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CN0337 Datasheet(PDF) 5 Page - Analog Devices |
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CN0337 Datasheet(HTML) 5 Page - Analog Devices |
5 / 8 page Circuit Note CN-0337 Rev. 0 | Page 5 of 8 Test Data Before and After Two-Point Calibration To perform the two-point calibration, a 100 Ω precision resistor is first applied to the input, and the ADC output code is recorded as Code_1. Then a 212.05 Ω precision resistor is applied to the input, and the ADC output code is recorded as Code_2. The gain factor is calculated by Code_1 Code_2 100 05 . 212 GF . The RTD resistance can now be calculated corresponding to any output code, Code_x, using the equation: ) ( 100 Code_1 Code_x GF R X . The error before calibration is obtained by comparing the ideal transfer function calculated using the nominal values of the components, and real circuit transfer function without calibration. The tested circuits have been built with ±1%, ±100 ppm/°C resistors with the exception of R1, R2, R8, and R9 which are ±0.1%, ±25 ppm/°C. The tests were conducted with the printed circuit board (PCB) at room ambient temperature. The graph in Figure 3 shows test results for few tested boards before and after calibration (without temperature changes). As it is shown, the maximum error before calibration is about 0.27% FSR. After calibration, the error decreases to ±0.037% FSR, which approximately corresponds to 1.5 LSB error of the ADC. RTD MEASUREMENT TEMPERATURE (°C) 0 0.30 0.25 0.20 0.15 0.10 0.05 0 –0.05 –0.10 50 100 150 200 250 300 ERROR BEFORE CALIBRATION ERROR AFTER CALIBRATION Figure 3. Circuit Error Before and After Calibration PCB Layout Considerations In any circuit where accuracy is crucial, it is important to consider the power supply and ground return layout on the board. The PCB should isolate the digital and analog sections as much as possible. The PCB for this system was constructed in a simple 2-layer stack up, but 4-layer stack up gives better EMS. See the MT-031 Tutorial for more discussion on layout and grounding and the MT-101 Tutorial for information on decoupling techniques. Decouple the power supply to AD8608 with 10 μF and 0.1 μF capacitors to properly suppress noise and reduce ripple. Place the capacitors as close to the device as possible, with the 0.1 μF capacitor having a low ESR value. Ceramic capacitors are advised for all high frequency decoupling. Power supply lines should have as large trace width as possible to provide low impedance path and reduce glitch effects on the supply line. The ADuM5401 isoPower integrated dc-to-dc converter requires power supply bypassing at the input and output supply pins. Note that low ESR bypass capacitors are required between Pin 1 and Pin 2 and between Pin 15 and Pin 16, as close to the chip pads as possible. To suppress noise and reduce ripple, a parallel combination of at least two capacitors is required. The recommended capacitor values are 0.1 μF and 10 μF for VDD1 and VISO. The smaller capacitor must have a low ESR, for example, use of a ceramic capacitor is advised. The total lead length between the ends of the low ESR capacitor and the input power supply pin must not exceed 2 mm. Installing the bypass capacitor with traces more than 2 mm in length may result in data corruption. Consider bypassing between Pin 1 and Pin 8 and between Pin 9 and Pin 16 unless both common ground pins are connected together close to the package. For more information, see ADuM5401 datasheet. A complete documentation package including schematics, board layout, and bill of materials (BOM) can be found at www.analog.com/CN0337-DesignSupport. High Voltage Capability This PCB is designed in adherence with 2500 V basic insulation practices. High voltage testing beyond 2500 V is not recommended. Appropriate care must be taken when using this evaluation board at high voltages, and the PCB should not be relied on for safety functions because it has not been high potential tested (also known as hipot tested or dielectric withstanding voltage tested) or certified for safety. |
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