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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: Audio Signal Conversion
Message-ID: <5170078@hplsla.HP.COM>
Date: 20 Mar 90 21:30:31 GMT
References: <22500.260029fb@kuhub.cc.ukans.edu>
Organization: HP Lake Stevens, WA
Lines: 42

>Could anyone tell me what the most efficient way to convert +1 to -1
>audio signals to 0 to +5 volts?  The analog to digital IC I'm using
>will only work on zero to five, and everything I've tried so far has
>had pretty crummy results. (Lots of noise and the likes.)


Lots of noise?  Keep leads short and power supplies clean.  As
far as about the simplest circuit for doing this, try one op
amp and four resistors.  I don't like to draw with ascii characters,
so:

R1:  from input sig to inverting opamp input.
R2:  from opamp output to inverting opamp input.
     (Take output from opamp output, of course.)
R3:  from some positive reference supply (say +5 volts)
     to opamp non-inverting input.
R4:  from opamp non-inverting input to ground/common.

R1 and R2 set a signal gain of 2.5, to get from 2 volt input
     range to a 5 volt output range.  Pick R1, say, 10K ohms.  Pick
     R2 = 2.5* R1; 24Kohms is probably fine, maybe even 22K ohms.

R3 and R4 set the bias on the non-inverting input to the opamp.
     Pick them so that +1 volt in and 0 volts out, voltage-divided
     across R1-R2, gives the same voltage as R3-R4 divide down to
     the non-inverting input.  Probably best to bypass the non-
     inverting input to ground with a capacitor; let R4 be 10K ohms,
     and try a bypass of somewhere around 27 to 100 microfarads
     (non-critical ;-).  

     e.g.,   5V *  R4/(R3+R4)  =  1V * R2/(R1+R2)
     Pick R4 as a convenient value, say 10K ohms, since the above
     will give only a ratio of R3:R4.  If R2 == 2.5*R1, then
     this works out to R4/(R3+R4) = .142 if I did my math
     right, and R4=10K ---> R3=60K -- 62K is a close 5% value...

Op amp should be capable of operation at about 0.5 volts input, and
its output should be able to swing the full 0-5 volts.  This can be
satisfied with some single-supply amps, and most any amp running on
+/- 15 volt supplies.  Op amp noise shouldn't be a problem at these
signal levels unless you have a really poor opamp...or very high
accuracy digitization following.