AD1298 and EEG - Noise & Implementation questions

Tony,

Thanks so much for the prompt reply.  I would love to see the recommended part numbers.  What do you think of the INA333, with it's integrated RFI/EMI  filter?  And I take it you still have no multiple IN amps in a single package?  I want to apologize again if some of these questions are basic, but I want to make sure I understand how to best implement the ADS1298 in an EEG application and to avoid making any stupid mistakes.  (Would love to see that EEG app note be released that I've heard so much about)

As to the other question, I was essentially wondering if the typical implementation is to use a single reference electrode as the negative input to all the channels?  And in the case of using a preamplifier, if it is sufficient to configure the ADS1298 to use a non-differential input (assuming a non-differential output of the preamplifier).

I'm not sure what the 'averaged point from the inputs' is on the schematic.  Can you point out a pin or component on the eval board that you are referring to?  

In the case of using preamplifiers, the other question is if a single RLD is generated from each of the preamplifiers (as in  http://openeeg.sourceforge.net/doc/modeeg/modEEGamp-v1.0.png which someone posted) and averaged, or if the RDL_INV output can be used even with preamplifiers?  I've looked at the sbaa188.pdf document, but it doesn't seem to answer this question.   

And, this is perhaps a bit obvious, but I want to make sure: the RLD connects to a discrete electrode on the scalp.  

According to sbaa188, connecting Vref (which I'm assuming is (VREFP-VREFN)/2) through a resistor helps with the CMR as well.  Have people implemented this strategy successfully?  

I suppose the simplest thing to do is hook up the eval board to the EEG electrodes and see if sufficient detail is visible which I'll try shortly.

Thanks again,

Reza

Hi Reza,

1 - INA114, INA115, INA131, and INA333 are low noise devices we have recommended in the past to customers. All of which are single amps per package.

2 - Your second question depends on what you want to measure. If the measurement is a differential measurement, then you would use two electrodes to record the voltage that is read back. If you are interested in a single ended measurement, for example a chest lead for ECG, the WCT is used to route to the negative terminal as a reference point. The reference point (WCT) for the ADS1298 can be routed internally using the ADC's internal mux unless you are in an application where you would need the preamplifier (INAs), requiring you to route pre-amp.

3 - You will want to use the RLD_INV and close the loop around the internal RLD amplifier to isolate the internal averaging from the RLD. Otherwise, you will create a voltage divider between the series resistances from the averaged inputs and the load on the RLD. 

4 - It is hard for me to comment on the performance when hooking it up to a patient, you will have to ask the other innovators on the forum. All of our testing has been done with patient simulators and we are not legally allowed to comment on and help with designing using a human.

Are you with a company working on an EEG solution? What is the end product you are developing?

Regards,

Tony Calabria

Tony,

Once again, thanks for the reply.  I looked over the amps you recommended, and the  best fit would be the INA333 for it's package size and built-in RFI filter, however would really prefer using a multiple-amp per package device.

Has anyone looked at the intersil ISL28470?  It's the only one that I can find with 4 amps in a package.  It's noise is slightly higher (60nV/rt(Hz)) vs the INA333 (50nV/rt(Hz)), but it looks OK otherwise.  Any other people on this forum look at it?

With regards to my second question, I suppose there are two ways to implement the system.  One would be to use a single electrode as a negative terminal reference for all the preamplifiers, or to do something similar to the WCT and average the signal from all the electrodes through a set of resistors and feed that to the negative terminal of the preamplifiers.  I was wondering if one method was preferred to another.   Either way, I would imagine that the input to the AD1298 would be single-ended after the preamplifiers, so the negative terminals of the inputs to the AD1298 would be Vref (mid-rail).

As for the RLD, I'm a bit confused by your reply.  My question is if I can use the signal form RLD_INV to generate the RLD signal using a low-noise amplifier, or if I would need to do so using the preamplifiers.  

I'm working at a startup, and this project is to make a wifi-enabled small (ambulatory) EEG device to monitor people while they are active.  

Thanks again,

Reza

Tony,

The more I've been looking at the design, the less the ADS1298 looks like it would be a good fit for EEG applications - the bulk of the benefits it provides are for ECG functionality.  I've started looking at using a high gain amp + your 16-channel 24-bit ADC.  The INA826 looks quite promising at high gains (~.5uV of RTI noise).  Have you recommended that amp to your customers for EEG applications?

Thanks,

Reza

hello,
Please I need your help for the ADS1299 circuit for an acquisition of EEG please!!