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Wireless biopotential measurement with CC254x: external ADS129x, or on-chip ADC + amplifier?

Graham Kelly
Prodigy 80 points
Other Parts Discussed in Thread: CC2541, INA333, ADS1291, ADS1299, ADS1294

Hi,

Feel free to move this to a different board if appropriate; it's kind of a cross-disciplinary question but I thought this board would be most likely to have someone knowledgeable about every part of it.

The topic says most of it: my group is working on a Bluetooth Low Energy biopotential acquisition system that must have an absolute minimum of components due to the experimental fabrication process we're using. We're currently only capable of using this process to fabricate single-sided circuits, so that places a premium on board space for interconnects--no vias allowed. We also want the end product to be as small as possible. We would thus like to use the CC2541's on-chip ADC, if possible, instead of an external biopotential AFE.

We'll be measuring primarily EMG and possibly EEG, if we can work out a way to get good enough signal quality. So, then: what is a good amplifier setup for the CC2541's ADC, given these constraints? It's been a long time since I had to design an EEG amplifier with discrete op-amps/in-amps, but I recall there being multiple preamplification and active filtering stages, even without a driven right leg system. Let's assume I'm misremembering that these multiple stages are crucial: could I use an instrumentation amplifier like the INA333 to directly amplify the signal, feed into a one-pole RC antialiasing filter, and use that to drive the CC254x's ADC?

Or would it, after all, actually take up less board space just to use an external AFE like the ADS1291?

Thanks,

Graham Kelly

  • TI__Guru 66466 points

    Hello Graham,

     

    I realize we still owe you a reply here.

     

    My initial thought would be that, for EEG specifically, it is essential to have the lowest noise and highest resolution system. The ADS1291 would be far too noisy to use for EEG. The ADS1294/6/8 are not as low-noise as the ADS1299, but may still work with a very low-noise gain stage in front.

     

    I took a quick look at the ADC inside the CC2541 as I am not familiar with it. I can tell you that the performance you get out of this ADC will not be in the same ballpark as what you'll get with a dedicated EEG front-end like the ADS1299. The AC specs speak for themselves and I do not believe this was ever the intended application.

     

    The ADS1299 also includes an embedded PGA with gains up to 24 V/V and an over-sampling ratio in the digital filter of up to 4096, reducing your input filtering requirements. I understand that space is a premium in your project, but the decision to go with a discrete solution will depend on the resolution of the biopotential signals you need to measure and the amount of noise you can tolerate. I am not confident that the CC2541's ADC will be sufficient for this application.

     

    Please feel free to get another opinion and reach out to the forums for the devices you mentioned above. I hope this helped!

     

    Best Regards,