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AD7193 bảng dữ liệu(PDF) 6 Page - Analog Devices |
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AD7193 bảng dữ liệu(HTML) 6 Page - Analog Devices |
6 / 9 page CN-0287 Circuit Note Rev. C | Page 6 of 9 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. Figure 7. Type K Thermocouple Temperature vs. Output Voltage with 0°C Cold-Junction 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. Figure 8. Error of CN-0287 Configured for Type K Thermocouple (VDD = 5 V, VREF = 4.096 V, Differential Input, Bipolar, Input Buffer Enable, Output Data Rate = 50 Hz, Gain = 32, Chop Enable. 60 Hz Rejection Enable, Sinc4) 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: = G R Code R REF N 2 (7) 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: reading of Error(%) % 01 . 0 100 μA 400 nA 6 3 ≈ × = (8) For a Pt100 with measurable range from −200°C to +850°C, this corresponds to a system accuracy of approximately C 1 . 0 0001 . 0 C / 385 . 0 400 ( ≈ × Ω Ω = ) C Accuracy (9) 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. 60 50 –500 0 500 TEMPERATURE (°C) 1000 1500 40 30 20 10 0 –10 0.20 0.15 0.05 1 6 11 16 21 26 31 36 41 46 51 INPUT VOLTAGE (mV) 0.10 0 |
Số phần tương tự - AD7193 |
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Mô tả tương tự - AD7193 |
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