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Current Reference IC for V & C Reference Standard Box

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t1d:

--- Quote from: David Hess on April 06, 2020, 10:27:14 am ---Digital pots tend to have poor absolute accuracy
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Do you mean beyond the know issue of not being able to select values that fall within the bracketed amounts? Example 10K Ohms/1024 steps... (I am a v-e-r-y novice coder.)

--- Quote from: David Hess on April 06, 2020, 10:27:14 am --- and are intended to be used as dividers.
--- End quote ---
I am not doubting you, here, but please point me to some materials that would teach about this effect.

--- Quote from: David Hess on April 06, 2020, 10:27:14 am ---A switched capacitor
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I know of variable capacitors, but what is a "switched" capacitor?

--- Quote from: David Hess on April 06, 2020, 10:27:14 am --- circuit can generate a precision variable current from a voltage within the limits of the accuracy of the capacitor but fixed resistors will be more accurate and easier to use.

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I know about using multiple resistors of the same value to average/lessen the tolerance error. I am also familiar with using a set value resistor and a trimmer. If I understand correctly, I can get an order of magnitude better accuracy with a trimmer.

It looks like the resistor/K.I.S.S. approach is best, for this project. Thanks for your input.

David Hess:

--- Quote from: t1d on April 11, 2020, 11:48:07 am ---
--- Quote from: David Hess on April 06, 2020, 10:27:14 am ---Digital pots tend to have poor absolute accuracy
--- End quote ---
Do you mean beyond the know issue of not being able to select values that fall within the bracketed amounts? Example 10K Ohms/1024 steps... (I am a v-e-r-y novice coder.)
--- End quote ---

For example the Analog Devices AD5293 is 35 ppm/C when used as a rheostat and and 5 ppm/C when used as a divider.  The absolute tolerance in rheostat mode is 7%.


--- Quote ---
--- Quote from: David Hess on April 06, 2020, 10:27:14 am --- and are intended to be used as dividers.
--- End quote ---

I am not doubting you, here, but please point me to some materials that would teach about this effect.
--- End quote ---

Just check any of the datasheets.  Accuracy is better when used as a divider than as a rheostat simply because a divider depends on identical resistors tracking while a rheostat depends on their absolute value plus the "wiper resistance".


--- Quote ---
--- Quote from: David Hess on April 06, 2020, 10:27:14 am ---A switched capacitor
--- End quote ---

I know of variable capacitors, but what is a "switched" capacitor?
--- End quote ---

If a capacitor is switched between two points, then it transfers charge between them proportional to the difference in voltage, capacitance, and frequency.  So it can act as resistance controlled by the frequency of switching.

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