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ADG738 Datasheet(PDF) 2 Page - Analog Devices |
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ADG738 Datasheet(HTML) 2 Page - Analog Devices |
2 / 9 page CN-0287 Circuit Note Rev. C | Page 2 of 9 Figure 1. 4-Channel Thermocouple and RTD Circuit (Simplified Schematic: All Connections and Decoupling Not Shown) CIRCUIT DESCRIPTION Temperature Measurement Introduction Thermocouples and RTDs (resistance temperature detectors) are the most frequently used sensors for temperature measurement in industrial applications. Thermocouples are able to measure very high temperatures up to about +2300°C and also have a fast response time (measured in fractions of a second). RTDs are capable of higher accuracy and stability than thermocouples, and the resistance of long wire lengths (hundreds of meters) to a remote RTD can be compensated for with 3- or 4-wire connections. 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 AD7193 REFIN2(–) REFIN2(+) AIN8 AIN7 AIN2 AIN1 1.69k Ω 1.69k Ω 1.69k Ω 1.69k Ω 1.69k Ω +5V 300 Ω 300 Ω 300 Ω 300 Ω 300 Ω 1.69k Ω 300 Ω 1.69k Ω 300 Ω 1.69k Ω 300 Ω 1.69k Ω 300 Ω 1nF 1nF +5V +5V 27nF 1nF 1nF +5V +5V +5V +5V +5V +5V 27nF D S IN ADG702 1kΩ R3 C2 C2 C2 S1 S2 S7 S8 SCLK D DIN SYNC ADG738 DOUT PWR-ON PRESET A W B SHDN VDD VSS CS CLK SDI GND AD5201 LOGIC CONTROL +5V +4.096V P2 P3 ADR3440 VOUT FORCE SENSE FORCE SENSE GND +5V REFIN1(+) REFIN1(0) +4.096V AD8603 ADT7310 SCLK DOUT DIN CT INT SCLK DOUT DIN CT INT COLD JUNCTION COMPENSATION SCLK GND CS ADT7310_CS SCLK DIN ADG738_CS SCLK DOUT DIN CS SCLK DOUT DIN AD7193_CS DGND AGND +5V SCLK DIN +5V +5V V– V+ 0.1µF 10µF +5V AINCOM AIN4 AIN3 AIN6 AIN5 JP1 1 2 3 1 2 3 1 2 3 4 1 2 3 4 JP4 +5V THERMOCOUPLE: RTD 2,3W RTD 4W CH 1 CH 4 4.02k Ω 0.1% 10ppm +5V 5.6V ZENER DIODE 0 Ω : ANALOG GROUND : DIGITAL GROUND DVDD AVDD GND1 GNDISO +5VISO VDD1 GND2 VDD2 VIA VIB VOA VOB ADuM1280 +5V CT INT CTISO INTISO GNDISO GND2 VDD2 GND1 VDD1 VOA VOB VIA VIB +5VISO +5V ADuM1280 ADT7310_CS ADG738_CS ADT7310_CS ISO ADG738_CSISO SCLK DOUT DIN ADuM5401 AD7193_CS AD7193_CS ISO GNDISO DINISO SCLKISO DOUTISO GNDISO VDD1 VISO GND1 VOA VOA VOC VID VIA VIB VIC VOD +5VISO +5V |
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