ADC research trends: Endangered performance parameters

Figure 1. Fraction of papers reporting a specific parameter.

SCIENTIFIC PRACTICES: In the previous post, your opinions on which parameters should be mandatory in an A/D-converter implementation paper were surveyed through a poll. In this one we will see what parameters actually get reported in scientific papers, and how the overall reporting practices have changed over time. We can also try to see how well Converter Passion reader’s opinions are in sync with harsh scientific reality 😉

Working with the ADC survey, I had noticed that the set of measured performance parameters authors choose to report or omit changed over time. I’ve gathered the statistics, and it will be interesting to hear what you think of it. Figure 1 shows the percentage of papers reporting signal-to-noise-and-distortion ratio (SNDR), signal-to-noise ratio (SNR), spurious-free dynamic range (SFDR) and total harmonic distortion (THD) each year. The good news is that dynamic single-tone performance is more frequently reported in today’s papers than ever before. In fact, SNDR was reported in 96% of all papers from 2011. Both SNDR and SFDR show noisy but steadily increasing trends, and SFDR is now reported in around 68% of all papers (2011).

At the same time, SNR is increasingly being abandoned in favor of simply reporting SNDR instead. We can also see that THD is becoming an endangered parameter. SNR and THD have been in a visible decline after year 2000, and THD was only reported in 12% of the papers in 2011. SNR went from 60% in year 2000 to 41% 2011. These numbers trouble me. If the trend continues, we will know less about the noise in our A/D-converters for each year. I can’t possibly see how that can be of benefit to “Science” or to the reader. If you can, be sure to post a comment below.

Figure 2. Fraction of papers reporting specific combinations of parameters.

Above we looked at the reporting frequency for each individual parameter. Authors also choose which simultaneous combination or set of performance parameters they include in a paper. This is shown in Fig. 2. Many works report SNDR together with SFDR, and 67% of all papers from 2011 reported at least these two parameters. There is a strong positive trend, so within a decade we may see SNDR+SFDR reported in nearly all published papers. The second most popular parameter pair is SNDR+SNR, with 38% coverage in 2011. Unfortunately, its reporting frequency is limited by the negative trend for SNR.

A more complete single-tone characterization is achieved with the three-parameter combination SNDR+SNR+SFDR. Only 21% of all papers published in 2011 offer this much information about the circuit. Even if there is currently a positive trend, it might soon become limited by the decline in SNR reporting, and eventually reverse.

The practice to report a full four-parameter set is very unusual – only 9% of the papers did that in 2011. It has actually never been really common, and if THD continues to disappear from papers, the four-parameter performance will be increasingly rare. Lots of credit to authors that are still reporting a full set! It is much appreciated here at Converter Passion.

Rare species

Finally, a few other rare species in the ADC parameter ecosystem are shown in Fig. 3. While the highly valuable parameter self-noise appears to be on its way to extinction, intermodulation distortion (IMD) as represented by the second– and third-order intercept point (IP2, IP3) seems to cling to life in the outskirts of the habitat.

So … what do you make of all this? Do you think it will have any long-term impact on our field of science? Does it matter to you what parameters are reported or not? Have your say in the comments below.

And, what happened around year 2000 to shift the trends so dramatically? Was it the dot-com boom or the Y2K bug? (I haven’t heard about that one for a while now)

Figure 3. Endangered species in the parameter ecosystem.

7 responses to “ADC research trends: Endangered performance parameters

  1. George Storm

    For me, not just what is reported that is important, but also the conditions under which it is reported. Some of the “progress” seems rather vestigial – 10GSample/s converters whose IP3 is only half-way decent below 500MHz…
    On the other hand, there is a case to be made for “horses for courses”: we do sometimes want to resolve small high-frequency content in a low-frequency background (without first separating the two).

  2. George: I agree completely – the mesurement/operating conditions should be reported as well. This particular post focused solely on a subset of parameters, and how some of them seems to disappear from our works. In an earlier post (What parameters should be reported in a good ADC paper?) I tried to address this a little bit broader.
    Being a bit more philosophical, one could perhaps ask what is the point in maintaining a field of experimental science if its scientist are reluctant to disclose a full set of relevant experimental data (including operating conditions) in their publications? I understand that doctoral students need publications to get their degrees, and that companies wish to market their technology. But still …
    Who looks after the field itself, and what’s good for “Science”?

  3. David San Segundo

    Self noise and THD are audio parameters, mostly. Maybe a plot of the applications which the published ADCs target, and the percentage of ADCs being for audio would correlate with the decline in these two parameters?

    • David: Interesting hypothesis. To the extent that authors have claimed a particular app, I do actually have that data, although slightly involved. I’ll see if I manage to make some sense out of it. It would be interesting to hear if other readers mainly associate THD and self noise with audio as well. If so, your hypothesis could very well be the explanation.

      Personally, why I like to see these two reported is because they give useful information about the circuit. Essentially, a more complete characterization.

      Do you have any idea why SNR is being abandoned (or at least is in decline)?

  4. George Storm

    Apologies that some of this is not directly on topic…
    “companies wish to market their technology”
    When it comes to actually using an ADC, you have to rely on the data sheet. These are rarely in the most advanced technology of the papers, and the design will be centred for yield rather than for best-case performance. Unlike the “scientific” papers, many selection guides list all of DNL, INL, SNR and THD for the top-end precision devices – but naturally you have to refer to the full data sheet to see the other information you need. Another issue is that typical performance is usually of little relevance…
    To my mind, ‘scientific’ papers have value in two cases: when they provide data on the performance of different topologies, and when they demonstrate the capability (and limitations) of process advances. In this regard, “not-good-this-way” can be as valuable as success – but unfortunately the level of detail provided is rarely sufficient to allow judgement whether the limitations are down to the basic technique/process combination rather than poor detailed design (I’ll accept some reduction of detail from publishers with proven track records of successes)

  5. George: I find scientific papers useful in many ways, but I agree with the two cases you list. The “not-good-this-way” papers are particularly underrated. As you say, they can be just as valuable. To systematically investigate the unknown is a cornerstone of real research. When you do so, it has got to be quite likely that your experiments do not always push the state-of-the-art in terms of absolute performance. From a scientific point of view they can be just as interesting because something new was explored and something can be learned from it.

  6. David: I took a quick look at papers claiming audio application. The numbers are very small (<10 papers any year) but it could be because that part of my survey needs some more work to get a consistent classification. I'm not really sure. The curve I get have similarities with the self noise curve in the post, but papers with ADCs tagged as 'audio' have a very week correlation with those reporting self noise and THD. Again, in the current state of my database.

    Therefore I did another test to see if scientific authors primarily associate self noise and THD with audio. If they did, it should not be common to report these parameters for ADCs with fs = 1MS/s and above. It actually turns out to be the contrary: The vast majority of papers reporting THD present MS/s ADCs, and more than half of those reporting self-noise. Looking at the last 10–15 years, such papers are almost exclusively about MS/s ADCs or above.

    So it doesn't seem like the decline in THD and the sparse reporting of self noise are correlated with audio ADCs.


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