[sdiy] LM358 datasheet VCO on spilt rails

[James Patchell]

Yes, you can run it on split rails, BUT, and it is a big BUT...

Because of the nature of the LM358/LM324, you may find that this causes 
more problems that it may solve.  The common mode input range of the 
LM358 includes ground, and the designer of this VCO took advantage of 
this fact.  Also, you will note, that the control voltage range is from 
0 to 2 * VCC - <some amount>.  If you were to run it from +/- 15 V, 
well, you can see the problem.

I used this VCO in the first synth I built, which was the circuits for 
the Paia 2700.  I did not like the VCO circuit that was in the magazine 
(Paia later made some major improvements on it), and when I built it 
discovered it worked a lot better.  The supply rails in t6he 2700 were 
+18, +9 and -9 volts.  I ran the VCO on 0 to +9, and with the 18 volt 
rail, I was able to provide the VCO with all the voltage it needed to go 
through its tuning range.

     My recommendation is to use it in the single supply mode.  If need 
be, use level shifters to get the signal swings (using op-amps running 
on split supply rails) to get what you need.

     Oh, yes, and this VCO is a true VCO.  That, of course, has it's own 
problems.  This VCO will never be anything more than a linear control VCO.

     -Jim

On 2/6/2012 7:31 AM, Dave Kendall wrote:
> Hi All.
>
> Wondering if any of you have tried the VCO on page 1 of the National 
> semiconductor LM358 datasheet, and if it could run on a spilt rail 
> instead of a single rail.
> http://www.hebertech.com/view/1448-national-semiconductor-lm358.pdf
> As it has V+ /2 going to the + input of the right hand opamp, could 
> that be replaced by GND, and all GND signals connected to -V?
>
> Am most interested in using it as a simple voltage controlled TRI wave 
> LFO.
>
> Any thoughts anyone?
>
> cheers,
> Dave
>
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