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ADC0816 Scheda tecnica(PDF) 9 Page - National Semiconductor (TI) |
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ADC0816 Scheda tecnica(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Applications Information (Continued) A good example of a ratiometric transducer is a potentiom- eter used as a position sensor. The position of the wiper is directly proportional to the output voltage which is a ratio of the full-scale voltage across it. Since the data is represented as a proportion of full-scale, reference requirements are greatly reduced, eliminating a large source of error and cost for many applications. A major advantage of the ADC0816, ADC0817 is that the input voltage range is equal to the supply range so the transducers can be connected directly across the supply and their outputs connected directly into the multiplexer inputs, ( Figure 9). Ratiometric transducers such as potentiometers, strain gauges, thermistor bridges, pressure transducers, etc., are suitable for measuring proportional relationships; however, many types of measurements must be referred to an abso- lute standard such as voltage or current. This means a system reference must be used which relates the full-scale voltage to the standard volt. For example, if V CC =VREF = 5.12V, then the full-scale range is divided into 256 standard steps. The smallest standard step is 1 LSB which is then 20 mV. 2.0 RESISTOR LADDER LIMITATIONS The voltages from the resistor ladder are compared to the selected input 8 times in a conversion. These voltages are coupled to the comparator via an analog switch tree which is referenced to the supply. The voltages at the top, center and bottom of the ladder must be controlled to maintain proper operation. The top of the ladder, Ref(+), should not be more positive than the supply, and the bottom of the ladder, Ref(−), should not be more negative than ground. The center of the ladder voltage must also be near the center of the supply because the analog switch tree changes from N-channel switches to P-channel switches. These limitations are automaticaly sat- isfied in ratiometric systems and can be easily met in ground referenced systems. Figure 10 shows a ground referenced system with a sepa- rate supply and reference. In this system, the supply must be trimmed to match the reference voltage. For instance, if a 5.12V reference is used, the supply should be adjusted to the same voltage within 0.1V. The ADC0816 needs less than a milliamp of supply current so developing the supply from the reference is readily ac- complished. In Figure 11 a ground references system is shown which generates the supply from the reference. The buffer shown can be an op amp of sufficient drive to supply the millliamp of supply current and the desired bus drive, or if a capacitive bus is driven by the outputs a large capacitor will supply the transient supply current as seen in Figure 12. The LM301 is overcompensated to insure stability when loaded by the 10 µF output capacitor. The top and bottom ladder voltages cannot exceed V CC and ground, respectively, but they can be symmetrically less than V CC and greater than ground. The center of the ladder voltage should always be near the center of the supply. The sensitivity of the converter can be increased, (i.e., size of the LSB steps decreased) by using a symmetrical reference system. In Figure 13, a 2.5V reference is symmetrically centered about V CC/2 since the same current flows in iden- tical resistors. This system with a 2.5V reference allows the LSB to be half the size of the LSB in a 5V reference system. DS005277-11 FIGURE 9. Ratiometric Conversion System www.national.com 9 |
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Descrizione simile - ADC0816 |
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