Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!sun-barr!ames!lll-winken!tekbspa!optilink!elliott
From: elliott@optilink.UUCP (Paul Elliott x225)
Newsgroups: sci.electronics
Subject: Re: buffer amp
Summary: application notes
Keywords: buffer impedance
Message-ID: <2326@optilink.UUCP>
Date: 13 Sep 89 16:27:58 GMT
References: <2596@husc6.harvard.edu>
Organization: Optilink Corporation, Petaluma, CA
Lines: 47

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.