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ADT6503ARJN015 Datasheet(PDF) 10 Page - Analog Devices |
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ADT6503ARJN015 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page ADT6501/ADT6502/ADT6503/ADT6504 Preliminary Technical Data Rev. PrA | Page 10 of 16 APPLICATION INFORMATION THERMAL RESPONSE TIME The time required for a temperature sensor to settle to a specified accuracy is a function of the thermal mass of the sensor and the thermal conductivity between the sensor and the object being sensed. Thermal mass is often considered equivalent to capacitance. Thermal conductivity is commonly specified using the symbol Q, and can be thought of as thermal resistance. It is commonly specified in units of degrees per watt of power transferred across the thermal joint. Thus, the time required for the ADT6501/ADT6502/ADT6503/ADT6504 to settle to the desired accuracy is dependent on the characteristics of the SOT-23 package, the thermal contact established in that particular application, and the equivalent power of the heat source. In most applications, the settling time is probably best determined empirically. SELF-HEATING EFFECTS The temperature measurement accuracy of the ADT6501/ADT6502/ADT6503/ADT6504 can be degraded in some applications due to self-heating. Errors can be introduced from the quiescent dissipation and power dissipated when converting. The magnitude of these temperature errors is dependent on the thermal conductivity of the ADT650x package, the mounting technique, and the effects of airflow. At 25°C, static dissipation in the ADT650x is typically TBD μW operating at 3.3 V. In the 5-lead SOT-23 package mounted in free air, this accounts for a temperature increase due to self- heating of ΔT = PDISS × θJA = TBD μW × 240°C/W = TBD°C It is recommended that current dissipated through the device be kept to a minimum, because it has a proportional effect on the temperature error. SUPPLY DECOUPLING The ADT6501/ADT6502/ADT6503/ADT6504 should be decoupled with a 0.1 μF ceramic capacitor between VDD and GND. This is particularly important when the ADT650x are mounted remotely from the power supply. Precision analog products, such as the ADT650x, require a well-filtered power source. Because the ADT650x operate from a single supply, it might seem convenient to tap into the digital logic power supply. Unfortunately, the logic supply is often a switch-mode design, which generates noise in the 20 kHz to 1 MHz range. In addition, fast logic gates can generate glitches hundreds of mV in amplitude due to wiring resistance and inductance. If possible, the ADT650x should be powered directly from the system power supply. This arrangement, shown in Figure 19, isolates the analog section from the logic switching transients. Even if a separate power supply trace is not available, generous supply bypassing reduces supply line induced errors. Local supply bypassing consisting of a 0.1 μF ceramic capacitor is advisable for the temperature accuracy specifications to be achieved. This decoupling capacitor must be placed as close as possible to the ADT650x VCC pin. 0.1µF ADT650x TTL/CMOS LOGIC CIRCUITS POWER SUPPLY Figure 19. Use Separate Traces to Reduce Power Supply Noise |
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