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

David G. Dixon dixon at interchange.ubc.ca
Wed Nov 10 00:37:32 CET 2010


Sorry, Richie, I responded before seeing your post. 


> Regarding this CV driver output discussion, there is a solution that
> provides the best of both worlds:
> 
> http://www.richieburnett.co.uk/temp/cv_out.GIF
> 
> Resistor R1 limits the maximum output current under short circuit
> conditions
> and decouples the output of the op-amp from the capacitance of the cable
> and
> the load at the end of the cable.  (It's value isn't critical, but it
> needs
> to be significantly greater than the open-loop output impedance of the
> op-amp at the cross-over frequency so that it effectively decouples the
> output node from the capacitive lag at the load.)
> 
> Resistor R2 takes negative feedback from the actual output at DC.  This
> means that any voltage drop across R1 due to load current is inside the
> feedback loop and is therefore compensated out.  (Under load the op-amp
> servos it's output to a higher voltage to make up for the volts dropped
> across R1, and ensure the output voltage is still what you wanted.)
> 
> Capacitor C2 takes negative feedback directly from the output of the op-
> amp
> at high frequencies.  This stops the op-amp ringing or breaking into
> oscillation due to load capacitance causing the feedback signal to
> diminish
> and start to lag at high frequencies.
> 
> Essentially the source of feedback gradually shifts from the output cable
> to
> the op-amp output as you go up in frequency.  At DC the feedback comes
> from
> the actual output to compensate for the DC voltage drop across R1.  At HF
> the feedback comes from the op-amp output directly to ensure "in-phase"
> negative feedback and guarantee stability as the op-amp's gain rolls off.
> 
> Another way you can look at it is by thinking of R2/C2 as a phase-lead
> network.  C2 imparts a phase-lead into the feedback signal around the
> break
> frequency where the output capacitance combined with R1 would otherwise
> cause a phase lag to kick in.
> 
> I hope this helps,
> 
> -Richie,
> 
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