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Letting Go of the Old Ways

A while ago, one of my colleagues was trying to map a simple, common analog circuit onto one of the programmable system-on-chip devices that my company makes.

It's an interface to a passive infra-red (PIR) sensor, and variants of it have been appearing in data sheets and app notes for decades, not only from the sensor manufacturers but also from opamp vendors who rightly see it as a good example of how to do something very non-critical using some very cheap amplifiers and some very pedestrian passives. And indeed he was able to fit it into this new, low-end product in our range. Two opamps, check. Two comparators, check.

A powerful CPU to do some simple calculations and drive some new-fangled digital interface — that's a bonus! The resulting circuit took almost as many components, almost as much board area and product volume and — don't tell anyone I said this — cost more. The figure shows the circuit (or one very much like it), extracted from the Murata PIR data book (which is freely available online, no secrets there).

A smart marketer once wrote that you shouldn't confuse your customer's compelling reason to buy with your compelling reason to sell. This looked like substitution for substitution's sake, to get our part number onto the very busy schematic. It seemed to me that it completely neglected the real reason why you might think of using such a cool new technology as ours, which is to implement a cool new way of solving the problem at hand!

Now, you might argue that for such a humdrum application, there's no economic return in changing from a design that already uses components with prices so low that you need double precision arithmetic to represent them. Well, perhaps not quite. But, by coincidence, I had also started looking at this very circuit with a view to addressing its major problem: It's too darn big!

The design contains seven capacitors (not including the rail bypass caps) of value up to 22 uF and about 15 resistors and two diodes. Even if we put the opamps and comparators into a bare die version of the target PSoC device, the board would still be way larger than the sensor — and I wanted it to be smaller than the sensor, for obvious reasons we don't need to go into.

Well — skip to the end, this is only a blog, after all — I figured out that with so much digital processing capability available, we ought not to need anything like that amount of auto-pilot analog passivity. By combining a lightly filtered low gain front end with a low power ADC and a digital servo loop to filter off baseline drift, I managed to chop the passive component count right down (to five, in the simplest case), and eliminate any capacitors higher in value than a few nF. And make the system's transfer function highly programmable for sensitivity and time response into the bargain.

One of my bright, young colleagues implemented and optimized it on a PSoC 4100, and, hey presto, a PIR interface that will fit on the rear surface of a PIR sensor, if you use the small bare die. We're going to write a more detailed article on that some time, but I only use this to make a point: Sometimes the old ways aren't the best ways. I'm an analog guy through and through by training and experience. But I'd be letting down my company and my customers if I wasn't open to the possibility that a new way of working might be better in some respects.

So, don't let people browbeat you into thinking that retro is the only respectable approach in analog. Paradigms shift discontinuously, and some analog engineers fear discontinuity. When you encounter a design problem, read between the lines. Or in this case, between the huge, old-fangled passive components.

There's a less-than-amusing coda to the story. I wanted to get some demo boards made up to show the solution off to some excited customers. We instructed an external group, which included several people with years of analog electronic experience, to do the job. But for some reason they never received the project describing the new design. They used their initiative and built… the old application circuit from the sensor data sheet. Hey ho.

13 comments on “Letting Go of the Old Ways

  1. eafpres
    March 22, 2014

    Hi Kendall–great little case study.  The eternal struggle between brute force and finesse.  I hope you eventually got the right boards made.

    This got me to thinking about electromechanical systems (and aren't most systems electromechanical these days?) and working very hard on component design and integration to lower the BOM count.   Things like using a plastic housing and some compressible foam to not only seal up the package (ex: a roof-mounted antenna module) but also hold things in place inside.  Addind divots onto the metal baseplate to align things without more parts.  Spot welding antenna radiators to a ground plan to avoid any kind of interconnect.  And so on.

    I've seen a lot of engineers design things they way they have always done.  My father drilled into me as a youth and teenager: If you are doing something the way it has always been done, you are probably doing it wrong.  He generally attributed this to Charles Kettering or Kelly Johnson.

  2. RedDerek
    March 23, 2014

    Kendall,

    Many circuits that are over 10 years old could be implemented in a smaller volume and probably at a lower cost. When using or redesigning an old circuit, one should look at how to make thing more efficient – by volume, cost, power, etc. The trick is to get the outside vendors on the same page.

    Converting from analog to a digital processor is becoming more of a reality. But one does have to look at the original specs and be sure the digital version does not generate any potential problems.

  3. Davidled
    March 23, 2014

    ->Converting from analog to a digital processor is more of reality. But one does have to look at the original specs and be sure the digital version does not generate any potential problems.

    In my view, analog world and digital word is coexisted in the industry.  Logic algorithm could be implemented in the digital logic, but signal condition circuit for filtering the signal in order to meet the spec would be designed in the analog circuit. For other example, the control of inductor motor with coil end winding requires a certain torque of rotor with high voltage MOSFET handling on switch transition. signal amplifier require almost analog circuit.  

  4. geek
    March 27, 2014

    ” My father drilled into me as a youth and teenager: If you are doing something the way it has always been done, you are probably doing it wrong”

    @eafpres1: That's a simple saying but it holds a lot of value particularly when you look at the electronics world. Given the rapid pace at which innovation is taking place, you cannot do things the same way. If you still do that, you'd be left behind like anything. You may be able to survive in some other industries but certainly not in this one.

  5. geek
    March 27, 2014

    “A smart marketer once wrote that you shouldn't confuse your customer's compelling reason to buy with your compelling reason to sell”

    @Kendall: That's a very apt saying and something that most electronics designers and manufacturers do not pay a lot of attention to. They are misled into believing that a high quality product will always be a success whereas that's not often the case. The demands of the customers may be very different from what you have to offer even if you have the best circuit. I wonder what can be done to bridge this gap.

  6. chirshadblog
    March 27, 2014

    @tzubair: I feel to bridge the gap best would be to pay personal attention as much as possible towards the customers. Then only you will be able to identify what they want. And then go for a common one after gathering the inputs from the customers. Not easy but have to start if it needs to climb the tree.  

  7. geek
    March 28, 2014

    “I feel to bridge the gap best would be to pay personal attention as much as possible towards the customers. Then only you will be able to identify what they want. And then go for a common one after gathering the inputs from the customers.”

    @chrishadblog: That's one way of going about it. But that doesn't always work in hi-tech products. Customers don't always know what they want and they'd never be able to tell you about it. You have to observe the market and see the trends yourself. As Henry Ford pointed out on his success, “If I had asked people what they wanted, they would have replied faster horses”.

  8. Sachin
    March 31, 2014

    Compared to most of the engineers that I work and interact with on a daily basis, as a young engineer I often find myself out of my depth when it comes to experience. This case study accurately represents a problem that I have to deal with on a near daily basis whereby whenever I come up with a new, insightful way to create a design, I get shot down and it gets done the old way. The result is almost always more costly than I would have made it and then I have to revise it once more.

  9. Sachin
    March 31, 2014

    @Kendal, interesting piece; I couldn't help laughing when I came to the end of this article. I wonder what they said when you finally showed them the boards that you had designed. This is a classic example of why the advancements in analog design are so slow compared to advancements in other fields of technology.

  10. yalanand
    April 30, 2014

    Re: Marketing

    It is interesting to learn about the coexistence of the worlds, that is analog and the digital world. I had not thought of it in that direction. All the same, it is always important to move at the same pace with others while at the same time ensuring that your customers' needs are met. Having the best and most recent circuits may not be the definite answer to meeting the demands of your customers, it may some times be important to remain with the old ways.

  11. yalanand
    April 30, 2014

    Well stated Zubair. Most consumers lack the in depth knowledge on high tech devices that can effectively aid them in coming up with ideas on the desired equipment that can meet their needs. It is therefore not advisable to ask the consumers what their needs are for the purpose of moving away from the old ways. It is important that the engineers observe the trends of the needs of the consumers and come up with implements that will effectively bridge the gap between the old equipment and the latest ones.

  12. yalanand
    April 30, 2014

    It's quite interesting how most of us feel at home with what we refer to as the norm. we are not quite open to change and we prefer our old ways simply because they are much simple and our systems have adjusted to using them. It is good that people have now opted to going the digital way which is quite faster and effective on my point of view. The end generally justifies the means and we should cut off our old ways.

  13. yalanand
    April 30, 2014

    @tzubair, I like your point about marketing any product regardless of its quality. A customer will only go for your product over the rest that fall under the same category of product, only if they know about the benefits that your product has over the rest. There has to be a reason for their action. Do not sit back and wait for a customer to do this research on their own. I am sorry, it will never happen and it has never happened.

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