【正文】 簽名： 月 日 教研室意見 指導(dǎo)教師。 consult the listing for additional information. With a 12 MHz processor clock and the resulting one microsecond instruction cycle, an eightbit conversion can be performed in under 300 microseconds. The unknown input voltage must be held constant for the duration of the conversion. Obvious disadvantages to the successive approximation analogtodigital converter presented here are the need for bipolar power supplies and the large number of microcontroller I/O pins required to control the DAC. The +15volt supply could be eliminated by replacing the LF355B op amp with a single supply, 5volt, functional equivalent with outputs that swing railtorail. The number of microcontroller I/O pins required to control the DAC could be reduced somewhat by substituting a seven or six bit DAC. The parallel input DAC could be replaced with a (more expensive) serial input DAC. Alternately, logic could be added to accept serial data from the microcontroller and present parallel data to the DAC. The software for this application may be obtained by downloading from Atmel’s BBS: (408) 4364309. Consult the ment block at the beginning of the source code file for detailed information on features and oper 功能特性概述： 指導(dǎo)教師意見 該生接受 “ 數(shù)顯式脈搏計 的設(shè)計 ” 課題 以來， 積極查閱并認真學(xué)習(xí)資料， 對 設(shè)計 要求有較為清晰地理解， 對系統(tǒng)各部分的組成 已有初步的 設(shè)計方案。 variations in VCC. The contributions to conversion error made by these sources can be expected to increase error to somewhat more than the value due to ponent tolerances alone. Successive Approximation AnalogtoDigital Converter This conversion method offers good resolution and accuracy and a short conversion time at the expense of increased ponent count. Successive approximation (SA) ADCs incorporate a digitaltoanalog converter (DAC), a parator and a successive approximation register (SAR). The SAR controls the conversion by performing a search for the binary code which, when fed to the DAC, will produce an output matching the voltage to be converted. The parator pares the DAC output to the unknown voltage and returns the result to the SAR. The SAR begins the search with the most significant DAC bit, which controls the widest output variation, and moves toward the least significant bit, causing the DAC output to “zero in” on the unknown value. The result of the trial is the binary code corresponding to the unknown value. In an eightbit SA converter, only eight iterations are required to find the correct binary code, resulting in relatively fast conversions. In this application (Figure 5), an AT89CX051 microcontroller with an integral analog parator performs the SAR function in software, reducing the ponent count. The DAC selected for the application is an MC14088, eightbit, current output type chosen for its low cost. Seven and six bit versions are available as the MC14087 and MC14086, respectively. The MC1408 series is guaranteed accurate to ull scale output current of milliamps. The relative accuracy of the MC14088 is better than %, assuring eightbit monotonicity and linearity. The DAC has an output settling time of 300 nanoseconds. The DAC contains binaryweighted, currentsteering switches which scale an input current by the applied binary code. The input current is derived from an precision voltage reference and a series resistor. The scaled current output is converted to a voltage by an LF355B operational amplifier wired as a currenttovoltage (I/V) converter. The LF355B op amp was selected for the I/V converter because of its low input offset voltage and high output slew rate. The voltage output of the I/V converter is fed into the AT89CX051 parator, where it is pared to the unknown voltage. When the programmed voltage exceeds the unknown voltage the output of the parator goes high, which is detected by software. A second op amp, wired as a noninverting, unity gain buffer may be inserted between the unknown voltage source and the input to the AT89CX051 parator to provide isolation. The reference provides a nominal output (Vref). The actual voltage may vary from to . The reference voltage and temperature coefficient may be trimmed using the method indicated in the data sheet. The nominal value of the current reference resistor (Rref) connected to pin 14 of the DAC is 1240 Ohms, yielding a reference current (Iref) of V / 1240 Ohms (Vref/Rref) = milliamps. The eightbit binary code applied to the DAC scales Iref by from 0/256 to 255/256, resulting in a current output (Io) of from zero (Iref?0/256) to milliamps (Iref ?255/256) full scale. Note that the sign of the DAC output current is opposite the sign of the reference (input) current. The output voltage is determined by multiplying the DAC output current (Io) by the value of the I/V converter gain resistor (Ro). Nominal full scale output voltage is mA?2500 Ohms (Io .?Ro) = . The circuit does not provide adjustments for offset or gain. Offset voltage adjustments should not be required, due to the low offset voltage specification of the LF355B op amp. If the offset voltage must be adjusted, add the offset trim circuit shown in the LF355B data sheet. The gain may be changed by changing the value of the I/V converter gain resistor (Ro). The resistor connected to the noninverting input of the op amp should be of the same value as the gain resistor for input bias current balancing. The 1240 Ohm resistor connected to pin 15 of the DAC and the 2500 Ohm resistor connected to pin three of the op amp may be eliminated with only a slight decrease in performance. The MC14088 DAC requires power supplies of + and to 15volts。 asymmetries between the charge and discharge portions of the cycl