[sdiy] parts watch

[Ian Fritz]

Hi Tim --

Well, that's a bit of an inconvenience.  The attenuation you need is from 
100k input resistor
to just under 1.8k feedback.  That's why we usually look for a 2k feedback 
resistor with a divider on the output.

You could go the the voltage divider design and just live with the small 
amount of tempco shift that gives you, or you could go to slightly smaller 
input resistors (91k ?).

Or you could use the design I recently proposed which can easily be 
modified for just about any resistance and which allows you to dial in the 
exact tempco.  But you would have to "waste" (gasp) two or three opamps to 
do that.

   Ian


At 12:50 PM 4/28/2003, Tim Ressel wrote:
>Yo,
>
>Well, semi-sharp eyes. Closer examination shows the
>0805 +3300 resistors top out at 1.8K. This would
>require a slight change in an expo convertor that
>calls for 2K. The price is 6.31 for 10.
>
>--Tim
>
>
>--- Ian Fritz <ijfritz at earthlink.net> wrote:
> > Hi Tim --
> >
> > Holy cow, how did we all miss that!
> >
> > Yes, +3350 is what you need to exactly compensate a
> > converter.  With the
> > 10% TC tolerance you will get at most about 250
> > ppm/K scale drift.  This is
> > good enough for most people, although a number of
> > crazy folks like me are
> > trying to get below 100 ppm/K.
> >
> > I would recommend the higher wattage (0805 profile)
> > to minimize any
> > possible self-heating effects.
> >
> > You could also get the 3900 ppm/K devices and trim
> > the tempco to the exact
> > value with a series non-tempco resistor.
> >
> > The only problem people might have is that the
> > minimum order is listed as
> > 10 pieces.
> >
> > Many thanks, Mr. Sharp-Eyes!
> >
> >    Ian
> >
> >
> > At 12:05 PM 4/28/2003, Tim Ressel wrote:
> > >Yo,
> > >
> > >I just noticed in the new Digikey catalog some
> > >Panasonic temco resistors. Sadly they are only
> > >positive temco, but they have +3300PPM resistors.
> > Are
> > >these useable somehow???
> > >
> > >Catalog T032 Page 758
> > >
> > >--Tim
> > >
> > >__________________________________
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> >
>
>
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