Do we still need schematic diagrams for analog circuits?

As a long-time analog-component and circuit “practitioner,” I view the schematic diagram as the starting point for almost any discussion of a design. It shows the signa flow, the primary ICs, other necessary but often underappreciated passive and active devices, the connectors, power source and distribution tree, and much more.

I sat down with the late circuit genius and prolific EDN contributor Jim Williams many years ago. See “This 30-ppm scale proves that analog designs aren’t dead yet” for just one of the many circuits he devised and explained in his articles published at EDN. As I sat down with him, he literally sketched out and talked through a clever yet conscientious design in real time; it was a master class in circuit explanation and exposition.

But lately I’ve been wondering to what extent we still need these schematics. In the era before large-scale analog ICs, a schematic was not only a diagram of what connected to what, but also a debug and troubleshooting guide. You could follow the signal flow from stage to stage, and separate the stages if needed, to see if a stage’s output corresponded correctly to its input.

A classic six-transistor AM-radio schematic makes this very clear (Figure 1).

Figure 1 This classic six-transistor AM radio has no processor, and its schematic diagram shows a linear signal flow from left to right, with RF stage/local oscillator, IF amplifiers, audio-stage driver, and audio power amplifier. Source: All American Five Radio

Things have changed; that’s for sure. Many electronic circuits now consist of a central processor, often with integrated analog I/O, plus perhaps a specialized network or other processor IC, along with connections to I/O including switches, displays, indicators, and similar. In other words, what we really need is an interconnection wiring diagram rather than a stylized schematic.

Consider a representative analog measurement function for an optical module for heart rate and blood oxygen (SpO2) measurement. Early units used a handful of individual devices, starting with the LED and its driver, going across to the phototransistor and its preamp, and then the rest of the signal chain that captured, conditioned, and digitized the output to show the relevant reading.

Now, the needed functions are largely integrated into a single IC such as the Analog Devices MAXM86161A, which includes the LED drivers, photodetector amplifier, analog and analog/digital functions, optical cancellation circuity, and I2C interface (Figure 2). All that’s missing is the user display, LED and photodetector, a soft on/off switch, and battery, plus a few capacitors and pullup resistors.

Figure 2 The MAXM86161A IC provides electro-optics for heart rate and blood oxygen monitoring, incorporating electrical and optical-friendly features. Source: Analog Devices

So, a product schematic diagram consists primarily of connections to that IC (Figure 3).

Figure 3 A highly integrated, tightly focused IC such as the MAXM86161A does not need many I/O connections. Source: Analog Devices

For this modest medical device, a hybrid block diagram/partial schematic is actually more informative, as it shows smaller-scale ICs with an independent processor (Figure 4).

Figure 4 A blend of conventional schematic diagram and high-level block diagram provides insight into system functionality and overall circuit action. Source: Microchip Technology

In many cases, the wiring diagram rather than the formal schematic is often of more use. When my microwave oven died, I opened the front panel out of curiosity and found both of those diagrams tucked inside (an unexpected but pleasant surprise). The below schematic (top) doesn’t really show what the wiring diagram clearly does (bottom). The innards are really a “mystery” control module with lots of connectors for keypad, display, safety interlock switches (lots of those), thermal overload switches and, of course, the magnetron tube that makes it all possible.

 

Figure 5 You can be the judge of which is more useful: the schematic diagram (top) or the wiring diagram (bottom) of this consumer microwave oven. Source: GE Appliances

Making it right

A good schematic tells the story of a design and shows how the different functional blocks relate to each other. For a not-so-good example, consider the one of Figure 6, also a heart rate and SpO2 monitor. It may have all the facts, but it certainly doesn’t tell the story at all.

Figure 6 This schematic of a heart rate and SpO2 monitor may be correct, but it’s hard to say; even if it is, it’s not very useful. Source: ResearchGate

It seems unnecessary to restate the obvious, but the guidelines for a good analog-centric schematic are simple. I have seen schematics from students which miss these points:

  • Have signal flow from left-to-right to the extent possible.
  • Use meaningful net names such as GND, SPI_CLK, SENSOR_OUT; designation such as Net_25 mean little. Even the circuit’s creator won’t remember these a few months later.
  • Group components by function, such as power, processor, sensors, and interfaces. My personal peeve is when all the bypass and bulk capacitors—and there can be dozens—are clustered in one corner of the schematic connected between the power rail and ground, without any indication of which IC a particular capacitor is supporting. That may be electrically correct, but it’s terrible in terms of the story, and useless for the inevitable debug and troubleshooting process.

But wait…there’s a counter to the story

On one side, the increasing use of large-scale analog-centric ICs with 40, 50, or more contacts is changing the function of the schematic diagram. Does this mean that conventional schematics are going the way of the six-transistor radio?

That was my fear, but then I realized I was only looking at the situation through one end of the telescope, so to speak. If today and the future are all about highly integrated, multifunction analog-centric ICs, why do vendors collectively release hundreds of single-function analog ICs every year (and that’s doesn’t include the countless power discrete devices, controllers, and management devices)?

Three examples show the reality. There’s the Analog Devices ADG2712 quad SPST switch (top), the Texas Instruments LVx886 zero-drift, low-noise op amp with multiplexer-friendly inputs (middle), or the STMicroelectronics TSC1801 current-sense amplifier (bottom) shown in Figure 7.

Figure 7 Despite the trend towards much highly integrated analog ICs, vendors still introduce many small, single-function ones such as this quad SPST switch, specialized low-drift op amp, and current-sense amplifier. Source: Analog Devices, Texas Instruments, and STMicroelectronics

Each one of these implements a basic function that is essential yet would be difficult, if not impossible, to implement with the needed performance in a larger multifunction IC. In most cases, doing so would require excessive compromise in one or more parameters such as leakage, bias current, on resistance, drift, and stability. It’s a long list. By offering single-function analog ICs, the compromises related to process, design, packaging, and cost are minimized or at least easier to manage.

Of course, once you have a circuit with these components, you’re going to need a real, story-telling, signal-flow schematic. So maybe I am too worried.

What are your thoughts about the future of schematic diagrams? Will it fade away, morph somewhat, become an entirely new technical entity? Or will it remain pretty much as-is?

Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.

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