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AD22103K Arkusz danych(PDF) 5 Page - Analog Devices |
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AD22103K Arkusz danych(HTML) 5 Page - Analog Devices |
5 / 6 page AD22103 REV. 0 –5– THERMAL ENVIRONMENT EFFECTS The thermal environment in which the AD22103 is used deter- mines two performance traits: the effect of self-heating on accu- racy and the response time of the sensor to rapid changes in temperature. In the first case, a rise in the IC junction tempera- ture above the ambient temperature is a function of two variables; the power consumption of the AD22103 and the thermal resis- tance between the chip and the ambient environment θ JA. Self- heating error in degrees Celsius can be derived by multiplying the power dissipation by θ JA. Because errors of this type can vary widely for surroundings with different heat sinking capacities, it is necessary to specify θ JA under several conditions. Table I shows how the magnitude of self-heating error varies relative to the environment. A typical part will dissipate about 1.5 mW at room temperature with a 3.3 V supply and negligible output loading. In still air, without a “heat sink,” the table below indi- cates a θ JA of 190°C/W, yielding a temperature rise of 0.285°C. Thermal rise will be considerably less in either moving air or with direct physical connection to a solid (or liquid) body. Table I. Thermal Resistance (TO-92) Medium θ JA (°C/Watt) τ (sec)* Aluminum Block 60 2 Moving Air** Without Heat Sink 75 3.5 Still Air Without Heat Sink 190 15 *The time constant τ is defined as the time to reach 63.2% of the final temperature change. **1200 CFM. Response of the AD22103 output to abrupt changes in ambient temperature can be modeled by a single time constant τ expo- nential function. Figure 7 shows typical response time plots for a few media of interest. TIME – sec 100 50 0 90 60 20 10 80 70 30 40 0 100 10 20 30 40 50 60 70 80 90 STILL AIR MOVING AIR ALUMINUM BLOCK Figure 7. Response Time The time constant τ is dependent on θ JA and the specific heat capacities of the chip and the package. Table I lists the effec- tive τ (time to reach 63.2% of the final value) for a few different media. Copper printed circuit board connections were neglected in the analysis; however, they will sink or conduct heat directly through the AD22103’s solder plated copper leads. When faster response is required, a thermally conductive grease or glue between the AD22103 and the surface temperature being measured should be used. MICROPROCESSOR A/D INTERFACE ISSUES The AD22103 is especially well suited to providing a low cost temperature measurement capability for microprocessor/ microcontroller based systems. Many inexpensive 8-bit micro- processors now offer an onboard 8-bit ADC capability at a mod- est cost premium. Total “cost of ownership” then becomes a function of the voltage reference and analog signal conditioning necessary to mate the analog sensor with the microprocessor ADC. The AD22103 can provide an ideal low cost system by eliminating the need for a precision voltage reference and any additional active components. The ratiometric nature of the AD22103 allows the microprocessor to use the same power sup- ply as its ADC reference. Variations of hundreds of millivolts in the supply voltage have little effect as both the AD22103 and the ADC use the supply as their reference. The nominal AD22103 signal range of 0.25 V to 3.05 V (0 °C to +100°C) makes good use of the input range of a 0 V to 3.3 V ADC. A single resistor and capacitor are recommended to provide im- munity to the high speed charge dump glitches seen at many microprocessor ADC inputs (see Figure 1). An 8-bit ADC with a reference of 3.3 V will have a least signifi- cant bit (LSB) size of 3.3 V/256 = 12.9 mV. This corresponds to a nominal resolution of about 0.46 °C/bit. USE WITH A PRECISION REFERENCE AS THE SUPPLY VOLTAGE While the ratiometric nature of the AD22103 allows for system operation without a precision voltage reference, it can still be used in such systems. Overall system requirements involving other sensors or signal inputs may dictate the need for a fixed precision ADC reference. The AD22103 can be converted to absolute voltage operation by using a precision reference as the supply voltage. For example, a 3.3 V reference can be used to power the AD22103 directly. Supply current will typically be 500 µA which is usually within the output capability of the refer- ence. A large number of AD22103s may require an additional op amp buffer, as would scaling down a 10.00 V reference that might be found in “instrumentation” ADCs typically operating from ±15 V supplies. USING THE AD22103 WITH ALTERNATIVE SUPPLY VOLTAGES Because of its ratiometric nature the AD22103 can be used at other supply voltages. Its nominal transfer function can be recal- culated based on the new supply voltage. For instance, if using the AD22103 at VS = 5 V the transfer function would be given by: VO = VS 5 V 0.25 V + 28 mV °C × T A 5 V 3.3 V VO = VS 5 V 0.378 V + 42.42 mV °C × T A |
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