Advertisement

Analog Angle Blog

Reference Designs: Strong Start but Weak Finish?

These days, nearly every released component is supported by vendor tools such as Spice or other models, evaluation software, critical code segments, and reference designs.

The term “reference design” covers a lot of range, of course. It can be a basic “how to apply” schematic; a detailed schematic with bill of materials (BOM); or, more likely, a schematic, BOM, PC-board layout, operating and test code, and more. Even a basic building block such as an op amp usually comes with some suggested circuits for likely applications, if not full reference designs.

You can't blame a design engineer for wanting to have reference designs. After all, who wants to re-invent the wheel? Who, in theory, should know more about a new component than its designer? There are occasional lapses in good quality by some vendors.

An example of a bad reference design. If you receive a reference design/eval board that looks like this, switch to another vendor.

An example of a bad reference design. If you receive a reference design/eval board that looks like this, switch to another vendor.

But that's the exception. Reference designs make a lot of sense, especially for a constrained application which is fully defined by an industry standard, such as one in the IEEE 802.xx family, where it is easier to evaluate a successful outcome.

A reference design, especially for analog/RF/power-related circuits, can be a good start but may also lull the designer into underestimating the actual project, for many reasons:

  • '…perhaps just one board was built…'

    “…perhaps just one board was built…”

    Just because you see the design in an application note doesn't mean that it was actually built, nor that it was adequately tested (don't be shocked). Or perhaps just one board was built and run through its paces, but the design was not evaluated for component tolerances, temperature drift, and other real-world factors which afflict designs;

  • A successful analog design requires lots of passive components, and many these of the have critical first- and second-tier parameters. For example, it is not enough to know that a 4.7mH inductor was used and thus is called out on the schematic and BOM; you usually also need to know its DC resistance, frequency-related specs, and more. So the reference design really needs to identify the vendor and model number of the part, so you can check out those other parameters;
  • While there is a lot of commonality among analog applications, there are also critical differences which may make the reference design an “almost OK” fit rather than a really good match — and the gap may be enough to require careful consideration. For example, a reference design for a 300-watt motor drive/control may not be a good fit for a 350-watt motor situation, although it may be a good starting point. Many things may need to be changed when going from 300 watts to 350 watts, depending on the margins and headroom of the lower-power design;
  • Finally, layout, layout, layout. Even if the reference design has been vetted and includes a PC board layout, most applications will also require other circuitry which is unique to the end-application in addition to the reference design. That other circuitry brings with it subtleties in power, grounding, system noise, IR drop, EMI, thermal drift… the list goes on. So even a very good, properly qualified reference design may need some moderate-to-major tweaking based on the ripple-effect of the rest of the design.

There's nothing wrong with using a properly designed and evaluated reference design. But it's a naïve engineer, or one under serious schedule pressure, or one with a manager who believes in miracles, who thinks such a design can just be dropped in and that's that, end of story. In some ways, a less-complete reference design is more realistic, because it doesn't lull its user into overconfidence with the veneer of completeness, and the engineer knows that a lot more work will be needed.

What's been your experience with using reference designs? Have you ever been able to use one “as-is”? Have you ever been badly mislead by one? Have they given you a good head start towards project completion?

Related posts:

11 comments on “Reference Designs: Strong Start but Weak Finish?

  1. etnapowers
    November 12, 2013

    “it is not enough to know that a 4.7mH inductor was used and thus is called out on the schematic and BOM; you usually also need to know its DC resistance, frequency-related specs, and more”

    That's an issue very common when an engineer has to build and debug an application board, because many times the choice of one particular inductor  from a specific vendor can create noise or malfunctioning on the application board, depending on the application to be implemented.

     

  2. etnapowers
    November 12, 2013

    “Just because you see the design in an application note doesn't mean that it was actually built, nor that it was adequately tested”

    I believe that this depends on the vendor, normaly big companies guarantee a complete debugging and testing of the products that they sell, for small vendors the situation is a little bit different: a not adequate testing or debugging is not impossible. 

     

  3. eafpres
    November 13, 2013

    @Bill–reference designs are an interesting part of the value chain in the electronics industry.  Consider a passives component company.  There isn't a lot to differentiate much of your product line.  So they often work hard to get onto that reference BOM, because it definitely increases their chances of being on the first customer design, which definitely increases their chances of being on the first production release.

  4. eafpres
    November 13, 2013

    A long time ago in another life my group designed antennas and antenna solutions.  We had a particularly challenging design in the very early days of 802.11b for a PCB antenna design that worked on a PCMCIA card WiFi adaptor.  Yes, it was so long ago that PCMCIA slots were widely used on laptops and laptops didn't have WiFi built in so companies made money selling PCMCIA WiFi plug-ins.

    Well, on that board were two large value inductors.  We made a serious error (when I say we, I mean I) by allowing sourcing to change the vendor of the inductor for production.  When the customer got the first production parts they didn't work as expected.  It was traced to the change in the non-inductive properties of the inductor.  Very painful lesson, which supports your point about needing to know other parameters for all the components before you start switching things out.

  5. eafpres
    November 13, 2013

    OK, so my last reference design story dates to the same early days of 802.11b.  There was an explosion of chip vendors trying to get business with comptuer and accessory OEMs.  Pretty much all those vendors were in Taiwan.  Most of them wanted a PCMCIA reference design since that was the hot form factor.  I spent collectively a month or so in Taiwan on multiple visits marching from vendor to vendor trying to get on their reference designs with my antennas (we had a SMT antenna for 2.5 GHz).  This required educating group after group about antenna matching, layout, gain, patterns, etc.  My design group did huge amounts of work taking circuit boards from these chip guys, and dealing with whatever layout they had done and whatever room they had left us to locate and get to work the antennas.

    The good news is we eventually sold a lot of those SMT antennas, and they are still in production today, even though I'm long gone.

  6. samicksha
    November 13, 2013

    @etnapowers: Thanx for summary, and that covers all, cross-sectional area, and resistivity of the conductor, temperature affects resistivity.

  7. etnapowers
    November 13, 2013

    @samicksha: You're welcome. As you correctly said , temperature affects resistivity, I want to add that temperature affects also reliability. Some components have reliability issues that other components, coming from a different vendor, don't have.

  8. amrutah
    November 13, 2013

    When it comes to selling chipsets having a working reference design is a big deal in winning a customer, but as Bill rightly put it, “Strong Start, but Weak Finish” -the engineer has to seriously understand the conditions/requiremtns of the applications before using it.

    The problem in understanding all the conditions of a reference design largely also depends whether the original IP designer is still available to provide the insights.

  9. amrutah
    November 13, 2013

    @etnapowers:

         Actually for smaller companies, having a refeence design in their kitty is a big advantage when it comes to selling a product.  They can better estimate the BOM, the duration or time-to-market of the next-gen product. As you mentioned “through testing and proper understanding of the application specifications” should immensely help them use the Reference designs.

  10. Davidled
    November 13, 2013

    I guess that automatic testing tool might be helpful for small company. Testing procedure will be process inside tool to debug the circuit or IC. For example, robotic based vision system would be installed on top of IC as part of testing. These tools might be available for rent.   

  11. etnapowers
    November 14, 2013

    @DaeJ: I think that an automatic test tool is really useful for standard products, in case of products having new features or technologies a customized test tool is required.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.