I’m currently evaluating the ADS1298 for an update and extension of a multi-modality data acquisition system. Modalities include eeg, emg, heart rate, GSR and temperature, but the main focus is EEG.
I want to know about sampling rates as well as several issues related to using the ADS1298 with an in-amp pre-amp.
Regarding sampling rates, is there a way I can get continuous mode power of two sampling rates (255 SPS, 512 SPS, 1024 SPS etc) perhaps by changing Fclk?
If not, what are the disadvantages to using single-shot mode? Is there a better explanation of the increased issues of aliasing effects alluded to on page 36?
Regarding using an in-amp pre-amp, from reading the discussion board I understand that the noise floor and amplification of the ADS1298 is insufficient for eeg taken on the scalp without the addition of a low-noise in-amp pre-amp.
I would like to know how to design a system that includes an RLD, Lead-off detection and EMI filtering while using a pre-amp. It appears there are some trade offs between relying on circuits that are built-in to the ADS1298 for RLD, Loff, and RFI filter and using off-chip circuits that may be better incorporated into the pre-amps.
Specifically:
RLD
In a typical in-amp based design the input from the RLD circuit is taken from the middle of the gain setting resistor in the in-amp. Regarding accuracy .1 % resistors are cheap and available and a third resistor in parallel helps to equalize the currents.
The input for RLD circuitry on the ADS1298 is taken from the gain setting resistors in the PGA. A big advantage of using the built in RLD circuitry is the ability to choose which channels are used to derive the RLD signal, i.e. the RLD source signal may be limited to appropriate channels in use. Whereas, an RLD derived from the sum of several in-amps might include an unused channel without adding addition of circuitry and control lines to turn an unused input off.
Question: In an EEG system which source will provide a higher quality RLD signal input one derived from the on-chip RLD or one derived from the middle of the gain setting resistor of the in-amp?
Lead-Off Detection
Can I use the on-chip lead-off circuitry to generate an electrode impedance measurement?
Please advise on the options for using the lead-off detection when using pre-amp.
Would I use the AC or DC measurement?
With AC measurement is the signal typically filtered in hardware or software?
Is it rectified?
Do you have a source code snippet for this procedure in software or a circuit diagram for doing it in hardware?
EMI Filter.
In an eeg/ekg design EMI/RFI circuitry is typically placed before the in-Amp to avoid offset and saturation caused by rectification of high frequency signals beyond the bandwidth of the in-amp. This issue will likely worsen in the future as wireless devices increase.
In an ADS1298 based EEG with an in-amp, the built-in EMI section of the ADS1298 would follow the in-amp. Seems that the in-amp should have EMI circuitry preceding it.
Will the EMI circuitry in ADS1298 cause any interactions with the pre-amp?
Would it be better to use an ADS1278?
