[En-Nut-Discussion] AD conversion resolution?

[Gerwin Voorsluijs]

Been investigating it a bit further based on the same data and I found 
that the two major gaps in my data show a "funny" symmetry:
255 - 304 and 719 - 768 both have a gap of 49 and 255+768 = 304+719 = 1023.
Been investigating the gaps in the data a bit more and to my surprise, 
this symmetry occurs throughout the data: check out the plot of the gaps 
versus the mean value of the two endpoints and the same data mirrored in 
1023/2 which I added to http://130.161.167.171/ethernut.zip

Anyone?


Gerwin Voorsluijs wrote:
> Hi Nic,
> 
> These conspicuous jumps are essentially what I'm getting at: I see jumps
> that don't make sense and come back all the time. I've made a
> log-file of the values that I get. If I plot them I clearly see white
> lines in my data: those are steps that I don't think can be explained by
> the data being noisy or my setup not being 100% grounded.
> Both files can be downloaded from http://130.161.167.171/ethernut.zip 
> (tried to attach them but then this msg didn't seem to get through)
> Plus I see the same jumps as I saw yesterday, with a completely
> different setup, actually the only thing that's the same is the Ethernut
> and some of the wires I used to set it up.
> 
> My setup: simple powersupply set at 4.5 V with a 100K pot meter (not
> used in previous test). That's all!
> 
> Hope someone can shed some light on this.
> 
> Gerwin
> 
> Nic Cave-Lynch wrote:
> 
>> Have to ignorantly throw in my 2c worth here.
>>
>> First, if it's a pot and the internal ref, no calibration should be 
>> needed, because the internal ref feeds the ADC as well as the pot: FSD 
>> == FSD no mater what. (assuming that the internal ref _is_ feeding the 
>> ADC in this case.
>>
>> Second: that big jump around 256: maybe coincidence, but I hate 
>> coincidences that are a power of 2. Could it be something to do with 
>> being interrupted while reading the ADC? having to read the high and 
>> low bytes in the correct order, with nothing else going on in between?
>>
>> I've used the ADC a lot in the Atmels, at times quite happily 
>> decimating to 12 bits resolution, and I've never had this sort of 
>> problem. These weren't using NutOS, but it shouldn't make a difference 
>> if the ADC reading is done properly.
>>
>>
>> Nic C-L
>>
>> Alexander Baranov wrote:
>>
>>> I strongly support. Don't use internal Ref if you would like to avoid
>>> calibration of each device.
>>> ----- Original Message ----- From: "Kathy Quinlan" 
>>> <katinkad at kaqelectronics.dyndns.org>
>>> To: "Ethernut User Chat (English)" <en-nut-discussion at egnite.de>
>>> Sent: Thursday, December 15, 2005 9:57 AM
>>> Subject: Re: [En-Nut-Discussion] AD conversion resolution?
>>>
>>>
>>>
>>>> Gerwin Voorsluijs wrote:
>>>>
>>>>
>>>>> Hi Kathy,
>>>>>
>>>>> You mean it could be noise? That would not explain why I don't see 
>>>>> such
>>>>> jumps on my voltmeter, nor does it explain why I see this behaviour 
>>>>> over
>>>>> a number of sweeps. Btw I use the internal reference voltage on the
>>>>> Ethernut.
>>>>>
>>>>> In the mailing list I saw that you are also using the ADC function of
>>>>> the Ethernut and I was wondering if you could look at the results you
>>>>> are getting to see if maybe I'm unlucky and my Ethernut is broken?
>>>>> I know that on the total range of the 10bit ADC (1024 values) a 
>>>>> jump of
>>>>> 50 is not that big (roughly 5% ), but for what I want to use it 
>>>>> for, it
>>>>> is vital.
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Gerwin
>>>>>
>>>>
>>>> You know that the internal ref is not accurate (or even close) a good
>>>> external Vref is essential)
>>>>
>>>> I have had ALOT of problems with the internal ref, and now only use
>>>> external ones.
>>>>
>>>> I would try to feed in an external Vref first.
>>>>
>>>> Regards,
>>>>
>>>> Kat.
>>>>
>>>>
>>>> -- 
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>>>
>>>
>>> 14/12/2005
>>>
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>>>
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