These devices were described as high performance, low power, 3½ digit A/D converters containing seven segment decoders, display drivers, voltage reference and clock. The chip doing all the heavy-lifting was likely to be the ICL7106 or ICL7107. 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). This had a 7 segment LED display, but the chip was again from the same family. Some more digging, and I found a digital panel meter. 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. 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. It could measure low voltage DC, high current DC, resistance, dwell angle, and engine RPM and ran off a single 9V battery. 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. Want to see another? This isn’t the first dual slope ADC we’ve seen. We used single slope ADCs to read analogue joysticks back in the day, but we certainly learned something here. 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. Pay attention to component matching and reference stability, and such a design can offer a very high resolution measurement. 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. This type of ADC measures an analogue value by counting how long it takes to charge a capacitor to that voltage. Best of all, it’s easy to understand, so there’s little of that analogue mystique to worry about. An entry in our Op-Amp Challenge from 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. 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.
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