[time-nuts] digital pots Part 2

[Bob kb8tq]

Hi

The advantage a wire wound pot has is that all the wire is (nearly) the same temperature coefficient. 
This ~$15 Bourns part:

https://www.digikey.com/product-detail/en/bourns-inc/3590S-2-103L/3590S-2-103L-ND/1088586 <https://www.digikey.com/product-detail/en/bourns-inc/3590S-2-103L/3590S-2-103L-ND/1088586>

(Buy 10 at a time and they go down to $13.25 each. )

Has a rated tempco of 50 ppm / C. That’s without taking any matching into account. With the matching
you probably are sub 1 ppm/C. 

The DS1804 has a “typical” temperature coefficient of 750 ppm / C. (from page 5 in the data sheet). 
Unfortunately there is no reason to believe that all the “bits” have the same tempco. Indeed there are
good reasons to believe they vary more than a bit. You would be doing well with a heater that drops that 
to an effective 75 ppm / C.

A 10 turn pot is ground isolated. There are no side effects from power and ground connections. Never
underrate the value of isolation. 

Bob





> On Jan 29, 2020, at 8:34 PM, Perry Sandeen via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Learned Gentlemen,
> I appreciate all the reply's to my questions.
> Perhaps I should have added a bit more detail of my planned project after moving.  I have 5 Lucent XO's and two Lucent Rb's which I want to add panel mounted fine EFC controls.
> I did consider Bourn's Helipots, but found used on ebay were $20 to $30 each and none had more than two for sale.  I did look at the ebay Bourn,s *tribute* pots with dials from China which seemed to have the exact same item from $10 to $2 (with free shipping). I remembered that Charles Steinmetz had posted to the list that he found many of a lot he purchased were defective.
> So I went alookin (southern USA term for searching) for a viable alternative, hence the Maxim DS 1804 non-volatile 10K pot with 100 taps that has a simple toggle up or down interface. (I have no ability to do serial or computer control.  I found a Microchip AN 691 which showed how to use two or three digital pot in series/parallel configurations.
> Three DS 1804's and a good reference IC for less than $10. A simple TL431 or LM723 heater circuit would be $2.  So with 100 taps on each IC, I figured (with lots and lots of toggling). I could achieve sub-millivolt or better tuning steps.
> And I was fairly certain that fine EFC tuning was like grandma getting ready for church on Sunday.  She was ready when she was ready. No amount of fussing or cajoling could change that. It occurred on her time (slow) table.
> Comments appreciated. 
> 
> Regards,
> Perrier
> 
> 
> 
> 
> 
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