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ADR3440 데이터시트(PDF) 5 Page - Analog Devices |
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ADR3440 데이터시트(HTML) 5 Page - Analog Devices |
5 / 9 page Circuit Note CN-0287 Rev. C | Page 5 of 9 Protection Circuits Transient and overvoltage conditions are possible both during manufacturing and in the field. To achieve a high level of protection, additional external protection circuitry is necessary to compliment the IC’s internal integrated protection circuitry. The external protection adds additional capacitance, resistance, and leakage. These effects should be carefully considered to achieve a high level of accuracy. The additional protection circuitry is shown in Figure 5. Figure 5. Transient and Overvoltage Protection Circuit 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. Isolation 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. Figure 6. Noise Distribution Histogram of CN-0287 (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) 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: bits 8 . 16 V 192 . 8 V 24 . 2 400 216 , 777 , 16 log 2 = × = Resolution Free Noise (6) 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. +5V 1.69kΩ TVS 30V, 600W PTVS30VP1UP 300Ω OVERVOLTAGE UP TO 30V SCHOTTKY DIODES BAV199LT1G +5V 15mA +6V, −1V ADC INPUT +5.3V, −0.3V 3mA 5.6V ZENER DIODE NZH5V6B 110 100 90 80 70 60 NUMBER OF OCCURENCES 50 40 30 20 10 0 |
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