Synthesis of inputs Description of GHOU

The Hands-On Universe (HOU, handsonuniverse.org) project began in the 1990s when Carl Pennypacker, a supernova research astronomer at University of California Berkeley, invited a group of astronomers and educators to share ideas about how to make astronomy education better. The idea of making real telescopes and telescope images available to teachers and students took root. Along with that, image processing software tools became necessary to provide, as well as cooperation among astronomers and observatories to make telescope instruments available for use by schools. From the on-set, GHOU has been an international program, with contributions from many nations working together with common values, goals, and ethical standards. Telescope networks for use by all have been brought forward by participants in the UK, the US, and other nations. G-HOU has since promoted scientific modelling methods in the classroom, reproducing experiments including data collection and analysis. The mission of HOU is to train teachers on the use of modern tools and resources for science education and engage students in international scientific projects. We also aim to promote interactive science projects among HOU countries and engage educators and students in a truly global cooperation. No borders or frontiers. The project came in to Europe (EU-HOU) in 2004 with a specific focus on the training of teachers. The main aim of EU-HOU was to adapt the project to Europe with the creation of a series of new exercises with modern topics in Astrophysics Research and a new image processing software freely available to all educators. During the International Year of Astronomy, the Galileo Teacher Training Program, one of the cornerstones of the event, was created by the G-HOU team and since then over 100 countries have joined the effort, reaching over 50 000 teachers around the globe.

Our yearly conference was online in 2020 and gathered 85 countries. "GHOU is not a program or a project, it is a movement." A movement of people creating a network of human beings. Estimates are that over 1,000,000 students a year are reached with GHOU resources, although we want to reach many more.

The round table was organised in the context of GHOU 2020 conference. All participants of this round table are leading members of GHOU in their country. They were selected to cover uniformly different parts of the world (Europe, America, Africa, Asia).

Thematic focus: ___ Citizenship and participation__________ (we do not reproduce Prompt 1 here) Prompt 2. How education can support robust citizenship and participation practices.

Our discussion focused on science education that is the expertise of GHOU.

 Science and citizenship Scientific thinking means to observe, collect, analyse, compare data… Everyone is born as a scientist. But in school we forget because the teacher tells us "not ask so many questions, this is a silly question…". First we need to be interested. Second, we need to use our intuition, to create our own experience, or "I want to know". We need to make hypotheses, to test them, to discuss with each other, acknowledge every points of view. Then, we need to be able to communicate our point of view, accept that we might be right, and then wrong, and then right… This is how science works. If you embed that in your soul, if you accept your natural condition of scientist, then you will be a natural inquirer for the rest of your life. If someone tells you, "get up and close the door" and you do not ask why, then you are a fool. We have to ask. Even a teacher might be wrong. And if I am wrong, I have the right to learn, not to be told silly or that I should know. We have to question everything.

Education should help any student to identify what is scientific thinking rather than to speak as a scientist. Science is very well matched to democracy. One the one hand, as a scientist I believe only myself, that means I am free from other people. But on the other hand I am not independent from others. So I have to wonder what are the evidence, whether a theory matches or not the observations. We communicate and discuss. Finally, I decide by myself, this theory is correct, this experience is reliable. So the scientific way is making discussion and finally leaving each person deciding for itself. Scientific thinking is direct to the democratic system.

 Knowledge as models

We focus on acquiring knowledge for use. Knowledge for the sake of knowledge is great, but for the general population of students that will be tomorrow's citizens, they need knowledge that they can use and specifically knowledge they acquired themselves. Nowadays most of the knowledge is stored on the internet. People can scroll down the particular knowledge. Yet, it is not enough to have pieces of knowledge to use in our thinking. We need ways to structure that knowledge. That is what models are for, to understand and to connect. Students need to know how to use models, conceptual models. They need to know how to specifically build the model they need. Using scientific methods in the classroom, as described in the next prompt, one reproduces experiments, makes new experiments and uses scientific thinking. Prompt 3. Imagine new ways of supporting robust citizenship and participation practices.

Prompt 3.

 Acquiring data

We are in the 21st century and surrounded by a vast amount of data. Rumor mills on the internet talk about so many strange ideas that are not supported by data. Besides, nobody knows what the next problem is going to be. Hence, students need to know how to get data and how to use conceptual tools to make sense of those data in order to find patterns. They need to be not only data literate. They need to be data users, to acquire observations, to have a sense of observations. Students need to know how to think for themselves, how to get the data themselves or to fact-check what people tell them. Looking at data, or using other's data is not enough; generating data gives power, belief in ourselves. Overall, students need to review science or to replicate experiences that have already been made, but using real data that have given birth to this knowledge. Even better, students may participate to science by analyzing themselves the huge amount of data available in astronomy, using the same methodology as researchers and discovering the same objects (exoplanets, asteroids, supernovae…). Consequently, we always provide data or make students acquire data that is real and has all these real imperfections in it. We want to give them clean data but not perfect data because that's the world that scientists live in. When you draw a trend line or a gradient on a graph, not all the points on the graph sit on the trend line. And most important, the profession of a scientist is to make hypothesis and test them again and again until they find satisfactory answers that can be soon after overcome. Making a "mistake" makes science progress. So it is a good thing to make a mistake. We have to make sure students realize not all questions to all answers are just "clickable". Not everything we do is just black and white, not every bad politician is always bad, not every good politician is always good.

 Local knowledge. We need to link the scientific knowledge to what people are used to. As for astronomy knowledge, we are all under the same sky but we need to be careful to convince everyone about it. By accounting for local, traditional and cultural knowledge, we will introduce deeper knowledge and mutual understanding. As an example, some farmers in Morocco know that to start to sow, or to put grass, they need to wait for a specific constellation. This is a local Berbere knowledge inherited by generations. But when we explain to kids what constellations are, they have a better understanding of the sky that they can transfer to generations, without contradicting traditions.

 STEAM (Science, Technology, Engineering, Arts and Mathematics). STEAM accounts for science and art. It is a man-made thing to divide knowledge in different boxes. We are holistic human beings. A work of art in some fundamental way is a representation of the artist's vision. Such a representation, through dialog and study, can lead to larger models that encompass broader topics, and lead to inquiry, dialog, and learning. There is so much science in art and so much art in science. It is natural to merge them, while taking both fields seriously on their own terms. This is how you can get some really interesting things happening. Both can support and inspire each other to something greater. The world needs to be redesigned into a healthier place. You need the best from all worlds, the best from science and the best from art. That is sustainable design. You can get products (in the large sense), that are friendly for people, planet, prosperity, peace and partnership. Sustainable design means to be able to create something forever without depleting resources or polluting. This is the main definition of sustainability. To make the world a better place, we need STEAM.

Thematic focus: ___ Knowledge Production, Access and Governance __________ We did not use Prompt 1. Prompt 2. Thinking in terms of 'best practices', how does education ideally protect or support knowledge becoming more of a global public good? Prompt 2.

Lot of knowledge may be brought by scientists in education. But scientists often are not trained science communicators. On the other hand, educators have to deliver a message on topics that they are not experts on, and deliver cutting-edge science to the hands of children. Usually they are not mastering the technology, they are not mastering the science behind.

GHOU practice is to prepare the educators to be travel companions of the children, within the context of astronomy. Educators are helping the children to acquire the knowledge on the way while they are using their own knowledge. Once they get the training they become ambassadors and prepare other teachers. From the point of view of one specific school, all teachers from all subjects can collaborate together while this path is being walked because astronomy is such a discipline that allows you to integrate many diverse topics. The story can be told as an over view and theme. This a policy that has to be supported by the community, the scientists, the surrounding schools. So all of this will have the consequence that content learning will be acquired and the learning will be deeper than if the teacher was just following the steps of a curricula or a book. We propose to bring teacher training to a higher level, to bring support, community, STEAM, cutting-edge science to the hands of any teachers.

Beyond science, GHOU gives the opportunity to listen to each other. In every training, every classroom, someone will bring its cultural aspect. There is no translation for each word, just the meaning of things is universal. Looking at an observatory has a meaning without words. Our practice is facilitating this to happen in a very humane, free, volunteer and supportive way. Prompt 3. It's possible that education might need to be radically transformed so as to build the kinds of economic security and human-centered work that we desire for the future. Thinking imaginatively and creatively, what can you come up with as ways we should reimagine schools and education more broadly? Prompt 3.

 Decolonization of education Firstly, efforts have to be localized, because teachers and students react differently. Training and teaching have to meet a few "best practice" criteria but those should not assume a local or cultural knowledge. Second, as far as astronomy is concerned, teachers must promote children-centered activities, observations with naked eyes or with a (robotic) telescope, whatever possible in their own locality. Sometimes even inside one country different realities may emerge. Some are more culturally engaged in their community with local habits that one cannot alleviate. A moon may look like a rabbit or a cheese… So decolonisation of education is already there. You eliminate a lot of obstacles by doing so. Everyone starts wherever it is, at its own pace, without leaving anyone behind.

 Community

What I can create on my own is limited to my experience and my thoughts. If I collaborate with other people, the collective experience and ideas and thoughts are so much larger. In a creative process, we get the best from several worlds and probably the best solution. The professional scientific community is full of collaborations and smaller communities, at many scales. In many ways, GHOU models itself after aspects of the scientific community in terms of sharing data, ideas, scientific topics, and facilities. An excellent example is the exoplanet study programs evolving in some parts of GHOU. This involves students forming international collaborations, sharing data, being in communication, and developing long term friendships and bonds within the exoplanet collaboration team. All the while, local students need to interact with their local teacher, who can help support their learning, explain details of the data analysis that might not be explained very well, and support the students' learning and growth. If GHOU approached students in isolation without their teachers or other community member, the system might fail or at least work at a much less effective level. And the teacher involved can share his.her knowledge with other teachers. Other teachers would often appreciate learning that some of their students are searching for planets around other stars, and such a local excitement within the community helps support continual engagement in the team. It is important to give teachers and schools a community, so they are not doing it alone, because whatever we do, as science people, we do it better together. So communication and community become very important. The development and use of robotic telescopes in education allows us to provide additional online learning and support (via e.g. Zoom, Webex, Teams) allowing GHOU people to share more easily their knowledge and enthusiasm to teachers and educators. This is especially pertinent as we move toward 'Big Data' and incorporate new working practices mid-and post-CoVID.

Knowledge is best acquired when the teacher and students are inspired. GHOU takes teacher and student seriously in providing accurate, up to date, information and access to real telescopes. Equally important is a network supporting teachers and students, helping with providing the knowledge and inspiring by mere support.

One can alter the saying "It takes a village to raise a child" into "It takes a well-educated child to raise a village". One example is the family who was driving at night, and their child, who had just attended a GHOU-workshop asked if they could pull over and look at the stars. And the child shared so much knowledge about the moon to his parents and siblings, telling them things they didn´t know. It is inspiring to us, that students can be our teacher, they can teach their own surroundings.

 global citizenship through astronomy

In a fundamental way, astronomy teaches some of the most powerful and compelling basis for community -all humans come from re-cycled star matter, and there is little difference in the chemical origin of any of us. Essentially, in a kind of overstated way, we are all star children and all humans are star brothers and sister. This is a deep fact that many sciences cannot expand upon. And in fact, extending this "atoms from the stars" model a bit, we can begin to see how humans fit into a delicate ecosystem, where our atoms are continually exchanged in and out of the ecosystem, and each other. Hence boundaries between humans, on an atomic -system viewpoint are lowered somewhat. Students connect through a telescope in another part of the world where it is dark. It makes people connect and learn about each other. Teachers from different parts of the world come to workshops, learning about ways to educate people, our children, how to think scientifically, analyse things in a global way. This is not only about children and teachers, but also the general public. Programs like "dark sky", "Eratosthenes", promote a sense of belonging to this globe that we all live on.

 Happiness

We want our children to become thinkers and learners, and that will make them happy. They will know they can figure stuff out. Children genuinely love to study and measure astronomy images, as certified by many of our internal researches. Education should be based on the word "critical" but also on the word "happy". Children should be happy exploring education, dreaming, assessing data. That's how the future could be. Education system has brought out a lot of pressure to the children. The beauty would be that education focus on the happiness of children and allow them to explore whatever they can.

III. Feedback on Consultation

Are there any other questions that should be posed in the context of the Futures of Education consultations?

Is there anything else you wish to relay to UNESCO and/or the International Commission?


William Sutherland

Sergio Ticul Alvarez-Castañeda

Tatsuya Amano

Roberto Ambrosini

Philip Atkinson

John Baxter

Alexander Bond

Philip Boon

Katherine Buchanan

Jos Barlow

Ensuring tests of conservation interventions build on existing literature

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.


Vincent Devictor

Yves Meinard
email: yves.meinard@lamsade.dauphine.fr

Empowering biodiversity knowledge

Introduction

Conservation sciences are inundated by academic warnings to humanity and calls for better environmental governance, presupposing that biodiversity science provides a descriptive picture on the basis of which decision makers can make cogent policy choices. We challenge this view and argue that discourses on biodiversity have become a rhetorical veneer hiding academic strategies to optimize funding and an opiate sustaining the comfortable belief that biodiversity knowledge unquestionably contributes to the mitigation of biodiversity loss. The hard truth is that the space separating scientists from policy makers is not a no-man's-land through which scientists can throw their recommendations to decision makers on the other side. This space is saturated by processes that distort, construct, and erase information, and are currently more efficient in consolidating the illusion that a global governance is being constructed, than in stanching the crisis. Rather than turning a blind eye to these processes, we need to invent new ways to empower biodiversity knowledge.

Ubiquity of Politics in Biodiversity Science

Biodiversity science has always been intermingled with political agendas. The emergence of concern for biodiversity coincides with the abandonment of teleological reference states to explain ecosystem dynamics. Population dynamics and individual species selection and adaptation replaced higher systemic levels as focal units, compromising the hope that the course of nature could be predicted and managed. The Odumian vision, according to which ecological succession is an orderly and predictable process, was replaced by neo-Gleasonian

Both the authors contributed equally to this work. Article impact statement: Biodiversity knowledge, which is increasingly reduced to rhetoric hiding academic strategies and an anxiolytic, needs to be re-empowered. Paper submitted February 27, 2019;revised manuscript accepted June 7, 2019. nonequilibrium theories [START_REF] Pickett | The ecology of natural disturbance and patch dynamics[END_REF]. This development shifted the focus of conservation toward individual species, their habitats, threats, and conservation status [START_REF] Forsyth | Critical political ecology: the politics of environmental science[END_REF].

In this context, articulating conservation issues in terms of data, in the wake of Wilson's original estimates of species richness, was presented as a means to "translate good science . . . into good politics" [START_REF] Kelmelis | Proceedings of the U.S. Geological Survey Global change research forum[END_REF]. The emergence of the term biodiversity was part of a "political mission" to draw the U.S. Congress's attention on the "species extinction crisis" [START_REF] Takacs | The idea of biodiversity: philosophies of paradise[END_REF]. This advocacy rapidly gained scientific credence (Soulé & Wicox 1980), and calls for integrating political constraints on biodiversity quantification soon followed [START_REF] Walker | Biodiversity and ecological redundancy[END_REF]). The development of a "functional approach" that admits "global biodiversity concerns will ultimately reduce to a cost-benefit analysis" was endorsed by scientists and practitioners. Biodiversity was interpreted as a reservoir of functional attributes, liable to fulfil political and social needs. The difficult challenge to entrench the political acceptability of the moral basis of conservation [START_REF] Soulé | What is conservation biology?[END_REF] was bypassed by reconfiguring environmental issues in instrumental terms [START_REF] Bonneuil | Tell me where you come from, I will tell you who you are: A genealogy of biodiversity offsetting mechanisms in historical context[END_REF]. By considering biodiversity and ecosystem services as 2 faces of the same coin, succeeding initiatives (e.g., Potsdam Initiative, Millenium Ecosystem Assessment) embarked on a political crusade to reduce the value of biodiversity to its contribution to an abstract and vague human well-being.

The emergence of global data on and maps of biodiversity is itself politically oriented [START_REF] Devictor | From ecological records to big data: The invention of global biodiversity[END_REF]. Their seeming neutrality feeds a political fantasy of global environmental governance [START_REF] Turnhout | What does policy-relevant global environmental knowledge do? The cases of climate and biodiversity[END_REF], and global information platforms on biodiversity, presented as means to facilitate 1 biotechnological innovation (OCDE 1999), rest on a politically loaded vision of society and the role of science [START_REF] Biermann | Navigating the anthropocene: Improving earth system governance[END_REF].

Vacuous Rhetoric of an Allegedly Policy-Relevant Biodiversity Science

The importance of politics within biodiversity science reflects an academic legitimization rhetoric. Early in the history of biodiversity science, some ecologists stumbled on the idea they could sell their research to funding agencies by claiming that ecological knowledge is policy relevant because biodiversity is important for human well-being [START_REF] Dempsey | Ecosystem services: Tensions, impurities, and points of engagement within neoliberalism[END_REF]. This is a gratuitous idea, only vaguely buttressed on a schematic understanding of human well-being and barely integrated in a vision of how this supposedly policy-relevant knowledge could be used by policy makers. The scientific leader of the first forum on biodiversity and ecosystem function accordingly stated that endorsing the relationship between biodiversity and ecosystem services "provides a powerful link between research in basic ecology and the interests of the general public and policy makers" [START_REF] Mooney | The globalization of ecological thought (excellence in ecology)[END_REF].

Such a narrative carelessly touting policy relevance to capture funding also probably plays a role in other scientific fields. The direct benefit of conservation to society is, however, less clear than the industrial and commercial products of physics or chemistry, forcing important distortions of conservation discourses.

This legitimization rhetoric struck a chord with economists because it bolstered their promotion of the concept of ecosystem services [START_REF] Jepson | Governance and accountability of environmental NGOs[END_REF]. A synergy of academic legitimization emerged and unified economists and ecologists around research programs on, for example, the link between biodiversity and ecosystem functioning. A bonanza of funding was secured and resulted in an avalanche of supposedly policy-relevant publications and fed an overstretched narrative of promise and hope for global biodiversity governance. Ecologists left unmoored by the demise of Odumian teleology were thereby provided with an opiate that alleviated their anxiety (an anxiolytic) by entertaining their hope of finding a new balance with nature to replace the lost balance of nature [START_REF] Simberloff | The "balance of nature"--evolution of a panchreston[END_REF]. This anxiolytic conservation science [START_REF] Garnett | Conservation science must engender hope to succeed[END_REF], presenting environmental issues as reducible to manageable technical problems, even allowed championing enthusiastic and bombastic development goals for "people and the planet" [START_REF] Griggs | Sustainable development goals for people and planet[END_REF].

The scientists involved in the origin of the term biodiversity did not foresee these developments, but found themselves caught in a series of decentralized, undirected processes that caused biodiversity knowledge to drift in a certain direction, including the construction of databases disconnected from field work and the forging of tighter links between ecology and economic valuation.

Yet far from witnessing the practical success of this biodiversity knowledge, prominent supposedly policyrelevant academic tools, such as valuations of ecosystem services, are rarely used [START_REF] Laurans | Use of ecosystem services valuation for decision-making: Questioning a literature blindspot[END_REF]) and political initiatives to stop the erosion of biodiversity are either missing or insufficient. Biodiversity knowledge thus increasingly seems merely a powerless discourse proliferating in insolation from environmental policies.

Empowering Biodiversity Knowledge

We do not claim that biodiversity science is nothing but a vacuous rhetoric or that the biodiversity crisis is fantasy. Empirical evidence shows that many biophysical entities are imperiled by increasing anthropogenic impacts [START_REF] Godet | What conservation does[END_REF]. These biophysical aspects of the biodiversity crisis are not, however, the subject matter of a non-normative knowledge. Biodiversity, biodiversity crisis, and conservation science have normative aspects: people engaged in conservation are motivated by a moral or ethical call whose contours are a subject of ongoing philosophical debate and reflexive scrutiny by conservationists [START_REF] Vucetich | Evaluating whether nature's intrinsic value is an axiom of or anathema to conservation[END_REF]. The aspect of politically loaded discourses geared to fund raising we criticize is not the normative dimension per se, but rather the selective presentation of evidence and gratuitous promises. Evidence is selected to construct a discourse designed to capture funds for researchers and nongovernmental organizations, rather than to develop interventions to contravene the crisis. And this discourse is structured around grandiloquent but gratuitous promises to stop the erosion of biodiversity, which arguably explains why so many conservation scientist and activists, motivated at root by their moral and ethical impulses, endorse a strategy manifestly geared toward capturing money and personal recognition.

Major contributions by those who anticipated as early as the 1980s that biodiversity science was a vehicle for a political agenda can be used to identify how to get out of this predicament. In the context of neoliberal globalization, struggles for local autonomy and knowledge have relentlessly challenged attempts to impose governance models for biodiversity [START_REF] Escobar | Territories of difference[END_REF]. In a similar vein, anthropological studies show that in what we, Western scientists, disdainfully call "local communities," people can be incredibly discerning and creative in manipulating discourses and institutions that we ingenuously conceive as being devoted to biodiversity conservation [START_REF] Hanson | Engaging green governmentality through ritual[END_REF].

The first lesson we should learn from these approaches has to do with the words we use. Language choices can undercut our mission [START_REF] Johns | Caring, killing, euphemism and George Orwell: how language choice undercuts our mission[END_REF].

If euphemism and buzzwords are blindly accepted, unsubstantiated assertions about how biodiversity contributes to ecosystem function and human well-being invade our reasoning [START_REF] Goldstein | Functional ecosystems and biodiversity buzzwords[END_REF]. Beyond word usage, efforts to reveal the values and goals underlying biodiversity discourses are needed to overcome the debilitating myth of consensus [START_REF] Peterson | Conservation and the myth of consensus[END_REF].

As a first step toward acknowledging this normative complexity, we could venture interpretations of the political significance of our work by highlighting, for example, the need to couple ecological work on invasive species with socioeconomic reflections on globalization. In the same vein, we could make it a rule to attempt (informal and tentative) multidimensional impact assessments of our work. For example, instead of complacently highlighting conservation implications, we could highlight how many kilometers we traveled to develop our projects and promote them and the contributions of our work to global change.

Such information would usefully raise awareness that our actions are more equivocal than we assume. However, they should not feed the illusion that virtuous individual action is the key to solve global environmental problems. They should be part of a broader attitude through which we step back and critically analyze the settings in which we act and think. We need to take it upon ourselves to manipulate the available discourses and institutions by taking them as they are (that is, as elements of a strong and resistant politically loaded processes), rather than as we want them to be (that is, as faithful and transparent attempts to solve the biodiversity crisis).

Acknowledgments

We thank 2 anonymous reviewers and B. Callicott for their important suggestions and constructive criticisms.


Eric Delagnes

Dominique Breton

Francis Lugiez

Reza Rahmanifard

Keywords: Analog-digital conversion, Time measurement, Delay lock loop, Mixed analog-digital integrated circuits, Frontend electronics, CMOS

During the last decade, ADCs using single ramp architecture have been widely used in integrated circuits dedicated to nuclear science applications. These types of converters are actually very well suited for low power, multichannel applications. Moreover their wide dynamic range and their very good differential non-linearity are perfectly matched to spectroscopy measurement. Unfortunately, their use is limited by their long conversion time, itself limited by their maximum clock frequency. A new architecture is described in this paper. It permits speeding up the conversion time of the traditional ramp ADC structures by a factor of 32 while keeping a low power consumption. Measurement results on a 4-channel, 12-bit prototype using a 3.2 GHz virtual clock are then presented in detail, showing excellent performances of linearity and noise.

I. INTRODUCTION

HE trend in data acquisition systems for modern physics experiments is to digitize signals closer and closer to the detector. With the very high level of integration achievable with modern submicron technologies, the benefit of integrating inside the same chip the analog front-end, the digitization, and a part of the digital treatment is becoming more and more obvious. Nevertheless, the design of high performances ADCs remains a difficult task. As the nuclear science detector granularity is continuously increasing, and thus the number of channels, the readout circuits are becoming massively multichannel. For these two reasons, multi-channel integrated ADCs are becoming necessary. Multi-channel lower speed ADCs, associated with demultiplexing structures based on fast analog memories, may also replace a high speed ADC.

Today, a large spectrum of ADC architectures is available. But, for applications with a large number of channels and a dynamic range higher than 6 bits, flash architecture are excluded by power dissipation and area constraints. Semi-flash or pipelined architecture, at the basis of most of the modern commercial ADCs, are better suited. But they are more difficult to design, especially if good Manuscript received March 12, 2007. E. Delagnes and F. Lugiez are with CEA, DSM/DAPNIA, CE-Saclay, F-91191 Gif-sur-Yvette Cedex, France (email: eric.delagnes@cea.fr).

D. Breton is with CNRS/LAL, BP34, 91898 Orsay, France. R. Rahmanifard was with CEA/DSM/DAPNIA during the chip design, he is now with, L.E.A. Immeuble Lafayette 2 Place des Vosges 92051 Paris La Défense , France differential linearity performances are required. Successive approximation architectures are easier to design, but their area may become prohibitive if a large dynamic range and a good linearity are needed.

II. THE SINGLE RAMP ADC ARCHITECTURE AND ITS LIMITATIONS

At last, the single ramp architecture appears to be the easier to design and the most adapted to multi-channel circuits. It has been widely used in front-end ASIC [START_REF] Milgrome | A 12-bit analog to digital converter for VLSI applications in nuclear science[END_REF] for two decades. Several types of implementations of this architecture are possible. In the most efficient one, the voltage to digital conversion is performed by measuring the time between the start of a voltage ramp and its crossing, detected by the comparator, of the voltage to be converted. Classically, the time measurement is achieved by a counter started synchronously with the ramp as shown on Fig. 1. To avoid metastability effects, a resynchronization of the comparator output by the clock of the counter is required to stop or memorize the counter state. The main advantage of this particular implementation is that , as appearing on Fig. 2, the counter and the ramp generator can be shared between the channels so that the ADC part replicated in each channel can be reduced to a discriminator and a memory used to copy and memorize the counter state when the discriminator triggers. So the power consumption and the area used can be very small even for high dynamic range and the linearity, mainly dominated by that of the ramp generator, can easily be very good. But, unfortunately, the use

III. A NEW ARCHITECTURE TO SPEED UP SINGLE RAMP ADCS

A. Global Architecture and main choices.

In the new architecture, as in the usual one, the analog to digital conversion is performed by a time measurement, but with a better time resolution. To achieve this, we propose to virtually increase the counter clock frequency. For this purpose, a structure similar to those of modern Time to Digital Converters, also used in the field of High Energy Physics instrumentation [START_REF] Bailly | A 16-channel digital TDC chip with internal buffering and selective readout for the DIRC Cherenkov counter of the BABAR experiment[END_REF][START_REF] Mota | A High-Resolution Time Interpolator Based on a DLL and a RC Delay Line[END_REF], will be used to measure the ramping time. As in modern TDC, the most significant bits of the conversion are obtained by a counter operating at moderate Fck frequency while the least significant bits are obtained thanks the use of a Delay Lock Loop (DLL). Many different implementations of this principle are possible, we have chosen one based on the Nutt method also used in [START_REF] Bailly | A 16-channel digital TDC chip with internal buffering and selective readout for the DIRC Cherenkov counter of the BABAR experiment[END_REF].

As in the previous designs, the counter and the ramp are started simultaneously. When a comparator triggers, its output is synchronized by the clock to memorize the state of the counter. In parallel the asynchronous signal output of the comparator is sent to the input of the DLL. Then the DLL is frozen by the resynchronized comparator signal, so that the DLL memorized state is a measurement, with a 1/(m.F ck ) precision, of the elapsed time between the triggering of the comparator and the next clock edge used to memorize the state of the counter.

The main interest of this architecture is, for a given clock frequency, to decrease by a factor m the time required for the conversion and this without any power consumption penalty.

B. Main specifications for a demonstrator chip.

To validate this architecture a 4-channel demonstrator, named WILKY, has been designed in the AMS CMOS 0.35µm technology. Its specifications were defined to match those of the SAM chip [START_REF] Delagnes | SAM: A new GHz sampling ASIC for the HESS-II frontend electronics[END_REF] for the H.E.S.S.-II experiment:

-12-bit dynamic range (and precision) over a 2 V full range: LSB value of 0. This implies that in the WILKY chip the seven most significant bits will be obtained from the counter whereas the five less significant ones will come from the DLL.

IV. DETAILED DESIGN DESCRIPTION.

A. The Counter.

As in the previous designs [START_REF] Milgrome | A 12-bit analog to digital converter for VLSI applications in nuclear science[END_REF], the counter is using gray code. In this code, only one bit is changing between two consecutive codes. This minimizes the digital noise and decrease the power consumption of both the counter itself and the digital buffers needed to drive the counter outputs through the channels. This counter is based on a cascade of elementary modular asynchronous blocks.

Its depth is programmable from 6 to 10 bits with a nominal value of 7 bits.

B. The Synchronizer.

To deal with metastability effects, the block resynchronising the discriminator output is using two cascaded RS latch. The first one has its clock input connected to the clock, the second one uses the clock delayed by four cascaded elementary delays similar to those used in the DLL and using the same control voltage. This design minimizes the delay induced by the synchronizer.

C. The DLL.

As shown on Fig. 4, the total propagation time of the DLL must be larger than the clock period to compensate for the latency of the synchronizer and to deal with edges effects and unexpected delays in the design. For this purpose, m+p elementary delays, of 1/(m.F ck ) each, are used. In the WILKY prototype the value m=32 and p=8 have been chosen. The DLL elementary delay is the cascade of two starved inverters. The DLL states are memorized by RS latches, more compact than DFF, when the resynchronized comparator output is triggered.

The main advantage of this architecture including a DLL in each channel is that digital signals are propagating in the DLL only after the comparison is achieved. It minimizes digital activities and so power consumption and noise compared to alternative designs with master DLL continuously operating with the clock [START_REF] Bailly | A 16-channel digital TDC chip with internal buffering and selective readout for the DIRC Cherenkov counter of the BABAR experiment[END_REF]. But it requires special calibration phases during which the total delay of each DLL must be servo-controlled to (m+p)/(m . F ck ). To perform this operation, a calibration pulse is sent to the input of the DLL. The phase of the DLL output is compared with the one of the calibration pulse delayed by one clock period (by a DFF) plus p elementary delays.

The output of the phase comparator drives a charge pump providing the voltage controlling the elementary delay value. A minimum of two clock periods are required for this calibration phase, adding extra dead time in the conversion. Practically, at the "cold start" of the ADC operation, 70 calibration pulses are required to ensure the DLL convergence. Afterward, a 50µs calibration periodicity is enough to compensate the charge pump leakages. In the nominal operation, with a 700 kHz repetitive conversion rate, a 2-clock duration calibration phase only is performed at the beginning of each conversion.

D. The ramp generator.

The fast conversion implies a larger sensitivity to the timing parameters of the ramp generator. Both the slope and the offsets of the ramp must be stable with temperature. To achieve a good linearity, the ramp generator, shown on Fig. 5, is based on the integration of a constant current by an active integrator. To ensure the required stability with temperature:

-The current source is servo-controlled.

-All the injected charges are minimized or cancelled by use of dummy switches. -The differential input pair of the integrator operational amplifier is biased with a transconductance stable with temperature, in order to ensure a constant gain-bandwidth product of the OPAMP. The rms noise of the voltage ramp scales with the time as:

V n = 1/C i .( A . t + B . t 2 ) 1/2 (1)
where C i is the integration capacitor and A and B coefficients related to respectively the thermal and 1/f contribution to the noise of the current source.

For a given ramp noise, a faster conversion, like that possible with this design makes it possible to decrease the value of the integration capacitor and then to decrease the size of the ADC. In this design the value of this capacitor is only 5 pF.

E. The comparator.

The fast conversion makes this block really critical. Its delay must be very stable with both the input level and temperature variations. To achieve this, a first solution would be to use a very fast comparator. This solution was eliminated because of its huge power consumption, incompatible with the replication of the comparator in each channel. Instead, a moderate speed, low power structure has been chosen. As shown on Fig. 6, the comparator is based on the cascade of three moderate gain stages followed by a digital level restorer. Actually, this structure is the one offering the best speed-power trade-off. The three *10 gain stages of Fig. 7 (right side), are based on simple differential stages. A cascod pair is used to increase speed and to decrease sensitivity to the input level. These gain stages are biased with a temperature-stabilized current source. The active loads of these stages consist of a PMOS used as a diode in parallel with a second PMOS used as a voltage-controlled resistor. This load behaves like a resistor for small signals and limits the output swing for larger signals. The active resistor value is controlled by the voltage reference common to all the channels of Fig. 7 (left side). This reference uses a replica of the active load to servo-control the gate voltage of the PMOS resistor in order to set the quiescent output voltage of the gain stage and therefore the resistance of the active load. The carefully control of the output swings and output resistances ensure a good stability of their delays.

F. Maximum conversion rate.

The practical conversion time is (6 + 2 N c )/ F ck where N c is the number of bits obtained with the counter. The 6 extra clock periods are needed for the following reasons: -2 for DLL calibration before conversion -2 after the ramp start to wait for the ramp to enter its very linear region. -1 for the comparator output synchronization.

-1 for DLL encoding.

For a 100 MHz clock Frequency, the conversion time is then 1.34 µS, corresponding to a 746 kHz maximum rate.

V. PERFORMANCES OF THE PROTOTYPE CHIP.

The 4-channel WILKY prototype is very compact. The area of the common ramp and counter block is 300 µm x 300 µm whereas 1 mm x 34 µm are needed for each channel. The form factor has been optimized for a multichannel application.

On this prototype, the depth of the counter together with the slope and the offset of the ramp are tunable in order to explore several possible operation modes of the ADC.

The ADC prototype has been tested using a USB test bench based on a 16-bit 4-channel DAC with a 75 µV LSB. The DAC channels are used as quasi-static inputs for the ADC prototype.

A. Test in the Standard Mode.

The prototype has been firstly tested in his nominal configuration: 12-bit dynamic range, 100 MHz clock. In this configuration, with a power supply voltage of 3.3 V, the power consumption is only of 3.3 mW + 0.5mW/channel. The ADC LSB, calculated from the transfer of Fig. 8 function is 534 µV. It can be noticed that the range of the ADC is actually larger than 2V, and that the code delivered by the ADC can be larger than 4096 because the maximum counter depth was set to 8 bits. For each input voltage of the Fig. 8 transfer function, 512 acquisitions have been performed and standard deviations on these measurements have been calculated. They are plotted on Fig. 11 as a function of the mean ADC code obtained. The noise is smaller than 0.6 LSB and is increasing with the ADC code as expected from the theoretical noise behavior of the voltage ramp. As for INL, the noise characteristic also reveals a 32 ADC-count periodicity. The ADC Differential Non-Linearity (DNL) has been characterized using the statistical density of code method assuming a good enough linearity of the test DAC. On Fig. 12, the normalized density of code is plotted as a function of the ADC code. It is equivalent to 1 + DNL. The DNL value is smaller than +/-0.2 LSB peak to peak, or equivalent to 0.1 LSB rms. As for INL and noise, a zoom on the DNL characteristic reveals a pattern with a periodicity of 32. This pattern appears to be the major contributor to DNL. In Fig. 13, the statistical density of code method has been applied only to the 5 bits provided by the DLL. The characteristic is not flat: some codes are more probable than others. It means that the DLL is not perfectly linear. The DNL of the DLL can be estimated to +/-0.15 LSB, corresponding in the time domain to +/-50 ps peak to peak or 20 ps rms. This pattern is the main contributor to the ADC DNL and INL periodicity. It is similar on all the channels and is probably due to digital couplings inside the DLL.

The offset between channels is 15 mV peak to peak in good agreement with monte-carlo simulations. The spread of transfer function between channels is smaller than +/-1/1000, this is slightly larger than the expected value. No crosstalk between channels is measurable.

Fig. 14 demonstrates the good stability of the ADC measurement with time for a fixed input and Fig. 15 its low sensitivity to temperature variations. The drift due to temperature is less than 3 LSB in a 10°C range. It is better than our expectation and sufficient for our applications. The shape of the temperature characteristic appears to be identical on all channels.

The performances of the ADC used in the standard mode are excellent and comparable to those of a genuine 12-bit ADC. If we consider that the prototype is a real 12-bit ADC we can calculate its Figure of Merit (FOM) defined in [START_REF] Draxelmayr | A 6b 600MHz 10mW ADC array in digital 90nm CMOS[END_REF] by

FOM = (P / (2 NBits . 2 . F BW )) (2)
where F BW is the ADC Nyquist frequency -we will take equal to conversion frequency divided by 2 for the calculation -and P is the ADC power consumption.

The FOM is equal to 1.2 pJ/conversion in the case of a single channel ADC but is decreased to respectively 0.4 pJ/conv and 0.2 pJ/conv in the cases of a 4 or a 32-channel ADC as the power of the common block is shared between the channels. These FOM values are comparable to the one obtained on the best ADCs using deep-submicron technologies. This has to be temperate by the fact that there is no track and hold stage included in this architecture.

B. Tests in other conditions.

The ADC has been also successfully tested with 25 and 50 MHz master clocks. In theses conditions the results are comparable to those obtained at 100 MHz with only a 10 to 20% noise increase for the largest codes due to the largest noise of the longer ramps as expected from [START_REF] Milgrome | A 12-bit analog to digital converter for VLSI applications in nuclear science[END_REF].

A test has also been performed with a ramp slope decreased to obtain a larger LSB value of 1 mV, but over a dynamic range of only 8 bits. With F ck =100 MHz it means a 140 ns conversion time. The INL, measured in these conditions, is plotted in Fig. 15. It is smaller than +/-0.7 LSB, clearly dominated by the modulo-32 pattern. This is again due to the DLL non linearity contribution which remains unchanged in this case compared to the nominal one. The funny shape of the variance versus mean ADC code characteristic of Fig. 16, measured in this condition, is exactly the one due to quantization effects expected on a low noise ADC: the variance is 0 when the voltage is in the middle of the code and is reaching a maximum value of 0.5 when it is approaching the limit between two codes. A noise of 0.15 LSB rms corresponding to the ramp and the comparator noises have been computed from this characteristic. The chip has also been characterized with a smallest LSB of 270 µV over a dynamic range of 13 bits corresponding to a longer conversion time of 2.6 µs. In this configuration, the INL, plotted on Fig. 18 is smaller than +/-3 LSB. The noise plotted in Fig. 19 is increasing from 0.7 to 1.2 LSB rms with the ADC code. This is corresponding to almost the same noise voltages than in the standard mode. For this reason this LSB value seems to be the smallest practicable. To decrease it, the noise of the ramp generator at large ramp voltages should be decreased by increasing the integrating capacitor of Fig. 5.

VI. CONCLUSION.

A new multi-channel single ramp ADC architecture has been proposed, prototyped and tested. It permits increasing the conversion speed by a factor of 32 without power consumption or other performances penalties. The characterization of the prototype in its nominal 12bit/1.34µs configuration has shown excellent performances compatible with a real 12-bit operation together with a good stability. Its differential nonlinearity performances make it usable for spectroscopy applications. Considering these excellent results, a 64channel version of this ADC is going to be integrated on the next generation of a gigaHertz analog memory dedicated to the future Cherenkov Telescope Arrays [START_REF] Delagnes | SAM: A new GHz sampling ASIC for the HESS-II frontend electronics[END_REF].

Extra measurements, made on other configurations have shown that it is possible to easily adapt this architecture to other application as long as the LSB remains larger or equal to 0.5 mV. For a smallest LSB, the ramp generator should be re-optimized to reduce the noise. Some other configurations not tested here, as 5 bits / 100 ns, where the conversion is achieved by the sole DLL may also be interesting for tracker electronics.

Fig. 1 .

 1 Fig. 1. Chronogram of the standard single ramp ADC operation.

Fig. 2 .

 2 Fig. 2. Possible architecture for a standard multi-channel single ramp-ADC.

Fig. 3 .

 3 Fig. 3. Bloc diagram of the improved single ramp multi-channel ADC.

Fig. 4 .

 4 Fig. 4. Block diagram of the DLL and its servo_control.

Fig. 5 .

 5 Fig. 5. Principle of the common ramp generator.

Fig. 6 .

 6 Fig. 6. Principle of the comparator.

Fig. 7 .

 7 Fig. 7. Replica-based voltage reference (left side) and Gain stage (right stage) used in the comparator.

Fig. 8 .

 8 Fig. 8. ADC transfer function in standard mode (data and linear fit superimposed).

Fig. 9 .

 9 Fig. 9. Integral Non Linearity as a function of ADC output code. The Integral Non Linearity (INL) plotted on Fig. 9 shows the residue to a linear fit performed on the data of Fig. 8. The INL is less than +/-1 LSB over the 12-bit range. A zoom of the INL, shown on Fig. 10, reveals a periodicity of 32 ADC counts in the INL characteristic.For each input voltage of the Fig.8transfer function, 512 acquisitions have been performed and standard deviations on these measurements have been calculated. They are plotted on Fig.11as a function of the mean ADC code obtained. The noise is smaller than 0.6 LSB and is increasing with the ADC code as expected from the theoretical noise behavior of the voltage ramp. As for INL, the noise characteristic also reveals a 32 ADC-count periodicity. The ADC Differential Non-Linearity (DNL) has been characterized using the statistical density of code method assuming a good enough linearity of the test DAC. On Fig.

Fig. 10 .

 10 Fig. 10. Zoom on the Fig. 10 plot.

Fig. 12 .

 12 Fig. 12. Normalized statistical density code as a function of ADC output code.

Fig. 13 .

 13 Fig. 13. DLL codes probability density.

Fig. 14 .

 14 Fig. 14. Stability of the ADC measurement with time. Each plotted dot is the mean value of 512 measurements.

Fig. 15 .

 15 Fig. 15. Variation of the ADC measurements with temperature. Each plotted dot is the mean value of 512 measurements.

Fig. 16 .

 16 Fig. 16. ADC Integral Non Linearity as a function of ADC output code in the 8bit, 1mV LSB configuration.

Fig. 17 .

 17 Fig. 17. Zoom on the variance vs mean ADC measured in the 8-bit, 1mV LSB configuration.

Fig. 18 .

 18 Fig. 18. ADC Integral Non Linearity as a function of ADC output code in the 13bit, 270µV LSB configuration.

Fig. 19 .

 19 Fig. 19. ADC noise as a function of ADC output code in the 13bit, 270µV LSB configuration.

  Fck is the clock period of the counter. So for a 12-bit conversion, making use of a 100 MHz clock, which appears to be a maximum for reasonable power consumption, 40 µs are required. This time is prohibitive for a lot of applications.

	A Low Power Multi-Channel Single Ramp ADC
	With up to 3.2 GHz Virtual Clock

T of this kind of ADC is limited by its long conversion time. In fact, for an N bit conversion, it requires 2 N / Fck, where

  5 mV -Power consumption < 1 mW per channel -Easily extendable up to 64 channels. -For this design, nominal values of F ck = 100 MHz and m = 32 have been chosen. Thus the LSB time step is 312.5 ps and the maximum conversion time is 1.3 µs. It corresponds to a virtual frequency of 3.2 GHz. -Reasonable stability with temperature.