kiely@lownlab.harvard.edu (James P. Kiely) (09/12/89)
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.
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NAME: James P. Kiely USPS: Kiely Laboratories
USENET: ...!harvard!lownlab!kiely P.O. Box 624
DOMAIN: kiely@lownlab.harvard.edu Allston, MA 02134-0624
PHONE: +1 617 782 4136 USAelliott@optilink.UUCP (Paul Elliott x225) (09/13/89)
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 >...deleted... Just some comments: 1 GigaOhm at 10 KHz requires an input capacitance of approx 16 femtoFarads (did I get that right? 15.9 * 10e-15 Farads). I presume you are referring to a DC input R spec. If your source impedance is anywhere near this high Z, the input current to the amp is very critical parameter as well. I question the equiv input noise requirement at this impedance since even a 1 Meg resistor generates over 12 microvolts RMS at room temp and 10 KHz bandwidth. Yes, I realize that nobody is trying to match input and output impedances here, it's just that the 10 GOhm input Z spec seems a bit excessive for the probable application, or that if the source output Z really is so high as to require this, and the bandwidth really needs to be 10 KHz, then 0.05 microvolts input noise doesn't seem practical. Just the same, I am certainly no expert in this field, and would like to learn more about low-noise instrumentation design methodology and circuits. Are there any volunteers out there who would like to enlighten us? Or to poke holes in my analysis? By the way, the Crystal Semiconductor databook has a good app note on ADC (Analog to Digital Converter) input buffers. I am looking for an excuse to request some samples of their new A to D converters to play with (oops, I mean "evaluate"); they look pretty nice. You can contact Crystal Semi at (800) 888 5016, ask for the "Telecommunications IC's -- Analog / Digital Converter IC's" databook. I wish you success with your design. -- Paul M. Elliott Optilink Corporation (707) 795-9444 {pyramid,pixar,tekbspa}!optilink!elliott "I used to think I was indecisive, but now I'm not so sure." Disclaimer: Like Horton, I meant what I said and I said what I meant.
strong@tc.fluke.COM (Norm Strong) (09/15/89)
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)
abali@parts.eng.ohio-state.edu (Bulent Abali) (09/16/89)
In article <11131@fluke.COM> strong@tc.fluke.COM (Norm Strong) writes: >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 ) > ........... >}Application - EEG (electroencephalogram - brain waves) amplifier. > >Your required bandwidth and noise level are not possible at the present >state-of-the-art,.... >Norm (strong@tc.fluke.com) There is an interesting article titled "Biomagnetism ..." on detecting brain waves in the latest issue of Science vol.245 pp.1041-1043. Superconducting Interference Devices (SQUID) were used for detecting weak magnetic fields such as brain waves. -=- Bulent Abali : abali@baloo.eng.ohio-state.edu Ohio State Univ., Dept. of Electrical Eng. : Phone: (614)292-2452 2015 Neil Ave. Columbus, Ohio 43210 :