[volt-nuts] Matched resistors

[Andreas Jahn]

Hello Randy,

until now you have not written about your design accuracy needs and 
about your environment conditions.
(humidity + temperature controlled lab or industrial environment or 
simply your uncontrolled "lab" at home).

In your measurements you should also regard humidity as a significant factor
especially with resistors in plastic package like the LTC5400.
Also the epoxy pcb is creating mechanical stress due to humidity to the 
resistors
which can change the resistor value if not proper mechanical decoupled.
(Difficult to mandage with SMD parts).

I know that humidity is not easy to measure. And time constants are more in
the range of 4-7 days which makes it even more time consuming.

My personal opinion is that a LTC1043 based desing will perform better than
a desing with SMD metal film resistors like the LTC5400. And a shoot out 
between
vhd200 and LTC1043 would also be interesting depending on temperature range.

By the way. The term TC tracking which is used by the resistor 
manufacturers may
be not the same that you expect in a lab condition.
Usually T.C. of a resistor is measured at "3 cardinal points" -55 deg / 
25 deg / 125 deg.
T.C. is then calculated out of these 3 values.
You probably want to have the dR / dT tracking around 25 degrees which 
is not specified with this method.
So if you specifiy T.C. tracking you should do this with your 
temperature range.

I hope that we will get feedback on your results some day.

With best regards

Andreas



Am 26.07.2014 17:56, schrieb Randy Evans:
> I would like to thank all those you supplied ideas for matching resistors.
>   I have decided to test three approaches for now, the first is using Vishay
> vhd200 hermetically sealed foil resistors (three each at around $26 each),
> the second is using LTC5400 resistor arrays, the third is a hybrid approach
> using a Vishay vhd200 for the most critical resistor pair and LTC5400
> resistor arrays for the other two, and the fourth will likely be a LTC1043
> switch capacitor doubler  plus LTC5400s, although the latter is a lower
> priority.
>
> I plan on characterizing them over time and temperature to see the effects
> on output stability for the best approach considering cost, complexity, and
> accuracy.  Should be interesting if it works.  It's been taking longer than
> I wanted or hoped but there are only so many hours in the day and my day
> job is interfering with my play time.
>
> Thanks for the help,
>
> Randy
>
>
> On Fri, Jul 25, 2014 at 1:04 PM, Ivan.Cousins <ijcousins at frontier.com>
> wrote:
>
>> Randy,
>>
>> You might want to look at:
>>
>> Digikey PN 749-1052-1-ND (qty 1, $0.89)
>> or
>> Digikey PN Y4485-5K/5KBCT-ND (qty 1, $22.93)
>>
>> Both parts are from Vishay.
>>
>>
>> My advice is to build something and then measure that something.
>>
>> You are the best judge of your immediate design problem and measurement
>> capabilities.
>>
>> I have found, after many years, that waiting for an "Expert" to hand you
>> your answer does not work.
>>
>> No other "Expert" is as familiar with your present problem as you are.
>>
>> After a number of build-measure cycles, you may become the new "Expert".
>>
>> If one waits for the ultimate answer then you may wait forever.
>>
>> The art of engineering is to get close enough.
>>
>> One can remain in the "thinking" stage forever, it can become an endless
>> loop.
>>
>> Once you start the project by building and then measuring, you will be on
>> your way to an answer, your answer.
>>
>>
>>
>> I am reminded of a saying "everything is a transducer" used here before.
>> The interactions in this case are the usual temperature, stress-strain,
>> humidity, resistor metal migration, etc.
>> Notice that I did not include time, (long term drift), on purpose.
>> It could be explained by one of the above or other, measurable quantity
>> over a measurement interval.
>>
>> This is meant in the best spirit possible.
>> Best of luck in your project.
>>
>> John C.
>>
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