Analog to Digital Converter (ADC): A True Understanding

Back in the day where the microprocessor was our standard building block, we tended to concentrate on computation and processing of data and not so much on I/O. Simply put there were a lot of things we had to get working just so we could then read the state of an I/O port or a counter.

Nowadays the microcontroller has taken care of most of the system level needs with the luxury of built in RAM memory and the ability to upload our code. That leaves us able to concentrate on the major role of a microcontroller: to interpret something about the environment, make decisions, and often output the result to energize a motor, LED, or some other twiddly bits.

Often the usefulness of a small microcontroller project depends on being able to interpret external signals in the form of voltage or less often, current. For example the output of a photocell, or a temperature sensor may use an analog voltage to indicate brightness or the temperature. Enter the Analog to Digital Converter (ADC) with the ability to convert an external signal to a processor readable value.

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When Difference Matters: Differential Signaling

We have talked about a whole slew of logic and interconnect technologies including TTL, CMOS and assorted low voltage versions. All of these technologies have in common the fact that they are single-ended, i.e. the signal is measured as a “high” or “low” level above ground.

This is great for simple uses. But when you start talking about speed, distance, or both, the single ended solutions don’t look so good. To step in and carry the torch we have Differential Signalling. This is the “DS” in LVDS, just one of the common standards throughout industry. Let’s take a look at how differential signaling is different from single ended, and what that means for engineers and for users.

Single Ended

Collectively, standards like TTL, CMOS, and LVTTL are known as Single Ended technologies and they have in common some undesirable attributes, namely that ground noise directly affects the noise margin (the budget for how much noise is tolerable) as well as any induced noise measured to ground directly adds to the overall noise as well.

By making the voltage swing to greater voltages we can make the noise look smaller in proportion but at the expense of speed as it takes more time to make larger voltage swings, especially with the kind of capacitance and inductance we sometimes see.

Differential

diff4

Enter Differential Signaling where we use two conductor instead of one. A differential transmitter produces an inverted version of the signal and a non-inverted version and we measure the desired signal strictly between the two instead of to ground. Now ground noise doesn’t count (mostly) and noise induced onto both signal lines gets canceled as we only amplify the difference between the two, we do not amplify anything that is in common such as the noise.

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A Better Way to Plug a CPLD into a Breadboard

If you read my first post about a simple CPLD do-it-yourself project you may remember that I seriously wiffed when I made the footprint 1” wide, which was a bit too wide for common solderless breadboards. Since then I started over, having fixed the width problem, and ended up with a module that looks decidedly… cuter.

To back up a little bit, a Complex Programmable Logic Device (CPLD) is a cool piece of hardware to have in your repertoire and it can be used to learn logic or a high level design language or replace obsolete functions or chips. But a CPLD needs a little bit of support infrastructure to become usable, and that’s what I’ll be walking you through here. So if you’re interested in learning CPLDs, or just designing boards for them, read on!

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Amplifying the Body’s Own Electricity

Measuring the body’s electrical signals is a neat trick… if you can get your equipment dialed in enough to establish dependable measurements. The technique is called Surface ElectroMyography (SEMG) though you’ll hear many call this ECG. They’re essentially the same technology; the Electro CardioGraph instruments monitor the activity of the heart while SEMG Instruments monitor electrical signals used to control other muscles. Both types of hardware amount to an instrumentation type amplifier and some form of I/O or display.

This topic has been in my back pocket for many months now. Back in May we Hackaday’ites descended on New York City for the Disrupt NY Hackathon event. We arrived a day or so early so that we might better peruse the Korean BBQ joints and check out the other electronics that NY has to offer. On Saturday we gathered around, each shouting out the size of his or her t-shirt preference as we covered up our black Hackaday logo tees with maroon maroon ones (sporting the Hackaday logo of course) for a 24-hour craze of hardware hacking.

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There were two individuals at our tables who were both hacking away on hardware to measure the electrical field produced by the body’s muscles in some form or another. The electrical signals measured from the skin are small, and need careful consideration to measure the signal despite the noise. This is a fun experiment that lets you work with both Instrumentation Amplifiers and OpAmps to achieve a usable signal from the movement of your body.

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Blacksmith Forge Made From the Bathroom Sink

The sweltering heat had finally moved on and Giant Tick season was coming to a close (not kidding, they are HUGE here), when I decided to fire up my hacked together blacksmith forge made out of an old bathroom sink and aquarium stand.

In the age-old formula I needed to supply an air source to a fuel to create enough heat to make iron malleable. I got the idea that this particular bathroom sink might be a good candidate for a fire bowl after I banged my shin with it and then cursed at it. It was clearly made of cast iron and as proof it was clearly unfazed by my tirade of words which I hope my son has learned from the Internet and not from listening to me remodel the bathroom.

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Programmable Logic: Build Yourself a CPLD Module

A Complex Programmable Logic Device (CPLD) is a great piece of hardware to have in your repertoire. As its name implies, you can program these chips to serve the logic functions you need. This might be replacing an obsolete chip, or maybe just a way to learn and try different techniques. What better way to learn than to get your hands on a CPLD and give it a try?

I created a CPLD module with the intent of being able to plug it into lots of things including solderless breadboards, but I screwed up. It seems that the plugin space available on a solderless breadboard is 1.1”, I had made the footprint 1” wide leaving no room for a row of wires on both sides. Duh.

But let me back up and show more about what I’m doing , I wanted to make a programmable piece of logic that could be built as a kit one could easily solder at home, could be programmed in-circuit, and could work at 3.3 or 5 volts.

Image5bTo implement an easily solderable kit I went with an older CPLD part that also has 3.3v and 5v versions that will maintain its programming regardless of power. The logic itself is a CPLD IC from the Altera Max family with two versions that fit the board with either 32 or 64 macrocells. A macrocell is the basic logic building block and it is programmed with logic “terms” and then interconnected to other macrocells through a programmable interconnect.

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Learn and Build a High Side Switch

As electronics engineer I have a mental collection of circuits that I’ve gathered over the years, much like a mechanic collects specialized tools as they work. All engineers do this and the tools in their tool boxes usually represent their project history and breadth.

A useful circuit to have in designer’s toolbox is the “high side switch”. Like it sounds, this is a circuit that switches the “high side” or positive voltage to a load.

We usually tend to switch things to ground as seen by outputs such as an Open Collector output, the reason being that ground usually is a known entity and is usually low impedance and is at a known voltage. But there are advantages to using a high-side switch in your circuits.

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