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An Instrument on a Chip? The Minimum-Subsystem Instrument

This overview of the integration of electronic measurement instruments ends with a broader look at the strategy of maximum-integration instrument design. (A subsequent series on integration of a Z meter will demonstrate instrument integration issues in more detail.) We have what appears to be a goal of a four subsystem instrument for a broad range of instruments. The four subsystems are:

  • Analog IC
  • Microcomputer (single IC)
  • Power supply (semi-discrete)
  • User interface (semi-discrete)

This four-subsystem solution might be reduced further by combining the microcomputer (μC) and analog circuits on one IC. Then if the IC is not thermally limited, the supply might be included by standardizing on IC-compatible voltages. The user interface could even be eliminated by considering it a part of a larger system.

Instruments built to plug into back panels of computers use that computer interface through which the commands and acquired data move. A software interface program runs on the user computer from which the instrument is controlled. In this case, the user interface is not eliminated or integrated; the user computer is the fourth subsystem. Seeing the relationship of these subsystems differently might be all that is needed to make progress in how instruments are configured.

As it is, the four-subsystem instrument motif seems like an achievable and realistic goal. Four subsystems and not one shows recognition of the fact that integrated circuits are but a part of a larger reality, that of whole systems. What is to be accomplished by the whole system involves interfaces to human beings and power sources. Each of these expands the scope of technical considerations for integration considerably.

The highly integrated and digital (switching) nature of the μC means it is best kept in a somewhat isolated environment from analog to reduce noise. The isolation is provided by separation at the substrate level. Separate digital and analog ICs not in close proximity to each other solve problems that otherwise would require other discrete components such as shields. So in this case, semi-integration has its advantages.

Power supplies can vary considerably in what is needed, depending on the source. Battery supplies are the simplest and are not readily reducible to monolithic integration, nor would we want them to be if they are to be replaced periodically. Power-line-operated sources require converters. One way of minimizing the supply component is to standardize on various wall-mount supply voltages such as 12V.

Then in-system conversion is accomplished free from the safety compliance issues by dealing with this intermediate 12V bus input instead of 165V (or twice that in Europe). The problem with proliferating wall-mount (or power-strip-mount) and desktop converter boxes is another matter that should be considered in the overall strategy for more integration. However, they do simplify the design of what is in the integrated box.

Even with a static (DC) low voltage as input, the magnetic power components that have yet to be integrated are transductors (transformers and inductors) and capacitors. As switching frequencies increase, the possibility for a single-chip converter increase because the size of C and L decrease. We are entering a phase in converter design where multilayer ceramic capacitors are replacing electrolytic capacitors at the high end of the frequency range.

Research has been ongoing at places such as David Perreault's lab at MIT to integrate magnetic material in with the rest of the electronics. A smaller volume of magnetic material is required at higher frequencies. Suitable transformer coupling can be obtained such that the converters can transfer a given amount of energy each switching cycle from input (primary) to output (secondary) circuit. The transferred power, being the rate of energy, thus increases linearly with switching frequency. Similarly, sufficient inductance can be obtained to properly filter.

The obstacle in this trend is that as switching frequencies increase, so do power losses in magnetic materials. The best ferrites nowadays begin to have excessive losses over 500kHz, and the typical design range is in the 100kHz to 400kHz range for switchers of 100W or more. At higher switching frequencies, the per-cycle energy transfer does not have to be as large and the peak magnetic field density ripple is reduced to reduce power loss. If it is reduced too much, then more power is transferred at the same loss at a lower frequency.

All in all, integration of the supply in a monolithic form is not inconceivable, though it is not here yet. It will be best applied at first to ICs requiring a small amount of power, though most ICs cannot handle much over 5W to 10W anyway, even with a fan blowing directly down on them. With low power requirements, the inductor can be eliminated entirely by using switched-capacitor energy transfer. Therefore, the supply is conceivably integrable.

Finally, the user interface could most simply be integrated by avoiding the issue through an RF link or wired link with a fast serial protocol. This is not anything novel, and it solves the “front-panel” problem, yet it seems like a cheap way to avoid the problem of the final subsystem. If electronics technology were to trend toward a standard user interface, then any instrument (or other similar kind of device) need only communicate with it, and that can be accomplished already through the μC. This might be the final solution, though it is not really optimal for instruments intended to be highly portable, such as handheld units. Or is it? The default “standard user interface” nowadays is a desktop or laptop computer or tablet (iPad) device. These small computers are viable possibilities as front-panels for small instruments and can preserve the needed portability.

One of the ultimate limitations to integration is that as everything becomes smaller, human perception does not evolve fast enough to accommodate changes in technology. An LCD display integrated onto monolithic silicon would be hard to read, even with a magnifier, yet it might be a possibility – unless it is wafer-sized. Some full-color LCD displays selling for as little as $10 each are not much larger than some large monolithic chips. As an example, Newhaven makes a 1.8 inch, 128 × 160 dpi device.

The final integration of the four subsystems onto a single monolithic die of silicon is not preposterous, yet it is not optimal at this time. The intermediate step of integration — that of the four-subsystem scheme — seems to me to be the optimal goal and offers plenty of integration possibilities for present and future analog ICs.

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31 comments on “An Instrument on a Chip? The Minimum-Subsystem Instrument

  1. Vishal Prajapati
    June 25, 2013

    Your blog has inspired me to think about power supply. The most critical part to design is power supply. How would it be easy, if we just need to insert an IC and it will convert the AC supply in to DC right away. Even without need of transformer. The power supply will also be on board then.

  2. Brad Albing
    June 25, 2013

    @Vishal – that would be nice – but at present, probably impractical. You would need an IC built using a high-voltage process – with the corresponding high cost associated with that. And you would also need lower voltage sections of circuitry – so you either need two separate fab-processes – or do it all with the same high-voltage process. Expensive either way.

    Oh – and you'd need galvanic isolation between the input voltage and the output voltage. Also expensive.

  3. goafrit2
    June 25, 2013

    >> How would it be easy, if we just need to insert an IC and it will convert the AC supply in to DC right away. Even without need of transformer

    They present paradigm of external power supply is primitive. But I think the challenge has lack of ways to plug directly to mains. When we move to the realm of wireless charging, the IC for AC to DC will be done easily internally inside systems.

  4. goafrit2
    June 25, 2013

    >> , probably impractical. You would need an IC built using a high-voltage process – with the corresponding high cost associated with that. And you would also need lower voltage sections of circuitry

    Technically, this can be done easily. With the design technique used in Charge Pump, one can do this. Floating gates can also be used. Sure, there are some needs to modify processes. But the generation of HV can be done.

  5. Vishal Prajapati
    June 26, 2013

    Sir, I can understand that providing galvanic isolation is harder to implement in ICs specially when delivering higher power. Still few isolators are available from Analog Devices which uses transofmer kind of thing inside the IC to provide galvanic isolation.

    Sir, I want to know why high voltage ICs are not much? What is the constraint behind making high voltage ICs?

  6. Vishal Prajapati
    June 26, 2013

    There exists AC LEDs which can directly work on AC mains, why can't other ICs work on mains?

  7. CarlWH
    June 26, 2013

    @Vishal – there seems to be several questions here.

    I hope these pointers help

    1) You are correct there are AC LED's, I suggest though you take a look and see how these devices are implemented and how the power is dissipated. You should be able to find some indications of the difficulties.

    2) Galvanic Isolation – to me this means spark reduction in hazardous areas and is used in ATEX type devices. Isolation is required, as it would be on any device, (e.g PCB) where you are mixing voltages and signals.

    3) Available processes- there are high voltage processes available which will allow high voltage devices and logic. These are not expensive, but think about what you are doing when rectifying AC, then think about what this will mean for device sizes on chip. Given that your bridge would dominate the chip, would you really want to do this? Also another thing you may want to consider is capacitance and how easy that is to do on chip.

    When considering AC/DC conversion you really have to put some limits on what you are thinking of. What load you are driving, how much power, what peak voltages and currents, regulation etc?. It would be nice if all our AC supplies were ideal.

    4) A charge pump for me is a DC to DC converter and I am not sure what this comment was aiming at, sorry if I have missed the point.

    In summary, you can put it onto a chip, but you have to ask yourself does this make sense to do so?

  8. Vishal Prajapati
    June 26, 2013

    Thanks Carl, for elaborated response. I think it has cleared all my doubts. And for isolation, it is not just to reduce the spark reduction. For consumer products, device needs to have isolation to avoid electrical shock.

  9. jkvasan
    June 26, 2013

    @Carl,

    Spot on.

    Expecting everything on a chip is similar to using those ready-made coffee-cream-sugar mixtures. Customising taste and flavour is a no-no.

     

  10. Brad Albing
    June 26, 2013

    @VP – the short answer is cost . The higher voltage semiconductor process is more expensive. Then there are issues of certification (i.e., UL testing & listing) and liability if something goes wrong.

    Those ADI devices are very low-voltage parts for passing low level digital signals. Scaling those up to higher power and higher voltage is not easy.

  11. Brad Albing
    June 26, 2013

    @VP – Carl's answer is better than mine – lots more detail from him.

  12. Vishal Prajapati
    June 27, 2013

    @BA, you are right sir, those devices are for low voltage levels. But now newer materials are developing, by which I think there would be some material which can has higher electrical isolation capabilities. We might be able to cramp more magnatic density in an IC.

  13. PCR
    June 29, 2013

    Exactly Vishal, But I would like to add one more word to your comment I believe that it should be EFFECTIVE power supply science these days saving power is more crucial. 

  14. SunitaT
    June 30, 2013

    An LCD display integrated onto monolithic silicon would be hard to read, even with a magnifier

    @Dennis, thanks for the post. True its very hard to read on LCD display. But since the cost of LCD is so less they are still being used in many applications. What about new displays which use LED ? Do you think using such display screens is an option ?

  15. SunitaT
    June 30, 2013

    But now newer materials are developing, by which I think there would be some material which can has higher electrical isolation capabilities

    @Vishal, thanks for sharing this info. Could you please point to me what kind of materials will help us achieve higher electrical isolation capabilities ?

  16. SunitaT
    June 30, 2013

     I believe that it should be EFFECTIVE power supply science these days saving power is more crucial. 

    @Ranasinghe, yes saving power is criteria these days. Many developing nations face severe power shortages these days and it would be great if we develop a product which consume less power.

  17. SunitaT
    June 30, 2013

    Customising taste and flavour is a no-no.

    @Jayaraman, good example. I think that is the reason many people dont prefer putting everything on a single chip. Keeping some things off-chip has its own advantages.

  18. PCR
    June 30, 2013

    Exactly SunitaT I think what we should focus on is the effective way of of using power. And also les power consumptions as you said. 

  19. jkvasan
    July 1, 2013

    Sunita

    Such bottled solutions defeat the very purpose of a design engineer – the freedom. Also, it transforms a designer into system integrator, which many of us may not want to become.

  20. goafrit2
    July 17, 2013

    >>  What is the constraint behind making high voltage ICs?

    I think the key is process. Typical process cannot support transistors which are optimized for high voltage ICs.

  21. goafrit2
    July 17, 2013

    LEDs are made with special transistors which are not options in typical ICs. From my experience, the key differentiator in HV design is not just the circuit architecture but the process that is available for the desig,

  22. goafrit2
    July 17, 2013

    >> Also, it transforms a designer into system integrator, which many of us may not want to become.

    It does happen many times and it has happened to me. There is a new poject and my boss will ask me to modify an existing circuit in a product and use in my work. He knew I could do better but there is no time. In companies, the number killer of innovation is unreasonable deadlines.

  23. RedDerek
    July 17, 2013

    When I worked with Supertex, they dealt with high voltage – up to 400V. The trick is to provide ample guard band around the high voltage and not make sharp corners. But the fab house should be able to provide all that information.

  24. Brad_Albing
    July 18, 2013

    @RedDerek – You should post a layout here or in a blog (assuming you still have some documentation handy that's not proprietary) with dimensional info added so we can see what that would look like.

  25. jkvasan
    July 23, 2013

    @RedDerek,

    I too am eager to have a look at the layout or topology. Please post, if it is possible.

  26. RedDerek
    July 25, 2013

    Let me see what I can remember and share. I will set it up on my blog list. (Just got back from being in the back country for a week – no internet and it was bliss!)

  27. goafrit2
    August 6, 2013

    That is sill the way they design some of the HF systems today. The fab does provide process data which the design must use to design for reliability.

  28. goafrit2
    August 6, 2013

    If you have ever made a capacitor with metal and you surround with guard rings and bands, it is similar. Wish I can post a layout here but not sure who is reading from the firm. But is very close and the same process that is used to guard cap. The idea is to isolate the HV part to reduce noise in the substrate and the whole chip.

  29. jkvasan
    August 12, 2013

    RedDerek,

    Are you back from your retreat?

    Any simple idea or link about the topology before we read your blog would keep us prepared.

  30. RedDerek
    August 12, 2013

    I am back. Just been quite busy with house work and work. I have set up the blog, just need to write it out. I am scheduling a writing session for next weekend for several blogs.

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