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AD22100A bảng dữ liệu(PDF) 5 Page - Analog Devices |
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AD22100A bảng dữ liệu(HTML) 5 Page - Analog Devices |
5 / 6 page AD22100 REV. B –5– For example, with VS = 5.0 V, and TA = +25°C, the nominal output of the AD22100 will be 1.9375 V. At VS = 5.50 V, the nominal output will be 2.1313 V, an increase of 193.75 mV. A proportionality error of 1% is applied to the 193.75 mV, yielding an error term of 1.9375 mV. This error term translates to a variation in output voltage of 2.1293 V to 2.3332 V. A 1.94 mV error at the output is equivalent to about 0.08 °C error in accuracy. If we substitute 150 °C for 25°C in the above example, then the error term translates to a variation in output voltage of 5.2203 V to 5.2298 V. A 4.75 mV error at the output is equivalent to about 0.19 °C error in accuracy. MOUNTING CONSIDERATIONS If the AD22100 is thermally attached and properly protected, it can be used in any measuring situation where the maximum range of temperatures encountered is between –50 °C and +150 °C. Because plastic IC packaging technology is employed, excessive mechanical stress must be avoided when fastening the device with a clamp or screw-on heat tab. Thermally conduc- tive epoxy or glue is recommended for typical mounting condi- tions. In wet or corrosive environments, an electrically isolated metal or ceramic well should be used to shield the AD22100. Because the part has a voltage output (as opposed to current), it offers modest immunity to leakage errors, such as those caused by condensation at low temperatures. THERMAL ENVIRONMENT EFFECTS The thermal environment in which the AD22100 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 vari- ables; the power consumption of the AD22100 and the thermal resistance between the chip and the ambient environment θ JA. Self-heating error in °C 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 nec- essary to specify θ JA under several conditions. Table I shows how the magnitude of self-heating error varies relative to the en- vironment. A typical part will dissipate about 2.2 mW at room temperature with a 5 V supply and negligible output loading. In still air, without a “heat sink,” the table below indicates a θ JA of 190 °C/W, yielding a temperature rise of 0.4°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 AD22100 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. 100 0 100 30 10 10 20 0 60 40 50 70 80 90 90 80 70 60 50 40 30 20 STILL AIR TIME – sec ALUMINUM BLOCK MOVING AIR Figure 7. Response Time The time constant τ is dependent on θ JA and the thermal 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 AD22100’s solder plated copper leads. When faster response is required, a thermally conductive grease or glue between the AD22100 and the surface temperature being measured should be used. MICROPROCESSOR A/D INTERFACE ISSUES The AD22100 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 AD22100 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 AD22100 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 AD22100 and the ADC use the supply as their reference. The nominal AD22100 signal range of 0.25 V to 4.75 V (–50 °C to +150°C) makes good use of the input range of a 0 V to 5 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 5 V will have a least signifi- cant bit (LSB) size of 5 V/256 = 19.5 mV. This corresponds to a nominal resolution of about 0.87 °C. |
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