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problems with ADS1298R eval board

Timothy Starr
Expert 2445 points
Other Parts Discussed in Thread: ADS1298R

We are performing an evaluation of the ADS1298R. There are two issues we are seeing. First we checked the leads ECG and it was OK. When we select chest electrodes the waveforms show strong 60Hz interference. Generally, the chest signal is stronger so we expect lower noise. Is there any specific reason for that? Here is the screen shot: (see attached)

The second issue is the leads-off detection. The figure below shows the map of the detected leads-off buttons, but V6 leads-off is not be able to detect. (see attached). Is there a reason for that?

Thanks and Best Regards, Tim Starr on behalf of JL@ZMEKG waveforms 082015.docx

  • 0
    TI__Mastermind 38745 points
    Hey Tim,

    There is, in general, a lot of 60 Hz noise in ECG systems simply due to the fact that both the body and the electrode cables are essentially large antenna which pick up noise from fluorescent lights. They may be able to improve the 60 Hz noise by using the RLD Sense functionality of the RLD amplifier. Are they doing that already?

    For the lead-off detection, have they enabled the corresponding LOFF bits in the LOFF_SENS registers? If they have, do they observe similar behavior when other leads are removed?

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Expert 2445 points
    Hi Brian-

    We understand there is 60Hz noise, the question is why the limb leads shows good signal while chest leads show such high noise. The ECG signals level are about the same and 60Hz noise is also the same. They should show same waveform and noise level.

    For leads off detection we have tried enable all the register bit but V6 is still not detectable

    -Tim
  • 0 in reply to Timothy Starr
    TI__Mastermind 38745 points
    Hey Tim,

    The channels themselves should be nearly identical, so my guess is that it has something to do with the orientation or position of the electrodes relative to the noise source. You might be able test this by moving the chest electrode to the limb and vice versa to see if the pattern persists.

    For the false lead-off detection, you can try a few things. First, understand that the lead-off detection works by injecting a small current on to the input. Since the PGA input is high z, all the current should flow out onto the electrode. For a "single-ended" channel like V6, the current would conceivably flow to the output of the RLD amplifier if all the electrodes were connected correctly. Since the amplifier sinks that small current easily, the DC voltage at the input of the channel stays at it's natural level. If the electrode becomes unconnected (i.e. if the impedance grows too high), the current source will saturate the input and the voltage will rail, triggering a lead-off detection comparator which sends its status to the LEAD_STAT register and is also output with each new data point.

    The lead-off detection comparator threshold voltage is configurable. If you're using the EVM, you can change the threshold in the "Channel Registers" tab. Try lowering that threshold. It could be that some leakage current is preventing the channel voltage from getting high enough to trigger the comparator. If that doesn't work, try increasing the magnitude of the current sources. If the problem is leakage current, increasing the magnitude of the source current might finally overwhelm the leakage and cause the voltage on the input to sufficiently increase.

    I'm not completely sure how the lead-off detection works with the EVM software, so be sure to check the LOFF_STAT registers in the register map tab by refreshing those registers when you change something as your definitive source of information on whether or not lead-off detection is triggering.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Expert 2445 points

    Hi Brain-

    We found one of the problems. The chest electrodes signals are clean now on this board but V6 and RLD leads off are still not working. We adjusted detection level, current mode/pull up down mode and tried all the register bit. There is no effect. We are not sure if any jumper or configuration is responsible for that. We checked the user’s guide and cannot find anywhere specifies it. Any ideas?

    Thanks for all your help,

    -Tim & JL

  • 0 in reply to Timothy Starr
    TI__Mastermind 38745 points
    Hey Tim,

    You're not going to be able to use RLD lead-off detection while the RLD amplifier is powered on simply because the voltage at that pin will be maintained actively by the RLD amplifier. For the V6 electrode, what is the channel voltage when the the electrode is off but lead-off detection hasn't triggered? We would expect it to be high since the current from the lead-off current sources has no path.

    I'm curious; try removing the jumper shorting pins 3 and 4 of JP33. That way we can be sure that there is no leakage since that jumper is close to the inputs. Does lead-off trigger in that scenario? If not what is the output voltage?

    Regards,
    Brian Pisani