The cult of DSPism

The cult of DSPism

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Dennis Feucht discusses the cult of DSPism and some of its limitations or how and when MCUs can replace analog circuitry.
By eeNews Europe

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Adherents to the cult of DSPism believe that eventually there will be essentially no analog circuits and that all circuitry will be either purely digital or a mix of mixed-technology ADCs and DACs with microcontrollers (MCUs) or other digital processing elements in between them. This article explores the possibilities and limitations on this viewpoint.

Before MCUs became circuit components, there was a clear divide between digital and analog. Digital circuits consisted of the kinds of functions found in 7400-series TTL or 4000-series CMOS data books: flops, shift registers, decoders, and gates of all kinds. Analog data books have quite different parts: mostly op-amps, voltage references, combinations of them such as voltage regulators, and various borderline parts such as comparators, which are analog in, digital out; the 555 timer, which has mostly analog pins but also has digital out and reset in; analog switches, which have digital in; and multipliers, which can have a digital input in some applications. On closer inspection, much of what is in the analog data books has been partly digital all along. A comparator is a one-bit ADC, and a 555 timer is a kind of voltage-to-time converter, which is itself a kind of ADC.

So what exactly is digital and what is analog? The best way to define these words is in reference to waveforms, which are signals when they are encoded with a message in communications systems. Waveforms are electrical functions of time: v(t) or i(t) or even p(t). The definition could be extended to any dynamic (time-dependent) physical quantity – and even to social variables such as company cash flow or the U.S. debt as functions of time. Analog waveforms are simply continuous functions of time. The definition ultimately goes back to mathematical definitions of continuity and analysis. In math, analysis refers to continuous functions and is “analog math”. Wherever waveforms are continuous in time, we have analog electronics.

In contrast, digital is mathematically synonymous with discrete. Discrete functions have discontinuities in their numerical values and are not associated with the continuous number line of real numbers but with the integers, which leave gaping holes along the number line to be filled by the irrational numbers. Digital computers have waveforms that are discrete in both value and in time. Therefore, to simplify the definitions to their minimalist essence:

analog means continuous
digital means discrete

Binary digital waveforms are limited to two values, {0, 1} (which make digital waveforms Boolean functions) but this digital limitation can be partially overcome by grouping multiple scalar digital variables together into a vector quantity that can represent a number in base 2. The earliest MCUs did this with four bits (Intel’s 4004, the first MCU), and then progressed quickly to 8 bits, which became a dominant grouping of bits for processors in the 1970s and 1980s. Now 32-bit ARM processors are becoming commonplace.

Next: Take a nibble, chomp or gobble
 


As an aside, the groupings of bits have informally become associated with names that have stuck. The most common is the byte for 8 bits, and of lesser usage, the nibble for 4 bits. I would like to propose a fuller set of neologisms, retaining the ingestion metaphor of the language that has become associated with bit groupings:

As the number of bits increases, numerical resolution increases and approaches the continuum in the limit. At the same time that functions represented in software are gaining in resolution because of increasing MCU bit groupings, MCUs are also increasing in clock rate, causing their discrete-time characteristic to approach continuity. Both trends cause MCU apabilities to approach that of analog functions. Thus it might seem reasonable to suppose that MCUs with ADCs and DACs are all that should be necessary for electronics in the future.

Zeno’s paradox of a frog jumping half the remaining distance to a line but never arriving at the line suggests that this belief is in vain, that no matter how fast MCUs execute code, they are still digital in nature if the clock rate is finite. And in a physical world, it is. Yet in real-world engineering, many applications do not need infinite clock rate to adequately approach a continuous-time status, and as clock rates increase, more electronic systems that were once the sole domain of analog circuits are being implemented with MCUs. On the leading edge of this trend are parts like the Cypress PSOC5, which has attractively-performing op-amps (±0.3mV input offset voltage to over 2 x σ, and fT = 6 MHz) cohabiting a chip with an ARM processor.

Next: MCU-only electronics

 

 


So what is limiting MCU-only electronics?

Several factors work against an all-MCU world. First, as MCU speeds increase, operating voltages decrease to 3.3 V or lower. This is not an adequate voltage range for many applications including general-purpose T&M equipment. Pin currents are also limited so that any application requiring significant power will not be implemented practically by a MCU. Special-purpose MCU-based ICs are a possibility for addressing these kinds of limitations, with power DACs of various kinds.

Another basic limitation is speed. Analog waveform processing is still far faster than a fast MCU running a software program and driving a DAC or ADC. Analog multipliers can achieve multiplication to comparable accuracy and at greater resolution at a far greater bandwidth than a MCU.

Systems such as measurement instruments have modes – different structural configurations of circuits – that are switched. Most commonly, these are ranges for parameters. The analog switches that do the ranging can be many and varied in their requirements. Electromechanical relays are in some cases the best, and sometimes the only feasible, analog switch. The resistance values of op-amp circuits can be many and varied. Analog circuits often need capacitors, some large in value. Some have matched diodes or transistors. These can be implemented monolithically, but the generality of the circuitry is a problem. The PSOC5 has an analog bus but not a general switch matrix that can allow for arbitrary component interconnection. Even if it could, the switches would detract from circuit performance in many cases. A general-purpose analog-circuit matrix has been an elusive goal for the analog circuit as a programmable I/O block for a MCU.

High-performance analog – whether by precision or speed – is not likely to be replaced by a MCU. Do not expect to see oscilloscope front-ends or waveform generator output amplifiers to be all-digital. MCUs also cannot replace fast digital functions such as direct-digital waveform synthesizers.

Power electronics is not likely to be replaced by a MCU except some control functions. The power stage, consisting of power MOSFETs, gate drivers delivering pulses of 1A or more, magnetic components, and power capacitors are of a different electronics world than MCUs. The same applies to motor drives and pulsed-power electronics, including laser and ultrasonic probe drivers for medical instruments, and magnetizers, high-power transmission-line converters, and electric train drives, which use SCRs for switching. Exceptions to MCU application abound in analog electronics.

Next: Closer to home

 

 

 

 


The issue closer to home for many circuit engineers is not one of feasibility but of optimality. As the MCU realm expands, it is at the edges of its applicability that hard design decisions must be made. Often, these decisions involve choices between external analog circuitry or MCU code. For instance, in impedance (RLC) meter design, phase detection can be implemented with an analog translinear amplifier (purely analog) or analog switches followed by low-pass filters and switched by digital phased waveforms (semi-analog), or by acquiring the voltage and current waveforms with an ADC and calculating the real and reactive components of Z in software (MCUs digital). Radio designers are going through these same assessments, whether to use analog RF circuits or put a fast high-resolution ADC in the front-end of the receiver – and how close to the antenna? In audio, how good is digital amplification? Good enough for “golden ears”? For some, even bipolar-junction transistor analog is not good enough.

The clear design trend is one of replacing low-performing analog with MCUs while retaining analog circuits for high-performance applications, where performance is measured in speed, precision, power, functional specialization, or other intangible aspects such as the knowledge-base of the designer, analog or digital technology biases, parts count, maintainability in the field, or observability for testing. In conclusion, at the leading edge of MCUs, it is not always clear which design alternative is optimal, and the ability of the engineer can be tested in having to make such decisions.

Dennis Feucht has his own laboratory, Innovatia, on a jungle hilltop in Belize, where he performs electronics research, technical writing, and helps others with product development. He wrote a four-volume book-set on analog circuit design, has completed a book on transistor amplifier design and is working on a book on power electronics.

This article first appeared on EE Times’ Planet Analog website.

 

 

 

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