Tool/software:
Hello,
I am designing a 32 bipolar/19 single-ended EEG data acquisition system using 4 ADS1299's. The situation I am encountering is that I have certain electrode signals from the patient that need to be connected to different inputs of the same ADS1299 and also to other ADS1299's. To describe the problem, I will refer to each of the four ADS1299's as ADS1, ADS2, ADS3, and ADS4. For Example, I will have eeg_f7 electrode signal that needs to routed to ADS1_IN1N, ADS1_IN2P, ADS2_IN5P. If I make these connections, I am essentially paralleling the input impedances together from each of these 3 inputs to equal a third of the ADS1299 input impedance (1G/3 = 333M). With these connections, the single-ended input impedance of eeg_f7 will be ~ 333M OHMs, which is undesirable for EEG applications.
So the easy and straightforward approach of course is to use more ADS1299's, but I was wondering if there was any other clever way to isolate the inputs from each other for a situation like this one.
I thought of using high-impedance input buffers which does effectively isolate all channels, adds a bit of noise, but is manageable. The issue is I lose the DC and AC-leads off sensing functionality since the ADS1299 inputs are no longer connected to the patient. At which point, I thought well maybe I can create my own low-noise, precision current sources and pull-up resistors at each of the inputs but this sounds easier than it actually is to achieve for basically 64 single-ended channels. Using the highest impedance buffers with lowest noise out there, practically speaking, the highest single-ended input impedance I think I could achieve is 250M OHMs using a 600mV source and 100M OHM resistor. I would then start to worry about the noise contribution of the 600mV source considering I have a 5uVp-p RTI noise requirement. So I kind of view this as "not worth it"
Also, related question, in the case that I limit my eeg_f7 signal to only 2 connections of the same ADS1299 (ADS1_IN1 and ADS1_IN2), besides a reduction in input impedance by 2, could I expect DC or AC-leads off sensing to work properly? What other downsides in performance could I expect by making these connections?
Thank you in advance,
-E