[sdiy] Which opamp pins wiring is best for CV out ?? A , B or C ???

[Neil Johnson]

Hi,

> OK, so what y'all seem to be saying is that the small output  
> resistor is
> critical for neutralizing capacitive loads.  Fair enough.  I'm glad we
> finally reached a consensus on this issue, which we have been  
> arguing about,
> off and on, for the last year!

I wouldn't call it arguing, more "educating" :)

> On that basis, I did some more simulations, assuming a 560pF  
> capacitive
> (Neil's 4m cable at 140pF/m) load, and found that the ideal  
> resistor value
> is 220R.  Assuming a 100k input impedance downstream, this would  
> cause a CV
> error of 0.22%, or about the same as typical tuning errors in VCOs.

But only if driving a single input.  Now with a 4-way passive multi,  
or stacked bananas, and your error is now nearer 0.88%.  Oh dear.

> Also, it bears noting that this small resistor would do very little to
> protect the opamp from current surges, a rail-to-ground short- 
> circuit would
> generate about 60mA through this resistor, which is 2 or 3 times  
> higher than
> most opamps allow.

Well, indeed.  But that's not the purpose for the damping resistor,  
so its not surprising that it in itself does not protect the op-amp  
to that degree.  You can either add more circuitry to protect the  
output from over current, or use an op-amp that is tolerant of  
shorted outputs (e.g. thermal protection) and keep the sensitive op- 
amps isolated from the outputs.

Cheers,
Neil
--
http://www.njohnson.co.uk