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LTC2480 Datasheet(PDF) 38 Page - Linear Technology |
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LTC2480 Datasheet(HTML) 38 Page - Linear Technology |
38 / 42 page LTC2480 38 2480fc APPLICATIONS INFORMATION rejection identical as that for the 1x speed mode. The averaging operation still keeps the output rate with the following algorithm: Result 1 = average (sample 0, sample 1) Result 2 = average (sample 1, sample 2) …… Result n = average (sample n – 1, sample n) The main advantage of the running average is that it achieves simultaneous 50Hz/60Hz rejection at twice the effective output rate, as shown in Figure 42. The raw output data provides a better than 70dB rejection over 48Hz to 62.4Hz, which covers both 50Hz ±2% and 60Hz ±2%. With running average on, the rejection is better than 87dB for both 50Hz ±2% and 60Hz ±2%. Complete Thermocouple Measurement System with Cold Junction Compensation The LTC2480 is ideal for direct digitization of thermo- couples and other low voltage output sensors. The input has a typical offset error of 500nV (2.5μV max) offset drift of 10nV/°C and a noise level of 600nVRMS. The input span may be optimized for various sensors by setting the gain of the PGA. Using an external 5V reference with a PGA gain of 64 gives a ±78mV input range—perfect for thermocouples. Figure 44 (last page of this data sheet) is a complete type K thermocouple meter. The only signal conditioning is a simple surge protection network. In any thermocouple meter, the cold junction temperature sensor must be at the same temperature as the junction between the ther- mocouple materials and the copper printed circuit board traces. The tiny LTC2480 can be tucked neatly underneath an Omega MPJ-K-F thermocouple socket ensuring close thermal coupling. The LTC2480’s 1.4mV/°C PTAT circuit measures the cold junction temperature. Once the thermocouple volt- age and cold junction temperature are known, there are many ways of calculating the thermocouple temperature including a straight-line approximation, lookup tables or a polynomial curve fit. Calibration is performed by apply- ing an accurate 500mV to the ADC input derived from an LT ®1236 reference and measuring the local temperature with an accurate thermometer as shown in Figure 43. In calibration mode, the up and down buttons are used to adjust the local temperature reading until it matches an accurate thermometer. Both the voltage and temperature calibration are easily automated. The complete microcontroller code for this application is available on the LTC2480 product Web page at: http://www.linear.com It can be used as a template for may different instruments and it illustrates how to generate calibration coefficients for the onboard temperature sensor. Extensive comments detail the operation of the program. The read_LTC2480() function controls the operation of the LTC2480 and is listed below for reference. CS SCK SDO SDI FO 6 9 7 1 10 VCC 5V LTC2480 REF GND IN– IN+ 3 ISOTHERMAL 2 C7 0.1μF C8 1μF 4 R2 2k R7 8k 6 2 5 4 R8 1k 5 2480 F43 26.3C TYPE K THERMOCOUPLE JACK (OMEGA MPJ-K-F) 11 8 GND IN OUT G1 NC1M4V0 TRIM GND LT1236 + Figure 43. Calibration Setup |
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