有关ad模数)转换的外文翻译(编辑修改稿)

有关ad模数)转换的外文翻译(编辑修改稿)内容摘要：

ue Selection For optimum performance of the analog section, care must be taken in the selection of values for the integrator capacitor and resistor, autozero capacitor, reference voltage, and conversion rate. These values must be chosen to suit the particular application. Integrating Resistor The integrating resistor is determined by the full scale input voltage and the output current of the buffer used to charge the integrator capacitor. Both the buffer amplifier and the integrator have a class A output stage with 100 181。 A of quiescent current. They can supply 20181。 A of drive current with negligible nonlinearity. Values of 5181。 A to 40 181。 A give good results, with a nominal of 20 181。 A, and the exact value of integrating resistor may be chosen by: Integrating Capacitor The product of integrating resistor and capacitor should be selected to give the maximum voltage swing which ensures that the tolerance builtup will not saturate the integrator swing (approx. from either supply). For +5V supplies and analog COMMON tied to supply ground, a + to +4V full scale integrator swing is fine, and F is nominal. In general, the value of CINT is given by: A very important characteristic of the integrating capacitor is that it has low dielectric absorption to prevent rollover or ratiometric errors. A good test for dielectric absorption is to use the capacitor with the input tied to the reference. This ratiometric condition should read half scale , and any deviation is probably due to dielectric absorption. Polypropylene capacitors give undetectable errors at reasonable cost. Polystyrene and polycarbonate capacitors may also be used in less critical applications. AutoZero and Reference Capacitor The physical size of the autozero capacitor has an influence on the noise of the system. A larger capacitor value reduces system noise. A larger physical size increases system noise. The reference capacitor should be large enough such that stray capacitance to ground from its nodes is negligible . The dielectric absorption of the reference cap and autozero cap are only important at poweron or when the circuit is recovering from an overload. Thus, smaller or cheaper caps can be used here if accurate readings are not required for the first few seconds of recovery. Reference Voltage The analog input required to generate a full scale output is REFIN VV 2 The stability of the reference voltage is a major factor in the overall absolute accuracy of the converter. For this reason, it is remended that a high quality reference be used where highaccuracy absolute measurements are being made. Rollover Resistor and Diode A small rollover error occurs in the ICL7135, but this can be easily corrected by adding a diode and resistor in series between the INTegrator OUTput and analog COMMON or ground. The value shown in the schematics is optimum for the remended conditions, but if integrator swing or clock frequency is modified, adjustment may be needed. The diode can be any silicon diode such as 1N914. These ponents can be eliminated if rollover error is not important and may be altered in value to correct other (small) sources of rollover as needed. Max Clock Frequency The maximum conversion rate of most dualslope A/D converters is limited by the frequency response of the parator. The parator in this circuit follows the integrator ramp with a 3181。 s delay, and at a clock frequency of 160kHz (6181。 s period) half of the first reference integrate clock period is lost in delay. This means that the meter reading will change from 0 to 1 with a 50 181。 V input, 1 to 2 with a 150 181。 V input, 2 to 3 with a 250181。 V input, etc. This transition at midpoint is considered desirable by most users。 however, if the clock frequency is increased appreciably above 160kHz, the instrument will flash “1” on noise peaks even when the input is shorted. For many dedicated applications where the input signal is always of one polarity, the delay of the parator need not be a limitation. Since the nonlinearity and noise do not increase substantially with frequency, clock rates of up to ~1MHz may be used. For a fixed clock frequency, the extra count or counts caused by parator delay will be constant and can be subtracted out digitally. The clock frequency may be extended above 160kHz without this error, however, by using a low value resistor in serieswith the integrating capacitor. The effect of the resistor is to introduce a small pedestal voltage on to the integrator output at the beginning of the reference integrate phase. By careful selection of the ratio between this resistor and the integrating resistor (a few tens of ohms in the remended circuit), the parator delay can be pensated and the maximum clock frequency extended by approximately a factor of 3. At higher frequencies, ringing and second order breaks will cause significant nonlinearities in the first few counts of the instrument. See Application Note AN017。