Isolated 4-Channel, Thermocouple/RTD Temperature Measurement System with 0.5°C Accuracy
The RTD excitation current are is programmable for optimum noise and linearity performance.
RTD measurements achieve 0.1°C accuracy (typical), and Type-K thermocouple measurements achieve 0.05°C typical accuracy because of the 16-bit ADT7310 digital temperature sensor used for cold-junction compensation. The circuit uses a four-channel AD7193 24-bit sigma-delta ADC with on-chip PGA for high accuracy and low noise.
Input transient and overvoltage protection are provided by low leakage transient voltage supressors (TVS) and Schottky diodes. The SPI-compatible digital inputs and outputs are isolated (2500 V rms), and the circuit is operated on a fully isolated power supply.
Figure 1. 4-Channel Thermocouple and RTD Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)
A thermocouple consists of two wires of different metals joined at one end. This end is placed at the temperature which is to be measured, refered to as the measurement junction. The other end is connected to a precision voltage measurement unit, and this connection is referred to as the reference junction or alternately the cold junction. The temperature difference between the measurement junction and the cold junction generates a voltage (known as the Seebeck effect voltage) that is related to the difference between the temperatures of the two junctions. The signal generated is typically from several microvolts to tens of millivolt depending on the temperature difference.
For example, K-type thermocouples are capable of measuring −200°C to +1350°C with an output range of approximately −10 mV to +60 mV. It is important for the signal chain to maintain as high impedance and low leakage as possible to achieve the highest accuracy for the voltage measurement. In order to convert this voltage to an absolute temperature, the cold junction temperature must be accurately known. Traditionally 1°C to 2°C has been considered sufficient, although since the cold junction measurement error contributes directly to the absolute temperature error, a higher accuracy cold junction temperature measurement is beneficial.
An RTD is made from a pure material, such as platinum, nickel or copper, that has a predictable change in resistance as the temperature changes.The most widely used RTD is platinum (Pt100 and Pt1000).
One method used to accurately measure the resistance is to measure the voltage across the RTD generated by a constant current source. Errors in the current source can be cancelled by referring the measurement to the voltage generated across a reference resistor that is driven with the same current (i.e. a ratiometric measurement). Minimizing the leakage current through the current path is important for achieving high accuracy because the excitation current is typically only a few hundred microamps to prevent self heating.
For the industrial field applications both high performance as well as protection against both high-voltage transient events and dc over-voltage conditions are important design considerations.
How this Circuit Works
The circuit shown in Figure 1 is designed for precision temperature measurement applications in the industrial field environment and is optimized for flexibility, performance, robustness, and cost. This circuit uses the AD7193, low noise, 24-bit sigma-delta ADC to ensure high resolution and linearity for the entire circuit.
The AD5201, 33-position digital potentiometer, AD8603 op amp, and ADG702 single channel switch constitute a simple programmable current source and bias voltage buffer for the RTD and thermocouple measurements. The ADG738 routes the current source to the active RTD channel and allows wire resistance compensation for the 3-W RTD configuration.
The ADT7310 digital SPI temperature sensor has ±0.8°C maximum accuracy (+5 V supply) from −40°C to +105°C and is used for cold-junction compensation for the thermocouple measurement. The ADR3440 is a low noise and high accuracy 4.096 V reference connected to REFIN1(+)/REFIN1(−) of the AD7193 for the thermocouple measurements.
The AD7193 is a low noise, complete analog front end for high precision measurement applications. It contains a low noise, 24-bit sigma-delta (Σ-Δ) analog-to-digital converter (ADC). This ADC achieves high resolution, low non-linearity, and low noise performance as well as very high 50 Hz/60 Hz rejection. The data output rate can be varied from 4.7 Hz (24 bits effective resolution, Gain = 1), to 4.8 kHz (18.6 bits effective resolution, Gain = 1). The on-chip low noise PGA amplifies the small differential signal from the thermocouple or RTD with a gain programmable from 1 up to 128, thereby allowing a direct interface. The gain stage buffer has high input impedance and limits the input leakage current to ± 3 nA maximum. The gain of theAD7193 must be configured properly depending on the temperature range and type of sensors. The on-chip multiplexer allows four differential input channels to be shared with the same ADC core, saving both space and cost.
Programmable Current Source for RTDs and Bias Voltage Generator Circuit for Thermocouples
RTD measurements require a low noise current source that drives the RTD and a reference resistor. Thermocouple measurements, on the other hand, need a common-mode bias voltage that shifts the small thermocouple voltage into the input range of the AD7193. The circuit shown in Figure 2 meets both requirements and utilizes the AD8603 a low noise CMOS rail-to-rail input/output op amp with only 1 pA maximum input bias current and 50 μV maximum offset voltage, combined with the ADG702 single channel, CMOS low voltage 2 Ω SPST switch, and the ADG738 eight-channel matrix switch.
With the ADG738 opened and the ADG702 closed, the AD8603 acts as a low noise, low output impedance unity-gain buffer for the thermocouple application. The voltage from the AD5201 digital potentiometer is buffered and is used for the thermocouple common-mode voltage, usually 2.5 V, which is one-half the supply voltage. The 33-position AD5201 digital potentiometer is driven with the ADR3440 low drift (5 ppm/°C) 4.096 V reference for accuracy.
With the ADG738 closed and the ADG702 opened, the AD8603 generates the RTD excitation current, IEXC = VW/RREF.
Temperature measurement is a high precision and low speed application, therefore there is adequate settling time available to switch the single current source between all 4 channels, providing excellent channel-to-channel matching, low cost, and small PCB footprint.
The ADG738 is an 8-to-1 multiplexer that switches the current source between channels. In order to support the 2-, 3-, and 4-wire RTD configurations, each of the four channels need two switches.
In many applications, the RTD may be located remotely from the measurement circuit. The resistance from the long lead wires can generate large errors, especially for low resistance RTDs. In order to minimize the effect of the lead resistance, a 3-wire RTD configuration is supported as shown in Figure 3.
With S1 of the ADG738 closed and S2 opened, the voltage at the input of AD7193 is V1. With S1 opened and S2 closed, the voltage on the input of AD7193 is V2. The voltage across the RTD sensor is VRTD, the exciting current from the current source is IEXC. V1 and V2 contain the error generated by the lead resistance as shown below:
Assuming RW1 = RW2 = RW3 and combining Equations 1, 2, and 3 yields:
Equation 5 shows that the 3-wire configuration requires two separate measurements (V1 and V2) in order to calculate RRTD, thereby decreasing the output data rate. In most applications this is not a problem.
The 4-wire RTD connection requires two extra sense lines, but is insensitive to wiring resistances and only requires one measurement.
Figure 4 summarizes the connector configuration and jumper placements for RTD 2-wire, RTD 3-wire, RTD 4-wire, and thermocouple applications.
Leakage currents can have a significant effect on RTD measurements so should be carefully considered. Leakage currents can also create some error in thermocouple measurements in the case where long thermocouple leads have significant resistance.
In this circuit, the PTVS30VP1UP transient voltage suppressor (TVS) quickly clamps any transient voltages to 30 V with only 1 nA typical leakage current at 25°C. A 30 V TVS was chosen to allow for a 30 V dc overvoltage. A 1.69 kΩ resistor followed by low leakage BAV199LT1G Schottky diodes are used to clamp the voltage to the 5 V power rail during transient and dc overvoltage events. The 1.69 kΩ resistor limits the current through the external diodes to about 15 mA during a 30 V dc overvoltage condition. In order to ensure the power rail is able to sink this current, a Zener diode is used to clamp the power rail to ensure it does not exceed the absolute maximum rating of any of the IC’s connected to the supply. The 5.6 V Zener diode (NZH5V6B) is selected for this purpose. A 300 Ω resistor limits any further current that could flow into the AD7193 or the ADG738.
The ADuM5401 and the ADuM1280 use ADI iCoupler® technology provide 2500 V rms isolation voltage between the measurement side and the controller side of the circuit. The ADuM5401 also provides the isolated power for measurement side of the circuit. The isoPower technique used in the ADuM5401 uses high frequency switching elements to transfer power through a transformer. Special care must be taken with the printed circuit board (PCB) layout to meet emissions standards. Refer to AN-0971 Application Note for board layout recommendations.
Thermocouple Configuration Test Results
The performance of the circuit is highly dependent on the sensor and the configuration of the AD7193. The Type-K thermocouple output varies from −10 mV to +60 mV, corresponding to −200°C to +1350°C. The AD7193 PGA is configured for G = 32. The voltage swing out of the PGA is −320 mV to +1.92 V, or 2.24 V p-p. With chop enabled, 50 Hz/60Hz noise reduction enabled, and filter word FS[9:0] = 96, the noise distribution histogram for 1024 samples is shown in Figure 6.
The resolution of the AD7193 is 24 bits, or 224 = 16,777,216 codes. The full dynamic range of the AD7193 is 2 × VREF = 2 × 4.096 V = 8.192 V. The output voltage of the thermocouple after the PGA is only 2.24 V p-p and does not occupy all the dynamic range of the AD7193. Therefore the range of the system is decreased by a factor of 2.24 V/8.192V.
The noise distribution is about 40 codes peak-to-peak. The noise-free code resolution over the 2.24 Vp-p range of measurement is given by:
The full-scale temperature range of the Type-K thermocouple is −200°C to +1350°C, or 1550°C p-p. The 16.8 bits of noise-free code resolution therefore corresponds to 0.013°C of noise-free temperature resolution.
Thermocouple Measurement Linearity
Thermocouple Measurement Linearity Figure 7 shows the approximate linearity of the type K thermocouple system. The “cold junction” temperature is 0°C in this plot.
The precision voltage for calibration as well as testing is provided by the Fluke 5700A Calibrator high precision dc voltage source with a resolution of 10 nV. The voltage error in Figure 8 is within 0.2 μV of ideal, corresponding to about 0.004°C. This result is the short time accuracy result just after a system calibration at 25°C without the effects of temperature drift.The dominant error for this circuit is from the cold-junction compensation measurement. In this circuit the ADT7310 is used for cold-junction compensation and has a typical error of −0.05°C, and a worst case error of ±0.8°C over the −40°C to +105°C temperature range for a 5 V supply. The device has a ±0.4°C maximum error over this temperature range if a 3 V supply is used.
RTD Configuration Test Results
For a Pt100 RTD, the default ADC gain setting is G = 8, and for a Pt1000 RTD the default gain setting is G = 1. The reference voltage to the ADC is equal to the voltage across the 4.02 kΩ reference resistor. The temperature coefficient of a Pt100 RTD is approximately 0.385 Ω/°C, and at +850°C the resistance can be as high as 400 Ω. With a 400 μA default excitation current, the maximum RTD voltage is therefore about 160 mV. The reference voltage to the ADC is 4.02 kΩ × 400 μA = 1.608 V. For G = 8, the maximum RTD voltage is 160 mV × 8 = 1.28 V which is approximately 80% of the available range.
For a Pt1000 RTD, the maximum resistance at +850°C is approximately 4000 Ω. The default excitation current is 380 μA, yielding a maximum RTD voltage of 1.52 V. The reference voltage to the ADC is 4.02 kΩ × 380 μA = 1.53 V. A default gain setting of G = 1 is used, and the maximum RTD voltage utilizes nearly all of the available range.
The general expression for the RTD resistance, R, in terms of the ADC code (Code), resolution (N), reference resistor (RREF), and gain (G) is given by:
The leakage current from TVS, diodes, clamping diodes, and ADC are the largest sources of errors in the RTD measurement circuit, even though nanoamp devices were selected for the design.
The total leakage current for each of the inputs is 9 nA (3 nA from AD7193, buffer on), 5 nA from clamping diode and 1 nA from the TVS diode). All four channels will thus generate 36 nA maximum leakage current. The feedback loop in Figure 2 maintains a constant current through the reference resistor. This means that leakage currents affect the RTD excitation current, thereby producing an error. The default exciting current is 400 μA for Pt100 and 380 μA for Pt1000. The approximate worst case system error due to the leakage currents for Pt100 RTDs is:
For a Pt100 with measurable range from −200°C to +850°C, this corresponds to a system accuracy of approximately:
The amount of the error depends on the configuration of the input terminals. After an input configuration is established, a room temperature calibration can reduce the error even further.
An experiment was conducted to show the effects of leakage current. Each channel was first configured as a 4-W RTD. A 100 Ω fixed resistor was connected to Channel 1 in the RTD position. Zero ohm resistors were connected to the inputs of the other three channels.
The gain was set for G = 1, and the excitation current for 380 μA (Pt1000 configuration).
Data was collected, then the jumpers connecting Channel 4, Channel 3, and Channel 2 were removed sequentially, and data collected for each condition. The results are shown in Figure 9.
The ADC code changed from approximately 437,800 to 437,600 corresponding to a measurement change of 104.9015 Ω to 104.8627, or 0.0388 Ω. This represents a measurement error of approximately 0.1°C; however it can be removed by calibrating at room temperature with a fixed input configuration.
RMS isolation up to 5 kV is be available in the ADuM6401 digital isolator with dc-to-dc converter.
The following equipment is needed:
- A PC with a USB port and Windows® XP (32 bit), Windows Vista®, or Windows® 7
- The EVAL-CN0287-SDPZ circuit board
- The EVAL-SDP-CB1Z SDP-B controller board
- The CN-0287 SDP Evaluation Software
- The EVAL-CFTL-6V-PWRZ dc power supply or equivalent 6 V/1 A bench supply
- A RTD or thermocouple sensor or sensor simulator. (The evaluation software supports the following RTDs: Pt100, Pt1000; Thermocouple: Type K, Type J, Type T, Type S.)
Install the evaluation software by placing the CN0287 Evaluation Software into the CD drive of the PC. Using My Computer, locate the drive that contains the evaluation software.
See Figure 1 for the circuit block diagram and the EVAL-CN0287-SDPZ-PADSSchematic.pdf file for the complete circuit schematic. This file is contained in the CN0287 Design Support Package located atwww.analog.com/CN0287-DesignSupport. A functional block diagram of the test setup is shown in Figure 10.
Connect the 120-pin connector on the EVAL-CN0287-SDPZ circuit board to the CON A connector on the EVAL-SDP-CB1Z controller board (SDP-B). Use nylon hardware to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors. With power to the supply off, connect a 6 V power supply to the +6 V and GND pins on the board. If available, a 6 V wall wart can be connected to the barrel connector J2 on the board and used in place of the 6 V power supply. Connect the USB cable supplied with the SDP-B board to the USB port on the PC. Do not connect the USB cable to the Mini-USB connector on the SDP-B board at this time.
Turn on the 6 V power supply to power up the evaluation board and SDP board, then plug in the Mini-USB cable into the Mini-USB port on the SDP board.
Launch the evaluation software. After USB communications are established, the SDP-B board can be used to send, receive, and capture data from the EVAL-CN0287-SDPZ board.
Figure 11 shows a photo of the EVAL-CN0287-SDPZ evaluation board connected to the SDP board. Information regarding the SDP-B board can be found in the SDP-B User Guide.
Information and details regarding test setup and calibration, and how to use the evaluation software for data capture can be found in the CN-0287 Software User Guide.
Connectivity for Prototype Development
The EVAL-CN0287-SDPZ evaluation board is designed to use the EVAL-SDP-CB1Z SDP-B board; however, any microprocessor can be used to interface to the SPI interface through the PMOD connector J6. The pin definition of PMOD connector can be found in the schematics of CN0287 evaluation board in CN0287 Design Support Package. In order for another controller to be used with the EVAL-CN0287-SDPZ evaluation board, software must be developed by a third party.
Figure 11. EVAL-CN0287-SDPZ Evaluation Board Connected to the EVAL-SDP-CB1Z SDP-B Board
|AD8603||MicroPower RRIO Low Noise Precision Single CMOS Op Amp||
|ADUM5401||Quad-Channel Isolators with Integrated DC/DC Converter (3/1 channel directionality)||
|ADT7310||±0.5°C Accurate, 16-Bit Digital SPI Temperature Sensor||
|AD7193||4-Channel, 4.8 kHz, Ultralow Noise, 24-Bit Sigma-Delta ADC with PGA||
|ADR3440||Micro-Power, High-Accuracy 4.096V Voltage Reference|
|ADUM1280||3kV rms, Default High, Dual-Channel Digital Isolators (2/0 Channel Directionality)||
|ADG738||CMOS, Low Voltage, 3-Wire, Serially Controlled, Matrix Switch||
|AD5201||33-Position Digital Potentiometer||
|ADG702||CMOS Low Voltage 2 Ω SPST Switch||