Circuit Note CN-0271 Devices Connected/Referenced Full K-Type Range 0C to 50C Thermocouple AD8495 Amplifier with Cold Junction Compensation Circuits from the Lab reference circuits are engineered and Low Power, Unity-Gain Fully Differential tested for quick and easy system integration to help solve todays AD8476 Amplifier and ADC Driver analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0271. 16-Bit, Single-Channel, Ultralow Power, Sigma- AD7790 Delta ADC Ultralow Noise, 2.5 V, LDO, XFET Voltage ADR441 Reference with Current Sink and Source Thermocouple Temperature Measurement System with Amplifier-Based Cold Junction Compensation A fixed gain instrumentation amplifier in the AD8495 amplifies EVALUATION AND DESIGN SUPPORT the small thermocouple voltage to provide a 5 mV/C output. The Circuit Evaluation Boards high common-mode rejection of the amplifier blocks common- CN-0271 Circuit Evaluation Board (EVAL-CN0271-SDPZ) mode noise that the long thermocouple leads can pick up. For System Demonstration Platform, SDP-B (EVAL-SDP-CB1Z) additional protection, the high impedance inputs of the amplifier Design and Integration Files make it easy to add extra filtering. Schematics, Layout Files, Bill of Materials The AD8476 differential amplifier provides the correct signal levels CIRCUIT FUNCTION AND BENEFITS and common-mode voltage to drive the AD7790 16-bit, - ADC. The circuit shown in Figure 1 is a complete thermocouple signal The circuit provides a compact low cost solution for thermocouple conditioning circuit with cold junction compensation followed signal conditioning and high resolution analog-to-digital by a 16-bit sigma-delta (-) analog-to-digital converter (ADC). conversion. The AD8495 thermocouple amplifier provides a simple, low cost solution for measuring K type thermocouple temperatures, including cold junction compensation. +5V +5V +2.5V +5V ADR441 +5V 0.1F 10F +2.5V 10F 0.1F 0.1F VIN VOUT +V S GND +V S V DD REFIN 10k COLD V DD INTERNAL JUNCTION CLOCK COMPENSATION 0.01F 0.01F SENSE INP 10k OUT 100 16-BIT SERIAL 100 DIGITAL 1.0F 1.0F BUF THERMO- AD8476 ADC INTERFACE IN-AMP PGA +2.5V 10k COUPLE OUT INN +OUT 100 0.01F 100 1M AD8495 49. 9k 0.01F AD7790 GND GND V 10k REF S V S Figure 1. K-Type Thermocouple Measurement System with Integrated Cold Junction Compensation (Simplified Schematic: All Connections Not Shown) Rev. B Circuits from the Lab circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 20122013 Analog Devices, Inc. All rights reserved. 10598-001CN-0271 Circuit Note CIRCUIT DESCRIPTION 4.9 V log The thermocouple is a simple, widely used component for 6.6 (32 nV/ Hz ) 122.4 1.57 800 Hz measuring temperature. It consists of a junction of two dissimilar = =12.4 bits log(2) metals. These metals are connected at one end to form the measurement junction, also called the hot junction. The other The AD8476 is a very low power, fully differential precision end of the thermocouple is connected to the metal lines that amplifier with integrated thin film, laser trimmed 10 k gain lead to the measurement electronics. This connection forms a resistors for unity gain. It is an ideal choice for this application second junction: the reference junction, also called the cold because it presents a relatively high impedance load to the junction. To derive the temperature at the measurement junction AD8495. (TMJ), the user must know the differential voltage created by The AD7790 is a low power, complete analog front end for low the thermocouple. The user must also know the error voltage frequency measurement applications. It contains a low noise, generated by the temperature at the reference junction (TRJ). 16-bit, - ADC with one differential input that can be buffered or Compensating for the reference junction error voltage is called unbuffered. cold junction compensation. The electronics must compensate Test Results for any changes in temperature at the reference (cold) junction An important measure of the performance of the circuit is the so that the output voltage is an accurate representation of the amount of linearity error. The AD8495 output is accurate to hot junction measurement. within 2C from 25C to +400C. To achieve even greater The circuit uses the AD8495 thermocouple amplifier on a single accuracy when operating at or outside of this range, a linearity 5 V supply. The output voltage of the AD8495 is calibrated for correction algorithm must be implemented in software. The 5 mV/C. On a single 5 V supply, the output is linear between CN-0271 evaluation software uses NIST thermoelectric voltage approximately 75 mV and 4.75 V, corresponding to a temperature lookup tables to achieve an output error within 1C from 15C range of 15C to 950C. The output of the AD8495 drives the to 950C. noninverting input of the AD8476 unity-gain differential amplifier, Figure 2 compares the performance of the AD8495 with the which converts the single-ended input to differential outputs for CN-0271 system, and the result of applying the linearization driving the AD7790 16-bit, - ADC. correction to the ADC output. For details on how the algorithm A low-pass differential and common-mode filter before the was implemented in the software, see the AN-1087 Application input of the AD8495 prevents RF signals, which, if allowed to Note, Thermocouple Linearization When Using the AD8494/ reach the AD8495, can be rectified and appear as temperature AD8495/AD8496/AD8497. fluctuations. The two 100 resistors and the 1 F capacitor 2.0 form a differential filter with a cutoff frequency of 800 Hz. The two 0.01 F capacitors form common-mode filters with a cutoff 1.5 frequency of 160 kHz. A similar filter is used at the output of 1.0 the AD8476 differential amplifier before the signal is applied to the AD7790 ADC. 0.5 The AD8495 inputs are protected from input voltage excursions 0 up to 25 V from the opposite supply rail. For example, in this 0.5 circuit, with a 5 V positive supply rail and the negative supply AD8495 rail tied to GND, the part can safely withstand voltages at the CN-0271 1.0 CN-0271 WITH inputs from 20 V to +25 V. Voltages at the reference and sense NONLINEARITY CORRECTION 1.5 pins should not go beyond 0.3 V of the supply rails. This feature is of particular importance in applications with power supply 2.0 sequencing issues that can cause the signal source to be active before the supplies to the amplifier are applied. JUNCTION TEMPERATURE (C) Figure 2. Output Error of AD8495, Total CN-0271 Circuit Error, and The theoretical resolution of the system can be calculated from Total CN-0271 Circuit Error with Thermocouple Nonlinearity Correction the bandwidth, voltage noise density, and gain of the AD8495. The peak-to-peak (noise free code) resolution in bits is Noise Free Bits = V OUT MAX log 6.6 Voltage Noise Density Gain 1.57 Bandwidth log(2) Rev. B Page 2 of 5 ERROR (C) 50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 10598-002