Accurate Temperature-to-Bit Converter Solves Temperature Sensor Measurement Challenges

Figure 1: LTC2983 temperature accuracy

Digitizing these basic sensor elements requires specialized analog circuit design, digital circuit design and firmware development techniques. The LTC2983 combines these specialized techniques into a single IC that addresses each of the unique challenges associated with thermocouples, RTDs, thermistors and diodes. The device incorporates the necessary analog circuitry and temperature measurement algorithms and linearization data for each type of sensor to directly measure each sensor and output the measurement in ºC.

Thermocouple Overview

The voltage produced by a thermocouple is a function of the temperature difference between the thermocouple tip (thermocouple temperature) and the electrical connection point on the circuit board (cold junction temperature). To determine the thermocouple temperature, the cold junction temperature needs to be accurately measured, a method known as cold junction compensation. The cold junction temperature is usually determined by a separate temperature sensor (not a thermocouple) placed at the cold junction. The LTC2983 allows diodes, RTDs and thermistors to be used as cold junction sensors. To convert the voltage output from the thermocouple to temperature, a high-order polynomial (up to 14th order) must be solved (using a table or math function) to obtain the measured voltage and cold junction temperature. The LTC2983 has built-in polynomials for all 8 standard thermocouples (J, K, N, T, R, S, T and B), as well as user-set table data for custom thermocouples. The LTC2983 simultaneously measures the thermocouple output and the cold junction temperature and performs all necessary calculations before reporting the thermocouple temperature in °C.

Thermocouples: What's the big deal?

The output voltage produced by the thermocouple is low (<100mV at full scale) (see Figure 2). The voltage being measured must be low due to the offset and noise of the ADC. In addition, the voltage is an absolute voltage reading, which requires an accurate/low drift reference voltage. The LTC2983 contains a low noise, continuously offset calibrated 24-bit delta-sigma ADC (offset and noise <1µV) with a 10ppm/°C maximum reference (see Figure 3).

Figure 2: Thermocouple design challenges

Figure 3: Thermocouple measurement with diode cold junction compensation

The output voltage of the thermocouple can also go below ground when the thermocouple tip is exposed to temperatures below the cold junction temperature. This complicates the system by forcing the addition of a second negative supply or input level shifting circuitry. The LTC2983 incorporates a proprietary front end that can digitize the signal with a single supply referenced to ground.

In addition to providing high measurement accuracy, thermocouple circuits must also employ noise rejection, input protection and anti-aliasing filtering. The LTC2983 has a high input impedance and a maximum input current of less than 1nA. The device can use external protection resistors and filter capacitors without introducing additional errors. The LTC2983 includes a built-in digital filter and 75dB rejection of 50Hz and 60Hz.

Fault detection is an important function in many thermocouple measurement systems. The most common fault is an open circuit (damaged or not inserted thermocouple). In the past, current sources or pull-up resistors were added to the thermocouple inputs to detect this type of fault. The problem with this approach is that these induced signals introduce errors and noise and interact with the input protection circuitry. The LTC2983 includes a unique open circuit detection circuit that checks for damaged thermocouples just before the measurement cycle begins. In this case, the open circuit excitation current/resistor does not interfere with measurement accuracy. The LTC2983 also reports faults associated with cold junction sensors. The device also detects, reports, and recovers from electrostatic discharge (ESD) events, which can occur when long sensor wires are used in industrial environments. The LTC2983 also indicates through its fault reporting whether the measured temperature is above/below the expected temperature range for a particular thermocouple.

Diode Overview

Diodes are inexpensive semiconductor devices that can be used as temperature sensors. They are typically used as cold junction sensors in thermocouples. When an excitation current is applied to a diode, the voltage produced by the diode is a function of the temperature and the applied current. If two perfectly matched excitation current sources of known ratio are applied to the diode, the output is a voltage that is known to be proportional to temperature (PTAT).

Diodes: What's the big deal?

To generate a PTAT voltage with a known ratio, two highly matched, ratiometric current sources are required (see Figure 4). The LTC2983 relies on a delta-sigma oversampling architecture to accurately generate this ratio. The diodes and leads connected to the ADC contain unknown parasitic diode effects. The LTC2983 offers 3 current measurement modes, eliminating parasitic lead resistance. Different diode manufacturers specify different diode non-ideality coefficients. The LTC2983 allows the non-ideality coefficient of each diode to be set individually. Because absolute voltages are measured, the value and drift of the ADC reference voltage are critical. The LTC2983 includes a reference that is factory trimmed to a maximum of 10ppm/°C.

The LTC2983 automatically generates a proportional current, measures the resulting diode voltage, calculates the temperature using programmed non-idealities and outputs the result in °C. The device can also be used as a cold junction sensor for thermocouples. If the diode is damaged, shorted or inserted incorrectly, the LTC2983 will detect this fault and report it in the conversion result output word along with the corresponding thermocouple measurement if the LTC2983 is used to measure the cold junction temperature.

Figure 4: Diode design challenges

RTD: Overview

An RTD is a resistor whose resistance value changes with temperature. To measure an RTD, an accurately known low drift sense resistor is connected in series with the RTD. An excitation current is applied to the network and a ratiometric measurement is taken. The resistance value of the RTD in ohms is determined from this ratio. This resistance value is then used to determine the temperature of the sensor element by looking up a table. The LTC2983 automatically generates the excitation current while measuring the sense resistor and RTD voltages, calculating the sensor resistance and reporting the result in °C. RTDs can measure temperature over a wide temperature range, from as low as -200°C to as high as 850°C. The LTC2983 can digitize most types of RTDs (PT-10, PT-50, PT-100, PT-200, PT-500, PT-1000 and NI-120), has built-in coefficients for many standards (U.S., European, Japanese and ITS-90 standards), and provides user-set table data for custom RTDs.

RTD: What is important?

The resistance value of a typical PT100 RTD (see Figure 5) changes less than 0.04Ω for every 1/10ºC change in temperature, which corresponds to a 4µV signal level at 100µA current excitation. Low ADC offset and noise are critical for accurate measurements. The measurement is ratiometric with respect to the sense resistor, but the absolute values ​​of the excitation current and reference voltage are not as important when calculating temperature.

Figure 5: RTD design challenges

Previously, ratiometric measurements between the RTD and sense resistor were performed with a single ADC. The voltage drop across the sense resistor was used as the reference input to the ADC that measured the voltage drop across the RTD. This architecture requires a 10KΩ or larger sense resistor, which requires buffering to prevent voltage drops caused by dynamic current at the ADC reference input. Since the sense resistor value is critical, the buffer must be low offset, low drift, and low noise. This architecture makes it difficult to cycle the current source to eliminate parasitic thermocouple effects. The reference input to the delta-sigma ADC is more susceptible to noise than the input, and low reference voltage values ​​can cause instability. The multi-ADC architecture of the LTC2983 solves all of these problems (see Figure 6). The LTC2983 uses two highly matched, buffered, and auto-calibrated ADCs, one for the RTD and the other for the sense resistor. These ADCs simultaneously measure the RTD and RSENSE, calculate the RTD resistance, and use this data to look up a ROM-based table to ultimately output the RTD temperature in °C.

Figure 6: Measuring RTD temperature with the LTC2983

RTDs come in many configurations: 2-wire, 3-wire, and 4-wire. The LTC2983 offers all 3 configurations in a single, configurable hardware solution. The device can share a single sense resistor between multiple RTDs. Its high impedance input allows external protection circuits to be placed between the RTD and ADC inputs without introducing errors. The device can also automatically rotate current excitation to eliminate external thermal errors (parasitic thermocouples). In situations where parasitic lead resistance of the sense resistor degrades performance, the LTC2983 allows Kelvin sensing with Rsense.

The LTC2983 includes fault detection circuitry. The device can determine if the sense resistor or RTD is damaged or shorted. If the measured temperature is above or below the maximum or minimum temperature specified for the RTD, the LTC2983 issues a warning. When the RTD is used as a cold junction sensor for a thermocouple, three ADCs measure the thermocouple, sense resistor, and RTD simultaneously. RTD fault information is passed to the thermocouple measurement, and the RTD temperature is automatically used to compensate for the cold junction temperature.

Thermistor Overview

Thermistors are resistors whose resistance value changes with temperature. Unlike RTDs, the resistance value of a thermistor can change by many orders of magnitude over its temperature range. To measure a thermistor, a sense resistor is connected in series with the sensor. An excitation current is applied to the network and a ratiometric measurement is taken. The resistance value of the thermistor, in ohms, can be determined from this ratio. This resistance value is used to determine the temperature of the sensor, solving the Steinhart-Hart equation or lookup table data. The LTC2983 automatically generates the excitation current, measures the sense resistor and thermistor voltages simultaneously, calculates the thermistor resistance, and reports the result in °C. Thermistors typically operate over a -40°C to 150°C temperature range. The LTC2983 contains the coefficients necessary to calculate the temperature for standard 2.252kΩ, 3kΩ, 5kΩ, 10kΩ and 30kΩ thermistor resistors. Because thermistors are available in a variety of types and resistance values, the LTC2983 can be programmed using custom thermistor table data (R and T) or Steinhart-Hart coefficients.

Thermistors: What's the big deal?

The resistance value of a thermistor (see Figure 7) can vary by many orders of magnitude over its temperature range. For example, a 10kΩ thermistor at room temperature may be as low as 100Ω at the highest temperature and >300kΩ at the lowest temperature, while other thermistor standards may be over 1MΩ.

Figure 7: Thermistor design challenges

Typically, to accommodate large value resistors, very low excitation current sources and larger sense resistors are used. This results in very low signal levels at the low end of the thermistor resistance range. Input buffers and reference buffers are needed to isolate the ADC’s ​​dynamic input current from these larger resistors. But buffers don’t work very well close to ground without separate supplies, and offset/noise errors need to be minimized. The LTC2983 solves all of these problems (see Figure 8). The device combines a continuously calibrated proprietary buffer that can digitize signals at or below ground with a multi-ADC architecture. Two matched buffered ADCs simultaneously measure the thermistor and sense resistor, calculate (based on a standard) the thermistor’s temperature, and report the result in °C. Large value sense resistors are not required, allowing multiple RTDs and different types of thermistors to share a single sense resistor. The LTC2983 also automatically sets different excitation current ranges depending on the thermistor output resistance.

Figure 8: Measuring thermistor temperature with the LTC2983

The LTC2983 includes fault detection circuitry. This device determines if the sense resistor or thermistor is damaged/shorted. If the measured temperature is above or below the maximum or minimum specified for the thermistor, the LTC2983 issues an alarm. Thermistors can be used as cold junction sensors for thermocouples. In this case, 3 ADCs measure the thermocouple, sense resistor, and thermistor simultaneously. Thermistor fault information is passed to the thermocouple measurement, and the thermistor temperature is automatically used to compensate for the cold junction temperature.

Universal measuring system

The LTC2983 can be configured as a universal temperature measurement circuit (see Figure 9). Up to 4 sets of universal inputs can be added to a single LTC2983. Each set of inputs can be used directly to digitize 3-wire RTDs, 4-wire RTDs, thermistors, or thermocouples without changing any built-in hardware. Each sensor can use the same 4 ADC inputs and protection/filtering circuits and can be configured by software. All 4 sets of sensors can share a sense resistor and measure cold junction compensation with a diode. The input structure of the LTC2983 allows any sensor to be connected to any channel. Any combination of RTDs, sense resistors, thermistors, thermocouples, diodes, and cold junction compensation can be added to any and all 21 analog inputs of the LTC2983.

Figure 9: Generic temperature measurement system

in conclusion

The LTC2983 is a groundbreaking high performance temperature measurement system. The device can directly digitize thermocouples, RTDs, thermistors and diodes with laboratory-grade accuracy. The LTC2983 integrates three 24-bit delta-sigma ADCs and a proprietary front end to solve many typical problems associated with temperature measurement. High input impedance and zero input range allow direct digitization of all temperature sensors and easy input prediction. 20 flexible analog inputs enable the device to be reprogrammed through a simple SPI interface, so any sensor can be measured with the same hardware design. The LTC2983 automatically performs cold junction compensation, can measure cold junctions with any sensor, and provides fault reporting. The device can directly measure 2, 3 or 4-wire RTDs and can easily share sense resistors to save cost, while easily rotating current sources to eliminate parasitic thermal effects. The LTC2983 can automatically set the current source range to improve accuracy and reduce noise associated with thermistor measurements. The LTC2983 allows the use of user-programmable custom sensors. Custom table-based RTDs, thermocouples and thermistors can be programmed into the device. The LTC2983 combines high accuracy, an easy-to-use sensor interface and high flexibility in a complete single-chip temperature measurement system.