A/D Converter resolution and accuracy

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If 5Vref and 3V DC input is applied to A/D converter of PIC16F877 microcontroller or any controller then what will be the count??? And if we change the 3V dc input to any other voltage value till 5V then there will be any affect on the resolution and accuracy???

thank for your answer in advance...

Posted 17-Jun-11 7:52am

Prashant Sorathia

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Albert Holguin 17-Jun-11 14:41pm

univoter came around...

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Albert Holguin · Answer 1 · 2011-06-17T08:12:00

Solution 1

What will be the count? Don't understand that question... As far as resolution, well, the A/D has a finite number of bits, let's say 8bits. So the number of discrete states is 2^8 (256 states). Typically these numbers will be based on either 0 in the middle (signed) or 0-256 (unsigned). With that many states, you just divide your reference by that, lets say the maximum voltage swing for the A/D is 5V, then the best resolution is 5/256=0.019Volts (since the states are discrete, you'll always have some level of quantization noise). If you lower the reference, it should give you better resolution but there's other problems associated with that. Since your maximum voltage swing is smaller, you're more susceptible to noise (hence the invention of noise cancelling differential transmission standards).

Posted 17-Jun-11 8:12am

Albert Holguin

Updated 17-Jun-11 8:14am

v2

Comments

Albert Holguin 17-Jun-11 14:40pm

care to explain downvote anyone?

S Houghtelin 17-Jun-11 15:29pm

Compensated, and your answer is correct.

To explain what OP meant by count is; 1 count = 0.019 Volts, it is the non converted AD value. An 8 bit AD converters has 256 counts. You can then just multiply the count number by the conversion factor. 125 counts = 2.5 Volts.

Albert Holguin 17-Jun-11 16:08pm

never heard the term... I'm an electrical engineer

Albert Holguin 17-Jun-11 17:02pm

...and thanks for the upvote.. :)

Richard MacCutchan 18-Jun-11 5:44am

There are idiots everywhere. However, I think the more sensible among us are trying to compensate.

Albert Holguin 18-Jun-11 12:19pm

That's a fact... lol... thanks richard... :)

Sergey Alexandrovich Kryukov 17-Jun-11 18:16pm

...well, that was a high-rank uni-voter. I disagree and voted 5 -- this is a good answer. I would add there is a physical accuracy limitation as well. There is also one more essential aspect: some devices use unsigned format but use one bit as a sign bit, so the discreet accuracy is twice as bad. OP should address the manufacturer documentation.
--SA

Albert Holguin 17-Jun-11 21:34pm

true... there's other factors involved... there's also low resolution ADs that are commonly used for some applications (down to 1bit), they implement differential encoding on the sampled data.