[sdiy] Temperature stable lin-exp converter with a CA3086 or CA3046

[René Schmitz]

Thats how I see it too, either option has a trade. 
Currently I'm favouring the divider approach, despite the small error. I
seriously 
doubt that one will find a practical difference. As Ian has said, there are
other 
sources of drift that will probably swamp this small error.

I could notice a small drift on my VCO3s during warmup, those have the
"tempco" 
(A KTY81-120) in the feedback loop. I suspect the drift of Vos of the 
input summing opamp is responsible for the effect. Its drifting very
slightly until 
the circuit has warmed up, some fractions of a Hz. A newer design with the
divider 
scheme is free of that effect. 

Cheers,
 René


At 19:47 27.05.02 -0700, patchell wrote:
>    The way I see the issue of where to put the Tempco resistor is
basically, six
>of one, half dozen of the other.
>
>    If you put the tempco in the divider, the compensation is not exact,
but, the
>offset voltage of the opamp is a smaller portion of the output signal.
>
>    If you put the tempco in the feedback of the opamp (and since it is
difficult
>to get large values because of the fact that the tempcos are usually wire
wound
>using copper wire), the offset voltage makes up a large portion of the
signal,
>creating potential for more errors...although, this problem can be solved
by using
>a premium quality opamp.
>

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159