Hi Ti,
I'm currently researching wether the ADS1298 is suitable for our EEG application. An important requirement is that we have constant information about the connection of the electrodes, without having to stop our measurements. If I understand correctly, the ADS1298 can do this, though only with AC lead off detection, not DC. Am I right?
I believe using this method, the ads1298 injects a high frequency signal into the positive or negative electrode of a channel. Then by converting the incoming signal, the output of this signal can be analyzed and the state of the electrodes can be determined. This must be done however externally. Therefor, I have several questions.
1. In our EEG measurements, 18 signals are measured in comparison to 1 central electrode. Thus, to be efficient, I thought I would just connect this last electrode externally to each of the inamps of the other channels. Another possibility would be to use this 1 electrode as the input for my RLD signal and then reroute this RLD signal internally to all other channels. I'm now wondering how this could be combined with the lead off detection.
In the last case, I have got no idea how lead off detection could be implemented. Can I do this by measuring the RLD signal and injecting a high frequency signal on each of the other inputs?
In the first case, will I be able to get the information I need, just by putting a high frequency signal on each of the other inputs and measuring the first input?
Another thing to take into consideration, is that I will need 3 ADS1298's to have enough inputs for EEG. This might give problems with the RLD solution.
2. Maybe this issue could be resolved by using DC lead off detection, if it doesnt take to much time to process it. I need 250 SPS, can I do lead off detection with DC between two of these samples, without possible capacitive or inductive distortion of the data? But then again, I don't really see how i should connect my inputs, since DC mode uses the differential electrodes of each channel, and I plan not to use all of these electrodes.
3. If part 2 is possible, do I lose the ability to use RDATAC mode, to continously run my conversion? Actually I'm not really sure what the difference is between using continuous mode and just iteratively using pulsed mode. It seems to me in both cases you still need to send a START signal, only in continuous mode, it's not possible to send opcodes.
I realise I'm probably asking alot here, but I hope you can help me out a bit. Thanks in advance!
Pieter Willemen
Antwerp, Belgium