24-bit ADC, what's the best these days for linearity (and low offset) - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: October 23, 2015, 01:50:34 pm
content: Looking for a good 24-bit ADC with the best linearity characteristics available today. Low offset/drift is preferable also, of course...

author: Marco
author_rank: Super Contributor
timestamp: October 23, 2015, 02:38:53 pm
content: 20 bit ADCs are the best for linearity, or rather Linear's LTC237X-20 20 bit ADCs are the best for linearity.

author: Someone
author_rank: Super Contributor
timestamp: October 24, 2015, 12:11:34 am
content: Quote from: DiligentMinds.com on October 23, 2015, 03:46:27 pmAs you say, the DNL can be excellent which lends to a good SNR measurement and reproduction of a wide dynamic range. Paired with floating point processing the INL is down in the noise. Its not audiofool stuff.Quote from: DiligentMinds.com on October 23, 2015, 03:46:27 pmIt all depends on the sample rate needed, 23-24 bits are relatively easy at slow rates such as 1S/s. The OP really needs more detail to be able to provide any useful information.

author: promacjoe
author_rank: Regular Contributor
timestamp: October 24, 2015, 12:39:44 am
content: Quote from: SharpEars on October 23, 2015, 01:50:34 pmunless I missed something, he is looking for a analog-to-digital converter "ADC". Not a digital to analog converter "DAC".sorry I don't have an answer for you. Just wanted to clarify what you were looking for.

author: splin
author_rank: Frequent Contributor
timestamp: October 24, 2015, 04:58:34 am
content: Quote from: DiligentMinds.com on October 23, 2015, 03:46:27 pmWhilst I agree that most 24+ bit ADCs have INL specifications of 4ppm or more, equating to 18bit linearity at best, it has nothing to do with ENOB which is purely a signal to noise ratio (SNR) specification. You could conceivably have a very linear, but noisy, ADC with <0.1ppm INL error but < 16 effective number of bits. Or fabulous noise performance (i.e. resolution) but terrible linearity.As Marco posted, the 20 bit SAR LTC237x ADCs have the lowest specified INL of +/- 2ppm max, 0.5ppm typical of any ADCs that I am aware of, but lower ENOBs than most 24bit delta sigma convertors. They have much higher conversion rates though so the ENOBs can be considerably improved by filtering/averaging.QuoteThat is probably true but I suspect the reputation has a lot to do with HP publishing the results of their linearity testing in their in house magazine HP Journal. I don't know of any similar publication of linearity tests on other 8.5 digit meters (such as the Datron 1281/Solartron 7081/Keithley 2002 etc.) which, although they may have less impressive guaranteed specs, it may be that they aren't that much worse than the 3458A in reality. Anybody know how the Transmlle 8081 compares?Linearity errors at these levels are hard to measure so it's hardly surprising that there isn't much freely available information - only meteorology labs will be likely to have much interest in such data. Testing linearity at these levels against Josephson junctions is expensive so perhaps it was a smart move by HP to put the results into the public domain. Perhaps HP's competitors didn't have the resources to undertake linearity testing at these levels or didn't have the business case to do so.

author: deadlylover
author_rank: Frequent Contributor
timestamp: October 24, 2015, 05:31:45 am
content: In regards to ADC linearity on 8.5 digit DMM's, I came across this graph on the brochure of Advantest's R6581 showing its linearity. I think they also had a picture showing the unit being tested at a JJA, I wouldn't be surprised if they actually operated one.

author: Marco
author_rank: Super Contributor
timestamp: October 24, 2015, 01:38:16 pm
content: Quote from: promacjoe on October 24, 2015, 12:39:44 amWhy are you quoting me in this response? I never mentioned a DAC, that was DiligentMinds.com. I have nothing to do with what he's talking about, at a guess I would say he was commenting on Blueskull's DAC but this is clearly the wrong thread for that.OP wanted the most linear ADC and the LTC237X-20 are the most linear COTS ADCs at the moment, not 24 bit ... but they are fast enough to do 24 bit resolution at audio frequencies using oversampling and adding either noise or a structured high frequency signal.

author: branadic
author_rank: Super Contributor
timestamp: October 24, 2015, 02:47:50 pm
content: LTC2400 is still a good choice for an 24bit ADC with low INL. Its initial INL 4ppm can be calibrated to lower values (e.g. <1ppm) with simple resistor technics and a stable voltage source. See also:https://www.eevblog.com/forum/projects/oshw-24bit-adc-measurement-system-for-voltage-references/

author: promacjoe
author_rank: Regular Contributor
timestamp: October 24, 2015, 05:36:31 pm
content: Quote from: Marco on October 24, 2015, 01:38:16 pmbecause I pushed the wrong quote button. All the other groups have their quote button at the bottom of the text, This one is at the top. it is an easy mistake to make. it has been corrected.I was not referring to you as being wrong, I was referring to the original author and what they wanted. This discussion was going in the opposite direction from what the author requested. This would not help him at all. I just wanted to get things back on track.