ADC accuracy - Page 1
Forum section: electronics / beginners

author: Zorkish
author_rank: Newbie
timestamp: October 17, 2012, 08:07:15 am
content: Can anybody help me out with calculating ADC accuracy. I am looking for a low cost-ish 16bit DAC and found the AD5660 for example, however reading the datasheet has left me somewhat lost.The family specs seemed simpleAD5660: 16 bitsAD5640: 14 bitsAD5620: 12 bitshoweverAD5660: 16 bits ±16 LSB INLAD5640: 14 bits ±4 LSB INLAD5620: 12 bits ±1 LSB INLAre they any different from each other, other than bit count?Thanks

author: grumpydoc
author_rank: Super Contributor
timestamp: October 17, 2012, 08:27:09 am
content: It's a question of accuracy vs resolution.All are accurate to a 12 but count. the 14 and 16 bit ones have better resolution but are not more accurate.

author: Zorkish
author_rank: Newbie
timestamp: October 17, 2012, 08:46:35 am
content: That's what I am failing to understand, why bother with the 16/14 bit version of it.From my limited understanding, the 16 and 14 bit would not hit say 1 volt closer than the 12 bit?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: October 17, 2012, 08:53:18 am
content: QuoteNo, it wont.For example:5V/2^16=76.3µV +16LSB: 76.3µV*16=1.22mV5V/2^12=1.22mV +1LSB: 1.22mV*1=1.22mV

author: andyturk
author_rank: Frequent Contributor
timestamp: October 17, 2012, 09:18:39 am
content: Quote from: Zorkish on October 17, 2012, 08:46:35 amIf you're only interested in absolute measurements, then they're very similar. But if you care more about relative changes (i.e., did the signal increase or decrease), then the 16-bit parts let you detect smaller relative changes. At least, I think that's what "guaranteed monotonic by design" means, which appears several times in the datasheet.

author: Bored@Work
author_rank: Super Contributor
timestamp: October 17, 2012, 09:19:19 am
content: There is still the question of precision, aka repeatability. While the accuracy might be bad, a high precision (I didn't check the datasheet), ensures repeatability of measurements which in turn makes things comparable.E.g. if the ADC has a high precision (I didn't check the datasheet ...) and you get a value of 0x9123 you might not know accurately within +/- 16 LSB what voltage it represents, but if you again get a value of 0x9123 you know it is the same voltage within the precision limits. And if you get 0x9122 or 0x9124 you know it is not the same voltage.

author: Zorkish
author_rank: Newbie
timestamp: October 17, 2012, 09:31:30 am
content: Apologies, I was just wondering why people were talking ADC when I was asking about DAC.....Doh..typoNot that your help is any less relevant in this case.Can I assume that the AD56x0 DAC family is the same silicon with LSB's turned off (marketing?) or does the DAC settlement time increase accuracy/precision?

author: Kremmen
author_rank: Super Contributor
timestamp: October 17, 2012, 10:05:57 am
content: Ok, an analogy. you asre a busy businessman that needs to run in meetings all over the town. Which would be a more usable watch for you, one that pings once every hour or one that has a jerky minute hand that might be 5 minutes off? Bearing in mind that the hourly ping might be off as well? Kinda no brainer, or what?There are lots of applications where the dynamic range is at least as important as accuracy, if not more important. A music ADC or DAC might introduce some distortion if the response is not truly linear, but to reproduce the waveforms properly in the first place you need good resolution. Or say an industrial control loop may demand a certain resolution to enable the dynamics that make control loop stabilization possible. Even if stable, a too rough resolution may cause the control loop to go into limit cycling, possibly causing extra wear and destruction in the actual plant.Conversion between digital and analog worlds is a big subject, witnessed by the tons of books that exist on that topic. Just to mention the key items regarding precision of conversion, without going into what it signifies for each application:- first comes monotonicity. The conversion needs to be monotonic so that increasing source values produce increasing final values, whether it is and A/D conversion or D/A. Resistor ladder based solutions are vulnerable to non-monotonic error where a change in bit pattern may cause a step/transition in the oppsite direction from that intended. In some contexts this can be a grievous error, in others just a nuisance.- resolution may be prioritized over accuracy or vice versa in any specific case. Usually accuracy is good or spot on at each end of the conversion range and the deviations cause what should be a straight line to be a gentle curve either below or above the ideal line (or both, as in an S-curve). Often but not always the errors are largest near the middle of the range. In systems with feedback, reasonable nonlinearity usually results in no visible difference in the end result, since the feedback compensates for the error in accuracy. In non-feedback systems the result of course fully depends on the sum of all individual error sources in the solution.There are other sources of errors but lets not go there. Maybe this is at least some kind of answer why it sometimes makes sense to have more resolution even with the same overall accuracy.

author: SeanB
author_rank: Super Contributor
timestamp: October 17, 2012, 04:06:22 pm
content: They are all ther same IC, but they are sorted during manufacture into 4 bins. The best are 16 bit accurate devices, so go in the b16 bit bin. Nest are those that are not 16 bit, but will do 14 bits ok, so marked as 14bit, the third are not 14 bit but good enough for 12 bits, so go in the 12 bit bin. Those not good enough for 12 bit are recycled for silicon. If you look at the data sheet they all will take a 16 bit input, but you can send less bits with the LSD remainder held to 0. You will find the 12 bit will give a 16 bit output, but might not be linear at the low end.

author: Jeff1946
author_rank: Regular Contributor
timestamp: October 17, 2012, 04:18:10 pm
content: A good analogy would be a double pan scale with a set of weights. The larger the range of weights is like number of bits. The accuracy would correspond to the accuracy of the heaviest weight which might vary by more the the weight of the lightest weight. Say the largest weight was 10kg +/- 5gr and the smallest 1gr. So you could measure something that weighs 10 gr to the nearest gram but something that weighed say 10,500 gr only to the nearest 5 gr.