Approximating An ADC With Successive Approximation

October 12, 2024 by Al Williams 17 Comments

[Igor] made a VU meter with LEDs using 8 LEDs and 8 comparators. This is a fast way to get one of 8 bits to indicate an input voltage, but that’s only the equivalent of a 3-bit analog to digital converter (ADC). To get more bits, you have to use a smarter technique, such as successive approximation. He shows a chip that uses that technique internally and then shows how you can make one without using the chip.

The idea is simple. You essentially build a specialized counter and use it to generate a voltage that will perform a binary search on an unknown input signal. For example, assuming a 5 V reference, you will guess 2.5 V first. If the voltage is lower, your next guess will be 1.25 V. If 2.5 was the low voltage, your next guess will be 3.75 V.

The process repeats until you get all the bits. You can do this with a microcontroller or, as [Igor] shows, with a shift register quite simply. Of course, you can also buy the whole function on a chip like the one he shows at the start of the video. The downside, of course, is the converter is relatively slow, requiring some amount of time for each bit. The input voltage also needs to stay stable over the conversion period. That’s not always a problem, of course.

If that explanation didn’t make sense, watch the video. An oscilloscope trace is often worth at least 1,000 words.

There are, of course, many ways to do such a conversion. Of course, when you start trying to really figure out how many bits of resolution you have or need, it gets tricky pretty fast.

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A Brief Look Inside A Homebrew Digital Sampler From 1979

June 17, 2024 by Dan Maloney 12 Comments

While we generally prefer to bring our readers as much information about a project as possible, sometimes we just have to go with what we see. That generally happens with new projects and work in progress, but it can also happen with old projects. Sometimes very old indeed, as is the case with this digital sampling unit for analog oscilloscopes, circa 1979.

We’ve got precious little to go on with this one other than the bit of eye candy in the video tour below and its description. Luckily, we’ve had a few private conversations with its maker, [Mitsuru Yamada], over the years, enough to piece together a little of the back story here — with apologies for any wrong assumptions, of course.

Built when he was only 19, this sampler was an attempt to build something that couldn’t be bought, at least not for a reasonable price. With no inexpensive monolithic analog-to-digital converters on the market, he decided to roll his own. A few years back he recreated the core of that with his all-discrete successive approximation ADC.

The sampler shown below has an 8-bit SAR ADC using discrete CMOS logic and enough NMOS memory to store 256 samples. You can see the ADC and memory cards in the homebrew card cage made from aluminum angle stock. The front panel has a ton of controls and sports a wide-range attenuator, DC offset, and trigger circuit with both manual and automatic settings.

It’s an impressive build, especially for a 19-year-old with presumably limited resources. We’ve reached out to [Yamada-san] in the hope that he’ll be able to provide more details on what’s under the hood and if this still works after all these years. We’ll pass along whatever we get, but in the meantime, enjoy.

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Homebrew Circuit Explores The Mysteries Of Analog-to-Digital Conversion

October 8, 2021 by Dan Maloney 9 Comments

When it comes to getting signals from an analog world into our computers, most of us don’t give much thought to how the hardware that does the job works. But as it turns out, there are a number of ways to skin the analog to digital conversion cat, and building your own homebrew successive approximation register ADC is a great way to dispel some of the mystery.

From his description of the project, it’s clear that [Mitsuru Yamada] wasn’t looking to build a practical ADC, but was more interested in what he could learn by rolling his own. A successive approximation register ADC works by quickly cycling through all possible voltage levels in its input range, eventually zeroing in on the voltage of the input signal at that moment and outputting its digital representation. The video below shows how the SAR ADC works visually, using an oscilloscope to show both the input voltage and the output of the internal R-2R DAC. The ADC has an input range of 0 V to 5 V and seven bits of resolution and uses nothing but commonly available 74xx series logic chips and a couple of easily sourced analogs for the sample-hold and comparator section. And as usual with one of his projects, the build quality and workmanship are impeccable.

We love these sorts of projects, which are undertaken simply for the joy of building something and learning how it works. For more of [Yamada-san]’s projects, check out his 6502-based RPN calculator, or the serial terminal that should have been.

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Get To Know 3½ Digit ADCs With The ICL71xx

January 31, 2017 by Anool Mahidharia 29 Comments

Riffling through my box of old projects, I came upon a project that I had built in the 80’s — an Automotive Multimeter which was published in the Dutch/British Elektor magazine. It could measure low voltage DC, high current DC, resistance, dwell angle, and engine RPM and ran off a single 9V battery. Besides a 555 IC for the dwell and RPM measurement and a couple of CMOS gate chips, the rest of the board is populated by a smattering of passives and a big, 40 pin DIP IC under the 3½ digit LCD display. I dug some more in my box, and came up with another Elektor project from back then — a True RMS digital Wattmeter with a 3½ digit LCD display that could measure up to 2kW. It had the same chip too. Some more digging, and I found a digital panel meter. This had a 7 segment LED display, but the chip was again from the same family.

Look under the hood of any device with a 3½ or 4½ digit, 7 segment, LCD or LED from the ’80’s or ’90’s and you will likely spot this 40-pin DIP with the Intersil logo (although it was later also manufactured by many other fabs; Harris and Maxim among others). The chip doing all the heavy-lifting was likely to be the ICL7106 or ICL7107. These devices were described as high performance, low power, 3½ digit A/D converters containing seven segment decoders, display drivers, voltage reference and clock. In short, everything you needed to take a DC analog signal and display it. Over time, a whole series of devices were spawned:

7106 – 3½ digit, 7 segment LCD
7107 – 3½ digit, 7 segment LED
7116 – 3½ digit, 7 segment LCD, with display HOLD (freeze)
7117 – 3½ digit, 7 segment LED, with display HOLD (freeze)
7126 – improved 7106
7136 – improved 7126
7135 – 4½ digit, 7 segment LCD

There were many similar devices available, but the ICL71xx series was by far one of the most popular, due to its easy of use, low parts count and single chip implementation. Here are several parts (linking to PDF datasheets) to illustrate my point: the TC14433/A needed several peripheral devices, ES5107 (a clone of a clone — read below), CA3162 (which has BCD output, and needs the CA3161 or similar to interface to a display), or the AD2020 (which too needed a lot of support circuitry).

The ICL71xx was the go-to device for a reason. Let’s take a look at the engineering and business behind this fascinating chip.

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Analog To Digital Converter (ADC): A True Understanding

May 5, 2016 by Bil Herd 65 Comments

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.