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LF13509D データシート(PDF) 7 Page - National Semiconductor (TI) |
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LF13509D データシート(HTML) 7 Page - National Semiconductor (TI) |
7 / 16 page Typical Applications DATA ACQUISITION SYSTEM A SIMPLIFIED SYSTEM DISCUSSION Analog multiplexers (MUX) are usually used for multi-chan- nel Data Acquisition Units (DAU) Figure 5 shows a system in which 8 different analog inputs are sampled and convert- ed into digital words for further processing The sample and hold circuit is optional depending on input speed require- ments and on AD converter speed Parameters characterizing the system are System Channels The number of multiplexer channels Accuracy The conversion accuracy of each individual sam- ple with the system operating at the throughput rate Speed or Throughput Rate Number of samplessecond channel the system can handle For a discussion on system structure addressing mode and processor interfacing see application note AN-159 A ACCURACY CONSIDERATIONS 1 Multiplexer’s Influence on System Accuracy (Figure 6) a The error (E) caused by the finite ‘‘ON’’ resist- ance RON of the multiplexing switches is given by E(%) e 100 1 a RIN (RON a RS a DRON) where RIN e following stage input impedance D RON e ‘‘ON’’ resistance modulation which is negligible for JFET switches like the LF11508 Example Let RON e 450 X DRON e 0 RS e 0 TA e 25 C and allowable E e 001% which is equivalent to 12 LSB in a 12-bit system RIN min e RON(100 b E) E e 45 MX Note that if temperature effects are included some gain (or full scale) drift will occur but effects on linearity are small b Multiplexer settling time (ts) ts(ON) is the time required for the MUX output to settle within a predetermined accuracy as shown in Table I CS (Figure 6) MUX output capacitance a fol- lowing stage input capacitance a any stray ca- pacitance at this node TABLE I ERROR % BITS ts(ON) TO 12 LSB 02 8 62t 005 10 76t 001 12 9t 00008 16 118t t e CS (RON a RS) ll RIN ts(OFF) is the time it takes to discharge CS within a tolerable error The ‘‘OFF’’ settling time should be taken into account for bipolar inputs where its effects will appear as a worse case of doubling of the ts(ON) 2 Sample and Hold Influence on System Accuracy The sample and hold if used also introduces errors into the system accuracy due to Offset voltage of sample and hold Droop rate in the Hold mode TA Aperture time or time delay between the time of a digital Hold command and the actual Hold occurance Taq Acquisition time or time it takes to acquire an analog input and settle within a predetermined error band Hold step Error created during the Sample to Hold mode caused by an undesirable charge injected into the Hold capacitor Ch For more details on sample and hold errors see the LF198LF298LF398 data sheet 3 AD Converter Influence on System Accuracy The ‘‘accuracy’’ of the AD converter is the best possible system accuracy In most data acquisition systems the AD converter is the most expensive single component so its error will often dominate system error Care should be taken that MUX SH and input source errors do not exceed system error requirements when added to AD errors For instance if an 8-bit accuracy system is desired and an 8-bit AD converter is used the accuracy of the MUX and SH should be far better than 8 bits For details on AD converter specifications see AN-156 TLH5668 – 13 FIGURE 5 Random-Addressed Multiplexed DAU FIGURE 6 8-Channel MUX 7 |
同様の部品番号 - LF13509D |
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同様の説明 - LF13509D |
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