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PICList Thread
'high res adc - with sawtooth ?'
1997\03\07@155945 by Mike

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At 03:20 PM 7/03/97 -0500, you wrote:
>At 09:17 AM 3/7/97 -0700, you wrote:
>>OK, I'm really intrigued but I admit to not understanding this.  I am
>>currently in a prototype stage using a PIC17C42 and an external ADC.  I
>>would really like to get more resolution.  Is there any chance of a more
>>detailed explanation?  What I don't understand is how the square wave
>>increases the resolution when the ADC (an 8-bit for example) can only
>>report 256 decision levels.  Thanks!
>
>Oh, yeah, sorry.
>
>Take the sum of the samples.  For example, 16 samples will give you 4 extra
>bits, 32 samples 5 bits, etc.
>
>I usually have to throw away 2-3 bits, resulting in 4-8 times the number of
>samples, but that's with a cheapo 8-bit A/D in these chips, with little if
>any filtering of the analog line.  If you're using a better A/D and/or an
>analog filter, you might to better.

Wouldn't you lose less bits if you used a sawtooth instead of a square wave or
in fact any waveform that is repetitive without a step change or wild
discontinuity in it ?

Rgds

Mike

Some say there is no magic but, all things begin with thought then it becomes
academic, then some poor slob works out a practical way to implement all that
theory, this is called Engineering - for most people another form of magic.
                                                                      Massen

1997\03\07@221738 by Andy Kunz

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>Wouldn't you lose less bits if you used a sawtooth instead of a square
wave or
>in fact any waveform that is repetitive without a step change or wild
>discontinuity in it ?

Maybe.

You actually end up with a kinda sawtooth going into the ADC.  The resistor
and cap work that part of it.  The PIC applies a square wave which is
filtered into a sawtooth by charging the cap through the resistor.

FWIW, I have 5K pots that I'm reading with 1M R1 and .47 C1.

Andy

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Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
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