[En-Nut-Discussion] AD conversion resolution?

[Gerwin Voorsluijs]

Hi Lars,

Being mostly a software guy, I am struggling to follow your way of 
thinking, but I do understand your end suggestion of what might be 
causing the problem. I have two questions however:

1> How come it seems that I am the only one having this problem 
considering the fact that the problem (noise) originates on the Ethernut 
board itself?

2> How do I solve it?

Sincerely,

Gerwin

Lars Andersson wrote:
> Hi group,
> being a mostly hardware guy I feel I might contribute 2 cents of opinion
> here.
> 
> IMHO, what you see here is the result of noise uppsetting the comparator
> of the internal SAR A/D converter in the atmega chip. As the SAR works
> its way towards the LSB the voltage differences on the comparator
> gets smaller and smaller and chances for a mistaken reading increases.
> 
> Considering how SAR works one comparator mistake at a low value bit,
> say 3rd LSB, makes all bits of even lower value will be wrong and
> also of the same value, either 1 or 0.
> 
> There are three ways (or more?)  to introduce noise in an SAR AD, signal,
> reference and comparator power supply.
> 
> I am suggesting that the noise is possibly not on your signal or your
> reference
> but on your power supply to the analog part of the AVR chip (AVCC). 
> 
> I have got several good ideas from this group in the past, so hope this 
> might help someone,
> 
> Lars H. Andersson
> 
> 
> 
> 
> 
> 
> 
>>-----Original Message-----
>>From: en-nut-discussion-bounces at egnite.de 
>>[mailto:en-nut-discussion-bounces at egnite.de] On Behalf Of 
>>Arius - Rick Collins
>>Sent: Saturday, 17 December, 2005 16:02
>>To: Ethernut User Chat (English)
>>Subject: Re: [En-Nut-Discussion] AD conversion resolution?
>>
>>
>>At 06:11 AM 12/16/2005, you wrote:
>>
>>>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?
>>
>>
>>I have looked at your data and it is a bit mystifying.  In 
>>the one graph, I 
>>believe I am looking at a time graph of data values as you sweep the 
>>pot.  The first 40,000 readings look like they are with the pot 
>>undisturbed.  Since you seem to get so much of a jump which 
>>would be due to 
>>noise, you could investigate that further without the 
>>complication of the 
>>wiper.  I see three (maybe four) discrete levels in this range.
>>
>>Looking back at one of your original posts, it appears to me 
>>that your data 
>>bus may be scrambled.  If the data bus is serial, then I 
>>suggest that you 
>>look for a problem with timing in the data vs. clk.  If you 
>>are clocking on 
>>the wrong edge, you may see some problems like this.  I was almost 
>>convinced that it was a mix up of the bits on the bus until I 
>>looked at the 
>>raw data and realized that the four lowest readings were 0, 
>>1, 3, 7 and 
>>15.  Yes, I know that was five values, I can't count.
>>
>>I can't explain that sequence by a data bus scramble, but I 
>>can explain it 
>>by a problem with timing on a serial bus or possibly a 
>>problem with the 
>>voltage levels, pull up value or some other issue that would 
>>result in a 1 
>>bit smearing into other bit time windows.
>>
>>
>>
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>>
> 
> 
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