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Analog front end for an EEG

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jlenoach

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I'm trying to build an EEG using an ADS1298. This chip was built more for ECG than EEG and is a little too noisy to directly take an EEG signal with, so I'm trying to add some initial gain to get a better noise figure. My question is what is the best way to do this? This is a 24-bit A/D converter and I absolutely want the least noise possible.

The ADS1298 has differential inputs into the ADC. Currently, I just use LT1167 instrumentation amps. One input into the amp is the measurement electrode, the second is the reference electrode. The gain is set by an external resistor. I then feed the output of the amplifiers into the positive pin of the differential input on the ADS1298. For the negative input, I feed in a voltage that's half the input range of the ADC, coming from an op amp. This voltage is also tied to the reference pin of the instrumentation amps.

What I'm worried about is the fact that this makes the signal single ended, correct? The power rails will likely be noisy from the digital switching of the ADS1298, and this will affect the output of my divider opamp that feeds into the negative differential input. But will the voltage between the reference between pin of the LT1167 and its output remain constant for a given input despite any noise on the refence? If this is the case, there would not be any noise since the input is differential, correct?

Any advice would be greatly appreciate, let me know if you want to see the schematic.
 

bump............................................
 

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