Temperature, which everyone is familiar with, is difficult to measure accurately. Before the advent of the era of modern electronics, Galileo invented the basic thermometer that can detect temperature changes. Two hundred years later, Seebeck discovered the thermocouple, which can produce voltage as a function of the temperature change rate of different metals. Nowadays, it is more common to measure temperature electronically by using thermocouples and temperature affected resistance elements (RTDS and thermistors) and semiconductor elements (diodes). Although the method of obtaining temperature from these components is well known, measuring temperature with accuracy better than 0.5 ℃ or 0.1 ℃ is still challenging.

To digitize these basic sensor elements, we need special analog circuit design, digital circuit design and firmware development technology. Ltc2983 integrates these expertise into a single IC, addressing each of the unique challenges associated with thermocouples, RTDs, thermistors, and diodes. The device integrates the necessary analog circuit, temperature measurement algorithm and linearization data for each type of sensor to directly measure each sensor, and outputs the measurement results in ℃ unit.

Thermocouple overview

The voltage generated by the thermocouple is a function of the temperature difference between the thermocouple tip (thermocouple temperature) and the electrical connection point (cold contact temperature) on the circuit board. In order to determine the thermocouple temperature, it is necessary to accurately measure the cold contact temperature, which is known as cold contact compensation. The cold junction temperature is usually determined by a temperature sensor (not a thermocouple) placed separately at the cold junction. Ltc2983 allows diodes, RTDs and thermistors to be used as cold contact sensors. In order to convert the voltage output from the thermocouple into temperature, it is necessary to solve the high-order polynomial (up to 14 orders) to obtain the measured voltage and cold junction temperature. Ltc2983 has built-in polynomials for all eight standard thermocouples (J, K, N, t, R, s, t and b), as well as user set table data for custom thermocouples. Ltc2983 measures both thermocouple output and cold junction temperature, performs all necessary calculations, and then reports thermocouple temperature in ℃.

Thermocouple: what’s important?

The output voltage produced by the thermocouple is very low (100 mV at full scale). Due to the offset and noise of ADC, the measured voltage value must be very low. In addition, this voltage is an absolute voltage reading and requires an accurate / low drift reference. Ltc2983 contains a low-noise, offset continuously calibrated 24 bit incremental accumulation ADC (offset and noise 1 μ V), and has a maximum reference value of 10ppm / ℃.

When the tip of the thermocouple is exposed below the temperature of the cold contact, the output voltage of the thermocouple can be lower than the ground. This forces the system to add a second negative power supply or input level shift circuit, thus making the system more complex. The ltc2983 incorporates a proprietary front end capable of digitizing signals from a single ground based power supply.

In addition to providing high measurement accuracy, the thermocouple circuit must also use noise suppression, input protection and anti aliasing filtering. The input impedance of ltc2983 is very high, and the maximum input current is less than 1NA. The device can use external protection resistor and filter capacitor without additional error. The ltc2983 contains a built-in digital filter and 75 dB rejection for 50 Hz and 60 Hz.

Fault detection is an important function of many thermocouple measurement systems. The most common fault is an open circuit (thermocouple damaged or not inserted). In the past, a current source or pull-up resistor was added to the thermocouple input to detect such faults. The problem with this approach is that these induced signals cause errors and noise and interact with the input protection circuit. Ltc2983 includes a unique open circuit detection circuit, which can check whether the thermocouple is damaged at the beginning of the measurement cycle. In this case, the open circuit excitation current / resistor does not interfere with the measurement accuracy. Ltc2983 also reports failures associated with cold contact sensors. The device also detects, reports and recovers from electrostatic discharge (ESD) events, which may occur when long sensor wires are used in industrial environments. The ltc2983 also indicates through its fault report whether the measured temperature is above / below the expected temperature range of a particular thermocouple.

Diode overview

Diodes are low cost semiconductor devices that can be used as temperature sensors. These devices are generally used as cold contact sensors of thermocouples. When an exciting current is applied to a diode, the voltage produced by the diode is a function of temperature and the applied current. If two perfectly matched excitation current sources with known ratio are added to the diode, the output voltage is known to be proportional to temperature (PTAT).

Diode: what’s important?

In order to generate a PTAT voltage with a known ratio, two highly matched and proportional current sources are needed. Ltc2983 relies on the incremental oversampling architecture to accurately generate this ratio. The diodes and leads connected to the ADC contain unknown parasitic diode effects. Ltc2983 provides three current measurement modes, eliminating parasitic lead resistance. Different diode manufacturers specify different diode non ideal coefficients. Ltc2983 allows the non ideal coefficient of each diode to be set individually. Because the absolute voltage is measured, the value and drift of ADC reference voltage are critical. Ltc2983 contains a reference with a maximum of 10ppm / ℃ fine tuned in the factory.

Ltc2983 automatically generates proportional current, measures the diode voltage, calculates the temperature using the set non ideal data, and outputs the result in ℃. The device can also be used as cold contact sensor of thermocouple. If the diode is damaged, short circuited or inserted incorrectly, ltc2983 will detect the fault if the cold contact temperature is measured by ltc2983, and report the fault in the conversion result output word and the corresponding thermocouple measurement result.

Overview of RTD

RTD is a resistor whose resistance value changes with temperature. To measure an RTD, an accurately known low drift detection resistor is connected in series to the RTD. The excitation current is added to the network and the proportional measurement is carried out. The resistance value of RTD is in ohm and can be determined according to this ratio. Then, by looking up the table, the resistance value is used to determine the temperature of the sensor element. Ltc2983 generates excitation current automatically, measures the detection resistor and RT D voltage at the same time, calculates the sensor resistance, and reports the result in ℃. RT D can measure temperature in a wide temperature range from as low as 200 ℃ to as high as 850 ℃. Ltc2983 can digitize most types of RTDS (pt-10, PT-50, PT-100, pt-200, pt-500, pt-1000 and ni-120). It has built-in coefficients for many standards (American, European, Japanese and ITS-90 standards), and provides user-defined table data for customized RTDS.

RTD: what’s important?

The resistance value of typical PT100 RTD changes less than 0.04 Ω when the temperature changes 1 / 10 ℃, and corresponds to 4 μ V signal level when the current is 100 μ a. Low ADC offset and noise are very important for accurate measurement. The measurement is proportional to the detection resistor, but the absolute values of the excitation current and the reference voltage are less important when calculating the temperature.

Previously, the proportional measurement between RTD and detection resistor was performed with a single ADC. The voltage drop of the detection resistor is used as the reference input of the ADC to measure the voltage drop of the RTD. This architecture requires a detection resistor of 10 K Ω or more, so buffering is needed to prevent voltage drop caused by dynamic current of ADC reference input. Since the value of the detection resistor is critical, the buffer must be low offset, low drift and low noise. This architecture makes it difficult to rotate the current source to eliminate the parasitic thermocouple effect. The reference input of incremental accumulation ADC is more susceptible to noise than input, and low reference voltage may cause instability. The multi ADC architecture of ltc2983 solves all these problems (see Figure 1). Ltc2983 uses two highly matched, buffered and auto calibrated ADCs, one for RTD and the other for detection resistor. These ADCs measure RTD and RSENSE at the same time, calculate RTD resistance, look up a table based on ROM according to these data, and finally output RTD temperature in ℃ unit.

Figure 1: RTD temperature measurement with ltc2983.

RTDS come in many configurations: 2-wire, 3-wire, and 4-wire. Ltc2983 offers all three configurations in a single configurable hardware solution. The device can share a single detection resistor among multiple RTDS. Its high impedance input allows an external protection circuit to be connected between the RTD and ADC inputs without introducing errors. The device can also automatically rotate current excitation to eliminate external thermal error (parasitic thermocouple). When the parasitic lead resistance of the detection resistor reduces the performance, ltc2983 allows Kelvin detection with RSENSE.

Ltc2983 includes fault detection circuit. The device can determine whether the detection resistor or RTD is damaged or short circuited. If the measured temperature is above or below the specified maximum temperature, a warning is issued. When RTD is used as cold contact sensor of thermocouple, three ADCs measure thermocouple, detection resistor and RTD at the same time. The RTD fault information is transmitted to the thermocouple measurement results, and the RTD temperature is automatically used to compensate the cold contact temperature.

Overview of thermistor

A thermistor is a resistor whose resistance changes with temperature. Different from RT D, the resistance value of thermistor can change in several orders of magnitude in its temperature range. In order to measure the thermistor, a detection resistor is connected in series to the sensor. The excitation current is added to the network and the proportional measurement is carried out. The resistance of the thermistor is measured in ohm and can be determined by this ratio. This resistance value is used to determine the temperature of the sensor, and then solve the Steinhart Hart equation or query table data. Ltc2983 generates excitation current automatically, measures the voltage of detection resistor and thermistor at the same time, calculates the resistance of thermistor, and reports the result in ℃. Thermistors generally work at – 40 ℃ ~ 150 ℃. Ltc2983 contains the coefficients needed to calculate the temperatures of 2.252k Ω, 3K Ω, 5K Ω, 10K Ω and 30K Ω standard thermistors. Since there are many types of thermistors and resistance values, ltc2983 can be set with custom thermistor table data (R and T) or Steinhart Hart coefficient.

Thermistors: what’s important?

The resistance value of thermistor can change in several orders of magnitude in its temperature range. For example, a 10 K Ω thermistor at room temperature may be as low as 100 Ω at the highest temperature and as low as 300 K Ω at the lowest temperature, while other thermistor standards may reach more than 1 m Ω.

In typical cases, in order to adapt to the large resistance, the excitation current source with very small current and the detection resistor with large resistance will be used. This results in a very low signal level at the low end of the thermistor range. The input buffer and the reference buffer are required to isolate the dynamic input current of the ADC from these larger resistors. However, without a separate power supply, the buffer does not work well when it is close to the ground, and the offset / noise error needs to be minimized. Ltc2983 solves all of these problems (see Figure 2). The device integrates a continuously calibrated proprietary buffer and multi ADC architecture, which can digitize the signal at or below ground level. Two matched buffer ADCs measure the thermistor and detector resistor at the same time, calculate the (standard based) thermistor temperature, and report the results in ℃. It does not need a large resistance value detection resistor, thus allowing multiple RTDS and different types of thermistors to share a single detection resistor. Ltc2983 can also automatically set different excitation current range depending on the output resistance of thermistor.

Figure 2: measuring thermistor temperature with ltc2983.

Ltc2983 includes fault detection circuit. The device can determine whether the detection resistor or thermistor is damaged / short circuited. If the measured temperature is higher or lower than the maximum or minimum value specified by the thermistor, ltc2983 will give an alarm. Thermistor can be used as cold contact sensor of thermocouple. In this case, the three ADCs simultaneously measure the thermocouple, detection resistor and thermistor. The thermistor fault information is transferred to the thermocouple measurement results, and the thermistor temperature is automatically used to compensate the cold contact temperature.

General measurement system

The ltc2983 can be configured as a general temperature measurement circuit (see Figure 3). Up to four universal inputs can be added to a single ltc2983. Each set of inputs can be directly used to digitize 3-wire RTDs, 4-wire RTDs, thermistors or thermocouples without changing any built-in hardware. Each sensor can use the same four ADC input and protection / filter circuits, and can be configured by software. All four groups of sensors can share a detection resistor and use a diode to measure the cold contact compensation. The input structure of ltc2983 allows any sensor to be connected to any channel. On any and all 21 analog inputs of ltc2983, any combination of RTD, detection resistor, thermistor, thermocouple, diode and cold contact compensation can be added.

Figure 3: general temperature measurement system

conclusion

Ltc2983 is a pioneering high performance temperature measurement system. The device can directly digitize thermocouples, RTDs, thermistors and diodes with laboratory level accuracy. Ltc2983 integrates three 24 bit incremental accumulation ADCs and a dedicated front end to solve many typical problems related to temperature measurement. High input impedance and zero input range allow direct digitization of all temperature sensors and easy input prediction. With 20 flexible analog inputs, the device can be reset through a simple SPI interface, so any sensor can be measured with the same hardware design. Ltc2983 can automatically perform cold contact compensation, measure cold contact with any sensor, and provide fault report. The device can directly measure 2, 3 or 4-wire RTD, and can easily share the detection resistor to save cost. At the same time, it is very easy to rotate the current source to eliminate the parasitic thermal effect. Ltc2983 can automatically set the current source range to improve accuracy and reduce noise related to thermistor measurement. Ltc2983 allows the use of user programmable custom sensors. Custom RTDs, thermocouples and thermistors based on tables can be set into the device. Ltc2983 integrates high accuracy, easy-to-use sensor interface and provides high flexibility in a complete single-chip temperature measurement system.

Editor in charge: GT