Quad-Channel Thermocouple Embeds CJC, Polynomial Linearization

Thermocouples are widely used for temperature measurement due to their accuracy, repeatability, precision, physical ruggedness, stability, and wide-ranging temperature capabilities. However, they present an interface challenge: They need amplification of their microvolt/millivolt-level output and cold-junction compensation (CJC). Many applications also need some degree of linearization of their temperature/output-voltage transfer function for acceptable results (see “Thermocouple Perspective” below).

Recognizing these issues, Microchip Technology introduced the MCP9604 integrated thermocouple-conditioning IC that overcomes the thermal-measurement and integration barrier. It’s the first single-chip, four-channel I2C thermocouple conditioning IC to deliver up to ± 1.5°C accuracy.

The IC offers an alternative to discrete and multichip solutions that can introduce errors, add to system design traps, and lengthen the bill of materials (BOM). Due to its high level of integration, the I2C-based physical connection between the thermocouple sensor and the system MCU is trivial (Fig. 1).

But don’t let the simple connection mislead you, as there’s a lot of internal, invisible complexity (Fig. 2).

NIST ITS-99 Equations Drive Higher Accuracy

The MCP9604 executes its advanced measurement accuracy for four thermocouple locations by using higher-order NIST ITS-90 equations for linearization rather than the simplistic single-order linear approximations of earlier all-analog amplifier designs. It does so via a processor core that’s used to compute the EMF to degree Celsius conversion using coefficients derived from the NIST ITS-90 coefficients.

It supports Type K, J, T, N, S, E, B and R thermocouples as defined by the NIST ITS-90 standard. As a performance example, it achieves ninth-order accuracy with K-type thermocouples in one integrated chip by incorporating the analog-to-digital converters (ADCs), CJC temperature sensors, amplifiers, and other components required for the temperature-measurement and math-engine signal chain.

Incorporating Cold-Junction Compensation

Providing cold-junction compensation is often a practical problem for thermocouple systems. The CJC temperature sensor is usually a lower-accuracy, limited-range device (often not a thermocouple). Situated at the connection junction between the thermocouple wires and the “plain-old” system copper wiring, it’s needed to correct for the unavoidable thermocouple created by this junction.

Traditionally, the CJC sensor is located at the connector or barrier strip where the thermocouple and system wires meet (and thus also requires another wire pair for that sensor). What Microchip has done is place the CJC sensor in the MCP9604 package, so there’s no need for a discrete CJC sensor.

The user does have to follow some “best practices” guidelines for running PCB traces to the device — a good PCB layout is key for better thermal conduction from the PCB temperature to the sensor die, but these guidelines are fairly straightforward (Fig. 3).

The PCB provides thermal conduction from the die to the thermocouple cold-junction. Therefore, the component placement positioning and the copper layout techniques are key for optimum cold-junction compensation. The recommended implementation for optimum temperature sensitivity is to extend a copper ground pad around the device pins.

More than Just Accurate Measurement

The temperature converter comes with user-programmable registers to provide design flexibility for various applications. The registers allow for user-selectable settings such as low-power modes for battery-powered applications, adjustable digital filters for fast transient temperatures, and four individually programmable temperature-alert outputs that can be used to detect multiple temperature zones.

In addition, the device (and its single-channel family members) includes open-circuit and short-circuit detection features, where an alert signal is asserted when the thermocouple wire is broken or disconnected. Similarly, an alert signal is asserted when the thermocouple is shorted to ground or power.

The MCP9604 device is supported by a 59-page datasheet and the EV19L27A evaluation board (Fig. 4) with a 24-page user manual and free downloadable software. It can connect to and read information from as many as four thermocouples.  A microcontroller acts as a host system for receiving this information and plotting the selected thermocouple.

The MCP9604 thermocouple-conditioning IC is priced at $10.56 each (10,000-unit) and packaged in a 24-pin 5×5 land grid array (LGA). The supporting EV19L27A evaluation board costs $96.00.