude of the signal and divide it by the current magnitude you configured in the LOFF register.

    The ADS1299 is the lower noise device. However, for each device, to take advantage of the very low noise performance, you must sample slower. In general, the slower the sample rate, the lower the noise will be.

    The best way to evaluate the ADS1298 or ADS1299 is with their respective evaluation modules. You can order the ADS1298 EVM here and the ADS1299 EVM here.

    Regards,

    Brian Pisani

  • 0 in reply to Brian Pisani
    Prodigy 70 points

    Thanks Brian

    I will evaluate ADS1298 EVM

    Regards,

    Ramanjulu Koneni