When measurement instruments reach their specified performance limits, one comes to a gray zone separating reality from fantasy. Thanks to sensors, this zone is widening.
This region is approached in digital multimeters (DMMs) as the quantity being measured approaches zero. One major DMM company forces a zero reading on the display when the measurement is within a certain small band around zero. A similar issue exists in data acquisition (DAQ) systems, which are essentially multichannel DMMs.
The encroaching fantasy is being exacerbated by sensors, particularly thermocouples (TCs), which are conceptually simple sensors. When two dissimilar metals are joined, a voltage appears across their junction that is approximately proportional to temperature. Two wires — one chromel and one alumel, for instance — can be spot welded together at the sensing end by process chemists (who like to use TCs) and connected to the DAQ screwdown terminals, shown in orange below.
However, the DAQ-end connections also form thermocouples and generate voltages in series with the intended sensor junction. These additional undesired sources are called cold junctions. If the two wire-to-terminal connections are at the same temperature, the cold-junction voltages cancel each other. Therefore, the goal is to have the same temperature for both connections on the terminal strip. This is not an electronic design problem as such, but is one of the many opportunities for engineers to exercise versatility.
If the cold-junction connections are near each other and somewhat enclosed, they will be within 0.5°C of each other and typically within 0.3°C. Consequently, one would think this would set the limit on the accuracy specification for DAQ products. In reality, it does, but process chemists want to be able to resolve to 0.1°C or better. With filtering, the DAQ electronics can be designed to provide such resolution.
The trouble arises when the chemist wants to see a steady reading to within 0.1°C. Cold-junction temperatures vary with air movement and cannot be maintained to this resolution, yet some DAQ suppliers offer it. How is it achieved? A similar kind of zero-scale sophistry is applied to DMMs. It is not so illogical to consider a measurement so close to zero that no better can be achieved as a measured zero value. However, a constant non-zero reading can give the illusion of a solid measured value.
The zero-scale problem with cold junctions presents a semiconductor product opportunity. More significantly, an excellent opportunity for integrated analog on silicon exists to minimize the cold-junction problem by moving it to the silicon chip. One of the usually undesirable characteristics of monolithic silicon for circuits is its high thermal conductivity. In the case of cold-junction compensation, it might be possible to maintain to within 0.1°C the on-chip connections of two wires. One method for doing this is to take fixed lengths of TC wire — long enough to be out of the heat — and affix a processing chip with a digital or noise-insensitive processed analog output. TC wires are thin enough to be attached (but not die bonded) to large pads on a chip.
Alternatively, a tiny leadless chip package can be given an unusual pin-out to accommodate the TC wires. Two power pins and one (digital) or two (differential analog) output pins complete the chip connections, and the small processing granule is enclosed in a way suitable for the general environment — perhaps in a blob of epoxy. Such nodular TCs would reduce TC wire length and cost and could sell by the millions. True 0.1°C resolution (and maybe accuracy) could be achieved by TC nodules. The process chemists would benefit. DAQ electronics would be simplified. Sensor costs would drop, since TC wire is not cheap, and the market would reward the IC company offering this.
The manufacturing process for packaging TC nodules is the central design issue. For a greater challenge, the IC design goal could include elimination of the power pins by deriving chip micropower from the sensor itself, making use of the cold junctions to supply power. This would increase the analog integration needed on the IC.
Creative IC developers and vendors should be able to find the optimal combination of features for such a product (including TC linearization) so that they appeal broadly to process chemists, chemical engineers, and electronics engineers.