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.
First, a little bit of history. In 1977, Fluke introduced the model 8020A, their first handheld DMM. Inside it was a chip marked as 429100 — the A/D converter and display driver. This chip was developed by Fluke in collaboration with (the original) Intersil, Inc., founded by Jean Hoerni in 1967 to produce digital watch and calculator microprocessors. A year after the 8020A was launched, Intersil introduced the ICL7106, a clone of the 429100.
To make it functionally different, Intersil masked off the switch that allowed the 429100 to auto-range between 200mV and 2V ranges (which was a Fluke patent). This made the 7106 unsuitable as a direct, drop in replacement for the 429100. But Fluke found out that the silicon inside the 7106 still sported a Fluke logo, and a lawsuit ensued. This was settled out of court, since Fluke could not afford to lose Intersil’s fab facilities. But the 7106 was now available to all for use and would end up inside a large number of products built in those days. You can read [drtaylor] who worked at Fluke during the time reminisce about this over at the EEVblog forum.
If we’re to design a similar product today, the 7106 would definitely be replaced with a micro-controller and a more capable analog front end. And we could choose from a wide range and type of displays running off standard protocols such as SPI or I²C. But the 7106 and others in its range are still available today, almost 40 years after they were first introduced. It seems there may still be a lot of use cases where selecting this device is justified. Especially if the product requires a no-fuss design which doesn’t need to connect to a computer, must be handheld, consume low power, and not require any software calibration.
Under the Hood
A bunch of passive parts connected to the 7106 is all it takes to build a device running off a 9V battery capable of measuring a ±200mV input. A ±2V range requires some component changes (C2,R2,C3) and adding a divider at the input allows measurement of higher voltages. Calibration can be done using a trimpot (R4+R1) which adjusts the reference voltage to the chip.
The differential inputs and adjustable reference voltage allow sensors such as strain gauges, load cells or similar ratiometric bridge type inputs to be easily interfaced. A lot of these chips got used for weighing scales, for example.
To do what it does, the device uses the “integrating type analog to digital conversion” method — also known as dual slope conversion. This Wiki stub explains the process succinctly. The three part process requires 4000 clock cycles (or 40000 cycles in case of 4½ digit device) — 1000 cycles for auto-zero phase, 1000 cycles for signal integrate phase, and up to 2000 cycles for the de-integrate or reference integrate phase. In order to achieve 50Hz noise rejection, the signal integrate phase needs to be a multiple of 50Hz. For example, to get good rejection of 50Hz, 60Hz, 400Hz and 440Hz noise, the oscillator needs to be at 40kHz and will result in 5 readings every 2 seconds. The oscillator parts (R3, C4), thus, need special selection.
The other parameter that affects performance is the stability of its internal reference voltage. In many cases, a simple voltage divider is connected to the regulated supply rail to derive the reference voltage. But for improved performance, a more stable reference, using a zener diode or a dedicated reference voltage (band gap) device was highly recommended.
The LED versions — 7107 and 7117 — suffered another issue related to the instability of its onboard reference voltage due to self-heating. LED’s consume significantly higher current, compared to LCD’s, and the internal heating that this causes degraded performance a lot. To help counter this, Intersil offered a special device with the “S” suffix, only for the LED versions.
Walking down memory lane is quite fun, especially for parts like this which were hiding in so many kits and products alike. Do you folks have any interesting ICL71xx stories to share in the comments? This article builds on the idea [Jenny List] started with her Get to Know Voltage Regulators with the a 723. If you have a beloved part that would be perfect for this kind of treatment we’d love to hear about it below.