Unusual design of JWST ADC

Article By : Bill Schweber

The systems engineers and designers working on the James Webb Space Telescope project took a big chance on the SIDECAR approach rather than a standard ADC.

I’ve been following the spectacular success of the James Webb Space Telescope (JWST)—a partnership consisting of NASA, European Space Agency (ESA), and Canadian Space Agency (CSA)—as well as looking into aspects of its design. There’s lots of good information available, including photos of its design and assembly as well as high-level block diagrams (Figure 1).

Figure 1 The high-level block diagram of the JWST only hints at the telescope’s complexity while it introduces many new acronyms. Source: NASA

Looking at the systems, subsystems, and boards, and individual electrical and mechanical components—many of them fully or partially customized, of course—makes me marvel at how the project was “pulled together” to achieve its eventual success, especially as unlike the Hubble Space Telescope, there would be no second chances to fix or adjust it if things went badly.

Yes, it was billions of dollars over budget and years late, but I’m not sure how you even begin to work out a budget and schedule on a project of this type, with all the scientific and technical unknowns, except as a very rough number with at least +100% uncertainty for those figures.

I was especially interested in learning more about the analog/digital conversion (ADC) subsystem, which takes the outputs of the various images sensors and digitizes them. Given the number of new parts which had to be designed and validated for the JWST, I assumed that the designers would try to minimize that number by using space-rated versions of an available ADC for this function, one that had a “track record” in production and design-in idiosyncrasies.

I was totally wrong. Not only did they use a custom converter, but the design was also part of a larger IC which performed many other functions. It is not a standalone ADC but is part of a larger IC dubbed SIDECAR ASIC (System for Image Digitization, Enhancement, Control and Retrieval Application Specific Integrated Circuit), as shown in Figure 2.


Figure 2 The block diagram of the SIDECAR ASIC shows that the ADCs are only a small part of the overall IC (above) while the SIDECAR ASIC is shown mounted on its PB board with connectors (below). Source: SPIE, European Space Agency

It is located next to the detector, like a sidecar on a motorcycle, to minimize the distance the analog signal travels and thus reduce the system noise. The three instruments that will use the SIDECAR are the near infrared camera (NIRCam), near infrared spectrograph (NIRSpec), and fine guidance sensors (FGS).

There are many “players” involved in the SIDECAR ASIC. The ADC subsystem was designed by Dr. Lanny Lewyn, founder of Lewyn Consulting Incorporated (LCI), who was asked to create a 36-channel 19-bit ADC array that is embedded in the imager ASIC. The array of 36 A/Ds was required to fit within an allocated IC width limit. The power limit was 1.5 mW per converter at 100 kilobits/sec, with 16-bit ±2.5 LSB integral nonlinearity (INL) accuracy and satisfy a special imaging requirement of ±0.3 LSB differential nonlinearity (DNL). A 10× speed mode was also required, but at a lower resolution.

Dr. Lewyn used the dimensionless design methodology called Gamma rules that he developed for another ADC project. These dimensionless layout design rules can be ported to multiple technology scales and foundries and represent a significant improvement over an earlier dimensionless circuit design and layout approach (called Lambda rules) created by his advisor at Caltech, Carver Mead. Professor Mead’s textbook “Introduction to VLSI Systems,” co-authored with Lynn Conway, is widely credited for starting the VLSI revolution. The Tanner EDA design suite from Siemens was the primary design tool, with additional design and fabrication done at Rockwell International Corp. (now part of Teledyne Scientific Imaging).

The design uses a successive approximation register (SAR) architecture with a precision capacitor array for the most-significant-bits (MSBs) and a precision resistive divider for the least-significant-bits (LSBs), as shown in Figure 3. To comply with the DNL requirements, an algorithmic method was used for perfectly matching the voltage-switching boundaries of the MSB capacitors to those of the lower LSB resistors. Further, the DNL requirement specifies a random error of only one part in 218,453.

Figure 3 The layout of the SIDECAR ASIC provides additional insight into its many functions and real-estate allocations. Source: SPIE

Using conventional autocalibration techniques to correct for small A/D linearity errors was not an option because of the requirement for low-power operation and the non-random nature of the signals. The INL requirements were met by using a combination of centroiding algorithms that had been previously used by Dr. Lewyn. One of those algorithms, perfect centroiding algorithm, involved duplication and rotation of elements rather than the side-by-side transistor method.

TSIDECAR ASIC was evaluated for basic performance at various ground-based optical telescopes in Chile, Hawaii, and other sites, but that’s only the first step. It was also checked out in the cryogenic vacuum chamber at NASA to assess performance in the harsh environment of space.

Fortunately, the JWST is not a classified project, and there are many good references and resources available, ranging from broad overviews to detailed technical discussions. The systems engineers and designers working on this project obviously took a big chance on the SIDECAR approach rather than a standard ADC, but also decided it was the only way to meet the tight performance objectives which were bounded by stringent constraints. That’s a difficult decision but one that had to be made, tested, evaluated—and succeeded beyond its goals.

Meanwhile, I’m hoping for a well-written book about the project, its many problems and delays, re-invigoration, tradeoffs, and eventually, incredible success. I’d be thrilled to see a quality work similar to “Roving Mars: spirit, opportunity, and the exploration of the red planet” by Steven W. Squyres, the principal investigator on the Mars missions that landed the rovers Spirit and Opportunity in 2004; or “Voyager: seeking newer worlds in the third great age of discovery” by Stephen J. Pyne, a fascinating look at dual missions launched in 1977 and which is still sending back data from beyond the edge of our solar system, and beyond; or “The Right Kind of Crazy: A True Story of Teamwork, Leadership, and High-Stakes Innovation” by Adam Steltzner (with William Patrick) who led the Entry, Descent, and Landing team in landing the Curiosity rover on the surface of Mars. That would truly make my day and give the thousands of people who worked on the project the recognition they deserve.


This article was originally published on Planet Analog.

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.


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