Path: utzoo!attcan!uunet!pilchuck!amc-gw!richm
From: richm@amc-gw.amc.com (Rich Moran)
Newsgroups: sci.electronics
Subject: Re: Audio Signal Conversion
Message-ID: <1168@amc-gw.amc.com>
Date: 21 Mar 90 19:04:37 GMT
References: <22500.260029fb@kuhub.cc.ukans.edu> <5170078@hplsla.HP.COM>
Reply-To: richm@the-end.amc.com (Rich Moran)
Organization: Applied Microsystems, Redmond, WA
Lines: 48

In article <5170078@hplsla.HP.COM> tomb@hplsla.HP.COM (Tom Bruhns) writes:
}}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...
}

This circuit works slick, except that the audio signal is inverted at
the output.


-- 
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Rich Moran
richm@amc.com
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