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Column-parallel comparator noise correlation analysis

When dealing with column-parallel comparators in Single-Slope (and other) ADCs, where there are a ton of other noise contributors, it may not be so obvious how you can measure the contribution of the comparator itself externally. This post shows a quick noise correlation analysis to estimate comparator noise contribution in the signal chain, which is applied to column-parallel Single-Slope (Ramp) ADCs, but not limited to this architecture. The idea can be expanded and reused in other ADC architectures and column-parallel chains, where we can assume high correlation between other noise sources e.g. references, clocks, bias voltages etc...

In order to evaluate the noise performance of column comparators an adjacent noise cross-correlation analysis technique can be applied. Let's have a look at a principle noise path diagram which will be used as a reference further in text.

Noise path assumption diagram: adjacent column noise sources and their correlation (reference used further in text)

$R_{i}$ represents the noise sample contributed by the shared ramp reference, which is common for both columns. The same applies to $K_{i}$ which represents the noise induced by random clock jitter and counter fluctuations. The noise samples from two adjacent columns $C_{1_{i}}$ and $C_{2_{i}}$ represent the thermal noise induced by the comparator. Therefore, for this analysis it has been assumed that the correlation $\rho$ for $R_{i}$ and $K_{i}$ for both column data $D_{1_{i}}$ and $D_{2_{i}}$ equals 1, and that the thermal noise from $C_{1_{i}}$ and $C_{2_{i}}$ between adjacent comparators has a correlation coefficient of $\rho = 0$, or:

$$\overbrace{R_{i}}^{\rho = 1} + \overbrace{C_{1_{i}}}^{\rho = 0} + \overbrace{K_{i}}^{\rho = 1} = D_{1_{I}} \\ \underbrace{R_{i}} + \underbrace{C_{2_{i}}} + \underbrace{K_{i}} = D_{2_{I}}$$

By knowing the output samples for a converted static DC signal of adjacent columns we can compute the comparator noise statistically using subtraction of the random variables (statistically equivalent also to addition). To calculate the noise standard deviation of the comparator we must first compute the total mean $\mu$ of the adjacent column samples:

$$\mu = \dfrac{ \dfrac{\displaystyle\sum_{i=1}^{N} D_{1_{i}}}{N} + \dfrac{\displaystyle\sum_{i=1}^{N} D_{2_{i}}}{N}}{2}$$

where $D_{1_{i}}$ and $D_{2_{i}}$ are the data samples at the output of the ADC. Assuming full correlation $\rho_{R} = \rho_{K} = 1$ and zero correlation for $\rho_{C_{1}} = \rho_{C_{2}} = 0$ we can write the comparator standard deviation as:

$$\sigma_{comp} = \dfrac{ \sqrt{ \dfrac{\displaystyle\sum_{i=1}^{N} \Big((D_{1_{i}} - D_{2_{i}}) - \mu \Big)^{2} }{N-1} } }{2}$$

One may note that it has also been assumed that the thermal noise contribution of both comparators is identical, thus based on the summation rules of probabilities the noise magnitude should be halved in order to obtain the thermal noise contribution of a single comparator, hence the used denominator.

Column noise contribution: comparator only
Column noise contribution: total column

The figures above show output data measured on a real implementation (disregard the gaussian nonuniformity - it is due to some ADC DNL glitches) computed using the described method. The first figure shows the calculated comparator noise being of 1/3 lower magnitude as compared to the total output column noise which is shown in the second figure. The difference is assumed to be contributed by clock jitter, sampling thermal noise and ramp random walk noise. The measured comparator noise stdev in this measurement case equates to about 330 µV, which matches very well with transient noise simulations performed on the comparator earlier on.

In conclusion: this method is not limited to only column-parallel ADCs and/or single-slope data converters. It is the principle that matters and it can be applied to various other cases.

Date:Wed Aug 23 13:32:05 GMT 2017


The semiconductor industry in Bulgaria

Recently I got inspired by a report about the semiconductor industry in the UK, and thought that it might be cool to copy some of their chart ideas and create a family tree about the semiconductor industry in Bulgaria.

Bulgaria can not boast with a large IC community, but actually, we have had quite a few successful, and not so successful IC ventures during the past 30 years. It all began during the 70s with the establishment of the Institute of Microelectronics and the semiconductor fabs around Botevgrad which used to produce discrete Ge and Si active devices. Later on during the 70s and 80s the fab produced MOS logic ICs of the 4000 and 7400 series, and also successfully copied and manufactured the IBM 6502 under the name CM630. This 8-bit processor was used in most of the early bulgarian personal computers - Pravetz.

After the fall of communism in 1989, the institute could not keep up innovating (if it ever has...) which led to its closure. Consequently most of the skilled resources emigrated, while a few remained to form a branch of Silway semiconductor (at that time a French fab). Later during the 90s there have been quite a few movements and acquisitions, making an average acquisition re-organization time of about 5-6 years.

Here is a graph chart of what I have managed to find out by talking to people and exploring the world of the internet:

Family tree of semiconductor companies in Bulgaria

Please, if you know more on the topic let me know in the comments, so I can update the chart. I am sure there is some missing information there.

Date:Mon Aug 14 18:34:53 GMT 2017


1 year Transistorized

Yup, that's right! Today marks a year since I moved to this domain. Now this is calling for some stats to the rescue. Since June 2016 I wrote 23 posts, which compared to 2015 is exactly 50 % less. I hope this trend does not end up following Zeno's paradox, but hopefully 2016 would just be archived as a local minimum. The majority of these posts are an offspring of travels, hence the quality... and perhaps quantity in 2016.

Anyway, let me share something. The name of this domain (which I still think isn't sparkling of uttermost cleverness) was inspired by a word printed on the first ever very high volume mass-produced portable transistor radio, which pretty much revolutionized the way we receive information. This was the TR-55, behold:

The first very high volume mass-produced transistor radio - TR-55

See? - it's T R A N S I S T O R I Z E D :)

Btw, I am now partly working on my final "long" paper, and pretty much write circuit related stories all day every day. So yes, catch you in my next post which might again be a philosophical one... just releasing the steam with philosophy doctrines here...

Date:Thu Jun 22 21:39:48 GMT 2017


On the phenomenon of rubbish patents

a train of thoughts written on a train

You are starting your next venture with a brief search on what has been done before. Stumbling across some patents you get excited and start exploring the database vigorously. It isn't too late that you realize that something doesn't feel quite right – nearly 2/3 of the patents you come across are vague and do not make sense. You then morph into a questioning phase, but why?

All granted patents in the databases have a certain role. They are like the bureaucrats in the public sector: some are completely useless, others a bit pointless, you can also find a few good ones. But why is it that the majority is rubbish? The short answer would be that a patent's purpose is to protect someone's intellectual property from misuse, but a large portion are instead used for image forgery and lobbying. The long answer, however, is a bit more involved and perhaps cannot be answered by a few lines. But, let me try to elaborate on what I think can be defined as rubbish, and later on state my views for the utopian world of patents.

So, what's a rubbish patent anyway? These are patents which include methods or apparatus with false claims, have never been or ever will work. Usually submitted by universities or companies which are hopelessly trying to boost their sell value by enriching their patent portfolios. Similarly, university professors sometimes use the patenting strategy to provide that kick which makes the graduation of the lower performing PhD students possible, or perhaps why not even justify project money funded by the taxpayer, easy huh? In the end, who doesn't get impressed by a few US/EU patents backing-up a CV. These patents are generally harmless, except for the disturbances caused in the public monetary balance.

The other, and more prominent, type of bullshit patents are the ones filed by patent trolls, whose sole purpose is to milk someone legally. Patent trolling is usually done by individuals (typically well trained lawyers) who issue or buy patents from a bankrupted company and then attempt to enforce patent rights against accused infringers far beyond the patent's actual value or contribution to the prior art. Patent trolls do not really create products or supply any services based upon the patents in question. These are perhaps a bit less of a problem in Europe than in the U.S. because Europe has a fairer loser pays the trial costs regime. But still, this kind of trickery is harmful to all sensible companies who add value to society, and objectively, the end users are the ones who suffer and have always been covering the mess.

Patent trolling is just a product of a lawyer's imagination, does not contribute any value to society, should be regarded as crime; and must be eradicated. That's probably easier said than done, but here's my suggestion for Intellectual Property (IP) handling in an utopian world.

My vision for the patenting system in a dreamland is not to have it in the first place. Eradication of the whole scheme would mean two tings; all harmful trickeries rooting from the law system would vanish and competition would solely be driven by creativity, leading to an even faster technological development. Having no secrets in science and engineering means that to be better one will have no other option but to expand his/her creativity, instead of trying to hide, protect, and waste energy on lawsuits. You might argue that once we lift IP protection some companies will go bankrupt, while others will just cheat by keeping secrecy (which happens nowadays anyway and is totally fine). Well and here is my point, lifting the patenting barriers will only work if, and only if, IPs are always shared and open which isn't particularly easy to enforce.

The last is likely to be the most mountainous issue which is typically encountered in all utopian dreams – you can't re-program people's brains overnight. We need someone like Rotwang, popping out the dungeons of Metropolis to ring that global brain-resonance tuning bell, but this only exists on film reels. Until then, it seems like we'll have to accept things as they are, and stick living in a world stuffed with patent lawyers, bullshit patents and inefficiency.

Date:Thu May 23 06:44:49 GMT 2017