[En-Nut-Discussion] AD conversion resolution?

[Gerwin Voorsluijs]

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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