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Why Digital Is Analog’s Best Friend (or Should Be)

Although analog circuit design techniques continue to advance rapidly, and companies can still gain a competitive edge through artful circuit design, most observers anticipate a future of incremental gains in pure analog circuit design for individual functions.

Moreover, commercial pressure to achieve high analog performance using low-cost process technology, such as deep submicron CMOS, makes it even harder to deliver high performance. This is because the trend towards deeper submicron process technologies is characterized by lower-voltage supply rails, lower intrinsic gain of active devices, worsening Vt mismatch of MOS transistors, and poorer-quality passive structures.

While it is true that we don't always have to design our analog circuits in processes optimized for high-density digital designs, commercial pressure to deliver more analog performance with fewer and lower-cost IC process options is universal. As a result, companies that do deliver higher performance with less-exotic technologies are most likely to have the competitive edge.

Over the last decade, the balance of forces has changed. Low-area component matching has become more difficult, whereas the area and power required for digital calibration circuits has decreased with Moore's Law. In the face of such trends, it should come as no surprise that classical analog design techniques are being augmented by digital calibration, adaptive signal processing, and nonlinear correction methods.

Some common examples of digital algorithms that improve delivered analog performance are:

  • Dynamic element matching in ADCs
  • In-phase vs. quadrature signal path gain and phase correction
  • Digital predistortion (DPD)
  • DC offset correction
  • Digital frequency offset correction
  • Frequency-dependent group delay compensation and gain droop compensation

These algorithms should ideally be capable of adaption using mission-mode signals continuously (also known as background calibration). In some cases, foreground calibration using special purpose signals may be needed, but these should still be autonomous and not require factory test time. A nonintrusive example of foreground calibration could be initiated at power-on-reset and managed by an on-chip state machine. The least desirable solutions involve factory-test calibrations that add to test time and cannot adapt to changing voltage and temperature conditions in the field.

One difficulty in creating solutions where digital and analog circuits are highly interdependent is that engineers often are specialized in one domain or the other, and often will only seek solutions inside their specialization. The old adage applies: If you have a hammer, everything looks like a nail! To overcome this, the best development organizations will employ cross-functional teams with good communications skills and (ideally) a few key individuals who have strong skills in both domains. Equally important is to have an open culture where engineering managers facilitate cross-domain collaboration and don't favor one engineering discipline (theirs!) over others.

What do you think? Does analog circuit design do best when it keeps itself pure, or are the best modern analog designs to be had by analog and digital making friends through the incorporation of judicious use of digital signal processing techniques?

10 comments on “Why Digital Is Analog’s Best Friend (or Should Be)

  1. RJShank
    January 17, 2013

    I agree with your comments.  I'm primarily an analog guy but I do digital and software.  Sometimes it's just easier to do the job with analog, sometimes with digital, and sometimes with software.  It just depends on the customer's requirements, cost, physical room, and the amount of documentation necessary to satisfy the FAA.

    While I've learned over the years how to use a wrench as a hammer and a nail file as a screwdriver, nothing satisfies like the proper tool for the job.

    Cheers,

  2. Charles Razzell
    January 18, 2013

    I am glad you are open minded enough to apply the appropriate tools for different tasks.  Beyond that, one of the things I was hinting at is the power of a hybrid approach, (often known as digitally-assisted analog/RF). In such cases you can sometimes get a very powerful combination where the deficiencies of analog are taken care of by digital techniques, while retaining the unqiue advantages of the analog signal path.

  3. SunitaT
    February 28, 2013

    Analog is non-linear by nature and thus we face many issues like gain variations, DC offset accumulations. Any monitoring algorithm (implemented in digital) which can change some of its paramters on the fly will overcome analog circuit deficiencies.

  4. Charles Razzell
    February 28, 2013

    @SunitaT, 

    I agree, this is certainly the general idea. Making this work well for specific cases is not as simple as it sounds, but the rewards in performance often make it well worth the effort.

  5. SunitaT
    February 28, 2013

    Beyond that, one of the things I was hinting at is the power of a hybrid approach, (often known as digitally-assisted analog/RF).

    @Charles, true. Hybrid approach is the most powerful approach to overcome the shortcomings of both digital and analog. But while implementing Mixed mode design we face totally new issues. Digital switching causes lot of design issues in Analog and thus we should be careful while implementing mixed mode design.

  6. Charles Razzell
    February 28, 2013

    @SunitaT,

    I totally agree with your comment about the difficulty. I was aleady saying something very similar when our posts crossed!

  7. Brad Albing
    March 20, 2013

    A man after my own heart. I strongly favor analog solutions but I'm quite aware that there are digital solutions that will solve some of the problems I've dealt with in a superior manner. So best not to get stuck in one way of thinking

  8. Brad Albing
    March 20, 2013

    Just an addendum to the concepts involved here – having a digital process that can modify the systems behavior lends a high degree of flexibility, so that's pretty good. But you must be aware and constantly vigil with any controll mechanism like this. You have to carefully examine failure modes, looking for single point failures that could send to system into dangerous territory.

  9. Per Lowenborg
    September 5, 2013

    Charles, I have spent the last 14 years working with these topics and I can't agree more with your points.

    To me, analog/digital co-design is the way to go for high-performance analog in CMOS. I have numerous times witnessed proof of the fact that digitally-enhanced analog surpasses the limits of mere analog design.

    /Per

  10. Charles Razzell
    September 5, 2013

    Per,

    We are very much of the same opinion!  I was very happy to see your post in this thread.

    Best regards,

     

    Charles.

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