How to make temperature measurement simpler, more flexible and more accurate? These two signal conditioning solutions can be referenced

A thermocouple consists of two wires of dissimilar metals connected at one end, called the measuring ("hot") junction. The other end of the wires that are not connected is connected to the signal conditioning circuit traces, which are usually made of copper. This junction between the thermocouple metals and the copper traces is called the reference ("cold") junction.

Schematic diagram of the basic structure of a thermocouple

Converting the voltage generated by a thermocouple into an accurate temperature reading is not an easy task. The reasons include the voltage signal is too small, the temperature-voltage relationship is nonlinear, reference junction compensation is required, and thermocouples may cause grounding problems.

For example, the most common thermocouple types are J, K, and T. At room temperature, their voltage changes are 52 μV/°C, 41 μV/°C, and 41 μV/°C, respectively, and other less common types have even smaller temperature voltage changes. Such weak signals require a higher gain stage before analog-to-digital conversion. Because the voltage signal is weak, the signal conditioning circuit generally requires a gain of about 100, which is a fairly simple signal conditioning. The more difficult thing is how to distinguish between the actual signal and the picked-up noise on the thermocouple leads. Thermocouple leads are long and often pass through dense electrically noisy environments. The noise on the leads can easily drown out the weak thermocouple signal.

On the other hand, to get an accurate absolute temperature reading, the temperature of the thermocouple reference junction must be known. The reference junction temperature needs to be measured using another temperature-sensitive device—typically an IC, thermistor, diode, or RTD (resistance temperature device), and then the thermocouple voltage reading is compensated to reflect the reference junction temperature. The reference junction must be read as accurately as possible—by keeping an accurate temperature sensor at the same temperature as the reference junction, any error in reading the reference junction temperature will be directly reflected in the final thermocouple reading.

In addition, the nonlinear temperature-voltage relationship and the grounding problems that thermocouples may cause make thermocouple signal conditioning more complicated than signal conditioning for other temperature measurement systems. The time required for signal conditioning design and debugging may increase the product's time to market. Errors introduced by the signal conditioning section may also reduce accuracy, especially in the reference junction compensation section.

The following figure shows a K-type thermocouple measurement using the AD8495/AD8494 thermocouple amplifier, which is specifically designed to measure type K thermocouples. This analog solution is optimized for reduced design time, with a simple signal chain and no software coding required.

Optimized signal chain design for measuring K-type thermocouples

In the block diagram of the AD8495 thermocouple amplifier, amplifiers A1, A2, and A3 (and the resistors shown) form an instrumentation amplifier that amplifies the K-type thermocouple output with a gain that produces just enough to produce a 5 mV/°C output voltage. Inside the box labeled "Ref junction compensation" is an ambient temperature sensor. If the reference junction temperature rises for any reason while the measurement junction temperature remains stable, the differential voltage from the thermocouple will decrease. If the tiny (3.2 mm × 3.2 mm × 1.2 mm) AD8495 is brought close to the hot region of the reference junction, the reference junction compensation circuitry applies an additional voltage to the amplifier so that the output voltage remains constant, compensating for reference temperature changes.

AD8495 Functional Block Diagram

Similar to the AD8495, the AD8494 thermocouple amplifier has an on-chip temperature sensor, typically used for cold junction compensation, and can be used as a stand-alone Celsius thermometer by grounding the thermocouple input. In this configuration, the amplifier produces an output voltage of 5 mV/°C between the output pin of the on-chip instrumentation amplifier and the reference pin. However, the reference pin is now driven by the AD8538 op amp (configured as a unity-gain follower), so the 5 mV/°C voltage appears across R1. The current flowing through R1 also flows through R2, producing a temperature-dependent voltage across this series combination that is (R1 + R2)/R1 times the voltage across R1. Using the values ​​shown in the figure, it can be found that the output voltage changes by 20 × 5 mV/°C = 100 mV/°C, so a 20°C temperature change will produce a 2 V output voltage change. The new system has a resolution of 0.05°C/LSB, a 20-fold improvement over the original circuit. The AD8538 buffers this resistor network, driving the reference pin with a low impedance, thereby maintaining good common-mode rejection and gain accuracy.

High-resolution temperature measurement system reference based on AD8494

The figure below shows a schematic diagram for high-precision measurement of J, K or T type thermocouples. This circuit includes a high-precision ADC for measuring the small signal thermocouple voltage and a high-precision temperature sensor for measuring the reference junction temperature. Both devices are controlled by an external microprocessor using an SPI interface.

Signal chain system optimized for precision and flexibility

The system measures the thermocouple voltage by using the AD7793, a high-precision, low-power analog front end. The thermocouple output is externally filtered and connected to a set of differential inputs AIN1(+) and AIN1(–). The signal is then sent to an ADC through a multiplexer, a buffer, and an instrumentation amplifier (amplifying the small thermocouple signal), which converts the signal to a digital signal, removes noise, and amplifies the voltage.

The ADT7320 can measure the reference junction temperature with an accuracy of ±0.2°C over the temperature range of –10°C to +85°C when placed sufficiently close to the reference junction. The on-chip temperature sensor generates a voltage proportional to the absolute temperature, which is compared with the internal reference voltage and input to a precision digital modulator. The digitized result output by the modulator continuously updates a 16-bit temperature value register. The temperature value register is then read back from the microprocessor via the SPI interface and combined with the temperature reading from the ADC for compensation. Unlike traditional thermistor or RTD sensor measurements, it does not require a costly calibration step after board assembly, nor does it consume processor or memory resources due to calibration coefficients or linearization routines. It consumes only a few milliwatts of power, avoiding the self-heating problem that reduces the accuracy of traditional resistive sensor solutions.

Thermocouples can be used for high-precision temperature measurement, but they are difficult for design engineers to optimize the measurement accuracy through solid circuit design and calibration. The first solution provided in this article integrates reference junction compensation and signal conditioning in one analog IC AD8495/AD8494, which is easier to use; the second solution separates reference junction compensation and signal conditioning, based on the high-precision, low-power analog front end AD7793, making digital output temperature sensing more flexible and accurate.