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AD22103KT Datasheet(PDF) 4 Page - Analog Devices |
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AD22103KT Datasheet(HTML) 4 Page - Analog Devices |
4 / 6 page REV. 0 –4– AD22103 THEORY OF OPERATION The AD22103 is a ratiometric temperature sensor IC whose output voltage is proportional to power supply voltage. The heart of the sensor is a proprietary temperature-dependent resis- tor, similar to an RTD, which is built into the IC. Figure 4 shows a simplified block diagram of the AD22103. +VS VOUT Ι RT Figure 4. Simplified Block Diagram The temperature-dependent resistor, labeled R T, exhibits a change in resistance that is nearly linearly proportional to tem- perature. This resistor is excited with a current source that is proportional to power supply voltage. The resulting voltage across R T is therefore both supply voltage proportional and lin- early varying with temperature. The remainder of the AD22103 consists of an op amp signal conditioning block that takes the voltage across R T and applies the proper gain and offset to achieve the following output voltage function: VOUT = (VS/3.3 V) × [0.25 V + (28.0 mV/°C) × T A] ABSOLUTE ACCURACY AND NONLINEARITY SPECIFICATIONS Figure 5 graphically depicts the guaranteed limits of accuracy for the AD22103 and shows the performance of a typical part. As the output is very linear, the major sources of error are offset, i.e., error at room temperature, and span error, i.e., deviation from the theoretical 28.0 mV/ °C. Demanding applications can achieve improved performance by calibrating these offset and gain errors so that only the residual nonlinearity remains as a source of error. TEMPERATURE – °C 2.5 –2.5 2.0 0 –0.5 –1.0 –2.0 1.5 0.5 0 100 50 –1.5 1.0 VS = 3.6V VS = 3.3V VS = 2.7V Figure 5. Typical AD22103 Performance OUTPUT STAGE CONSIDERATIONS As previously stated, the AD22103 is a voltage output device. A basic understanding of the nature of its output stage is useful for proper application. Note that at the nominal supply voltage of 3.3 V, the output voltage extends from 0.25 V at 0 °C to +3.05 V at +100 °C. Furthermore, the AD22103 output pin is capable of withstanding an indefinite short circuit to either ground or the power supply. These characteristics are provided by the output stage structure shown in Figure 6. VOUT VS Ι Figure 6. Output Stage Structure The active portion of the output stage is a PNP transistor with its emitter connected to the VS supply and collector connected to the output node. This PNP transistor sources the required amount of output current. A limited pull-down capability is provided by a fixed current sink of about –100 µA. (Here, “fixed” means the current sink is fairly insensitive to either sup- ply voltage or output loading conditions. The current sink ca- pability is a function of temperature, increasing its pull-down capability at lower temperatures.) Due to its limited current sinking ability, the AD22103 is inca- pable of driving loads to the VS power supply and is instead in- tended to drive grounded loads. A typical value for short circuit current limit is 7 mA, so devices can reliably source 1 mA or 2 mA. However, for best output voltage accuracy and minimal internal self-heating, output current should be kept below 1 mA. Loads connected to the VS power supply should be avoided as the current sinking capability of the AD22103 is very limited. These considerations are typically not a problem when driving a microcontroller analog to digital converter input pin (see MICROPROCESSOR A/D INTERFACE ISSUES). MOUNTING CONSIDERATIONS If the AD22103 is thermally attached and properly protected, it can be used in any measuring situation where the maximum range of temperatures encountered is between 0 °C and +100°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 conductive epoxy or glue is recommended for typical mounting conditions. In wet or corrosive environments, an electrically isolated metal or ce- ramic well should be used to shield the AD22103. Because the part has a voltage output (as opposed to current), it offers mod- est immunity to leakage errors, such as those caused by conden- sation at low temperatures. |
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