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From: strong@tc.fluke.COM (Norm Strong)
Newsgroups: sci.electronics
Subject: Re: buffer amp
Keywords: buffer impedance
Message-ID: <11131@fluke.COM>
Date: 15 Sep 89 15:28:25 GMT
References: <2596@husc6.harvard.edu>
Sender: news@tc.fluke.COM
Organization: John Fluke Mfg. Co., Inc., Everett, WA
Lines: 26

In article <2596@husc6.harvard.edu> kiely@lownlab.harvard.edu (James P. Kiely) writes:
}I am looking for an IC or circuit with the following characteristics:
}        - high impedance single ended input ( >= 1 Gigohm )
}                (differential with one input grounded should be ok)
}        - low impedance balanced output ( <= 50 ohms) (to drive twisted pair)
}        - SNR of at least 98 db
}	- equivalent input noise <= 0.05 microvolts
}        - gain of 1
}        - high frequency cutoff 10 KHz or higher
}        - any other spec's necessary to make the circuit usable as a front
}                end to a true 16 bit data acquisition system
}I would like this all in one inexpensive IC if possible.
}If not a simple inexpensive circuit will do.
}Application - EEG (electroencephalogram - brain waves) amplifier.

Your required bandwidth and noise level are not possible at the present
state-of-the-art, when combined with the extraordinarily high input
impedance.  You are asking for a noise level of 500 picovolts per root Hz.
To get this kind of noise level is next to impossible at zero source
impedance, much less at the impedances common in biological circuits.  

Even ten times that noise level would be difficult--and certainly not
cheap.  Rethink your noise requirements.
-- 

Norm   (strong@tc.fluke.com)