Electronic Design

Digitize Thermocouple Output Near Sensing Point

The thermocouple is a popular sensor for industrial temperature measurement because it's accurate, cost-effective, widely available, and suitable for a wide range of temperatures. It consists of two wires of different metal or metal alloys, welded together at the sensing end (usually called the hot junction). The thermocouple output is a voltage difference across the other ends of the wires (called the cold junction), which must be maintained at a known temperature.

If required by an application, the cold junction actually can be warmer than the hot junction. In that case, the polarity of the thermocouple's output voltage simply reverses. Thus, thermocouples measure the temperature difference between their hot and cold junctions. To obtain the absolute temperature of the hotjunction sensing point, you must measure the cold-junction temperature and adjust the thermocouple output accordingly. This technique is called cold-junction compensation.

When thermocouples were first used in the mid-1800s, an absolute measurement was obtained by maintaining the cold junction in an equilibrium mixture of ice and distilled water, thereby establishing a true 0°C reference point. Today, the cold-junction isothermal, located at the input of the thermocouple's signal-processing-block, is usually maintained by a small slab of material with high thermal conductivity. Copper, whose thermal conductivity is 381 W/mK (watts per meter-degree Kelvin), is the ideal material. The input connections must be electrically isolated, but thermally linked, to the slab. Ideally, the whole signal-processing block should be included in this isothermal environment.

Like other low-level circuitry ( thermocouple signals are in the μV/°C range), the thermocouple's signal-processing block is sensitive to electromagnetic interference (EMI), and thermocouple wires are often exposed to EMI. EMI increases uncertainty in the received signal and undermines the accuracy of the acquired temperature data. On the other hand, the special thermocouple cable needed for this connection is expensive, and if inadvertently replaced by another cable, the resulting situation can be difficult to diagnose.

The usual options for minimizing noise include moving the controlling circuitry nearer the sensing point (since EMI is proportional to the wires' length), adding a remote board with local intelligence near the sensing point, or introducing complex signal filtering and cable shielding. A better solution is to digitize the thermocouple output near the sensing-point (Fig. 1). This is especially useful if the supervising microcontroller is distant from the sensing point.

In this example, the signal-processing circuitry consists of a low-level dc amplifier, a temperature sensor, a cold-junctioncompensation circuit, an analog-to-digital converter (ADC) with internal reference, an open-thermocouple detector and alarm indicator, and a digital-output interface. All of these functions are now integrated in small ICs, such as the MAX6674 and MAX6675. Their serial output is a digital representation of temperature at the thermocouple's sensing point.

Circuitry internal to the MAX6674/6675 thermocouple-to-digital converters is scaled for use with Chromel-Alumel (type K) thermocouples. The MAX6674 has a range of 0°C to 128°C with a resolution of 0.125°C, while the MAX6675 has a range of 0°C to 1024°C with a resolution of 0.25°C. Both ICs contain an SPI-compatible interface that works under the supervision of a microcontroller or similar local intelligence.

IC1's SPI is driven by a local pulsesequence generator (IC2 and IC3), which forces IC1 to generate a train of asynchronous serial characters at 4800 bauds (four characters per second): one start bit, 11 data bits (13 for the MAX6675), and one stop bit. The 11 (or 13) data bits include 10 (or 12) straight-binary data bits for temperature (most-significant bit first) and one bit to warn of an open thermocouple.

The crystal-stabilized oscillator guarantees baud-rate accuracy for the data. The thermocouple's sensing tip must be electrically insulated for the circuit to operate correctly. You must also ensure that IC1 remains within its operating temperature range of 20°C to 85°C at all times.

The circuit connects to a distant power supply and data receiver via twisted-pair cable, which routes power to the circuit and brings digital data back to the data receiver. A temperature measurement consists of a single conversion by IC1's internal 10-bit ADC and is presented to the cable as a serial data word. Figure 2 shows an example of temperature data transmitted over a twisted-pair cable that's 3000 ft long. The data indicates a thermocouple in good working order, sensing a temperature of 21.875°C.

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