A High Precision ADC That You Can Understand!

April 21, 2023 by Jenny List 8 Comments

In a world where an analogue to digital converter is all too often an integrated peripheral buried inside a microcontroller, it’s easy to forget how simple these devices can be when built from first principles. An entry in our Op-Amp Challenge from [NNNI] demonstrates this perfectly, it’s a high resolution multi-slope ADC for instrumentation purposes, constructed using a mixture of op-amps, logic chips, and a Raspberry Pi Pico. Best of all, it’s easy to understand, so there’s little of that analogue mystique to worry about.

This type of ADC measures an analogue value by counting how long it takes to charge a capacitor to that voltage. A simple version that measures charge time has a few drawbacks, so this project goes from single slope to multi slope by measuring both charge and discharge times compared to the voltage. Pay attention to component matching and reference stability, and such a design can offer a very high resolution measurement.

The value in this project lies not only in the design itself, but also in the extremely comprehensive description of its operation, which should teach most readers a thing or two. That curvy-line PCB is rather nice, too. We used single slope ADCs to read analogue joysticks back in the day, but we certainly learned something here. Want to see another? This isn’t the first dual slope ADC we’ve seen.

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.

Continue reading “Get To Know 3½ Digit ADCs With The ICL71xx” →

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.