【正文】 ded for unipolar operation and connected to the 10 volt reference for bipolar operation.DRIVING THE AD574 ANALOG INPUTFigure 2. Op Amp – AD574A InterfaceThe output impedance of an op amp has an openloop value which, in a closed loop, is divided by the loop gain available at the frequency of interest. The amplifier should have acceptable loop gain at 500 kHz for use with the AD574A. To check whether the output properties of a signal source are suitable, monitor the AD574’s input with an oscilloscope while a conversion is in progress. Each of the 12 disturbances should subside in sorless. For applications involving the use of a sampleandhold amplifier, the AD585 is remended. The AD711 or AD544 op amps are remended for dc applications. SAMPLEANDHOLD AMPLIFIERSAlthough the conversion time of the AD574A is a maximum of 35 ms, to achieve accurate 12bit conversions of frequencies greater than a few Hz requires the use of a sampleandhold amplifier (SHA). If the voltage of the analog input signal driving the AD574A changes by more than 1/2 LSB over the time interval needed to make a conversion, then the input requires a SHA. The AD585 is a high linearity SHA capable of directly driving the analog input of the AD574A. The AD585’s fast acquisition time, low aperture and low aperture jitter are ideally suited for highspeed data acquisition systems. Consider the AD574A converter with a 35 ms conversion time and an input signal of 10 V pp: the maximum frequency which may be applied to achieve rated accuracy is Hz. However, with the addition of an AD585, as shown in Figure 3, the maximum frequency increases to 26 kHz.The AD585’s low output impedance, fastloop response, and low droop maintain 12bits of accuracy under the changing load conditions that occur during a conversion, making it suitable for use in high accuracy conversion systems. Many other SHAs cannot achieve 12bits of accuracy and can thus promise a system. The AD585 is remended for AD574A applications requiring a sample and hold.Figure 3. AD574A with AD585 Sample and HoldSUPPLY DECOUPLING AND LAYOUTCONSIDERATIONSIt is critically important that the AD574A power supplies be filtered, well regulated, and free from high frequency noise. Use of noisy supplies will cause unstable output codes. Switching power supplies are not remended for circuits attempting to achieve 12bit accuracy unless great care is used in filtering any switching spikes present in the output. Remember that a few millivolts of noise represents several counts of error in a 12bit ADC.Circuit layout should attempt to locate the AD574A, associated analog input circuitry, and interconnections as far as possible from logic circuitry. For this reason, the use of wirewrap circuit construction is not remended. Careful printed circuit construction is preferred.UNIPOLAR RANGE CONNECTIONS FOR THE AD574AThe AD574A contains all the active ponents required to perform a plete 12bit A/D conversion. Thus, for most situations, all that is necessary is connection of the power supplies (+5 V, +12 V/+15 V and –12 V/–15 V), the analog input, and the conversion initiation mand, as discussed on the next page. Analog input connections and calibration are easily acplished。2 LSB.) If the offset trim is not required, Pin 12 can be connected directly to Pin 9。15 mV of offset trim range.The fullscale trim is done by applying a signal 1/2 LSB below the nominal full scale ( for a 10 V range). Trim R2 to give the last transition (1111 1111 1110 to 1111 1111 1111).BIPOLAR OPERATIONThe connections for bipolar ranges are shown in Figure 5. Again, as for the unipolar ranges, if the offset and gain specifications are sufficient, one or both of the trimmers shown can be replaced by a 50 W 177。圖1所示為AD574A的方塊結構圖。接著，控制部分停止時鐘，把輸出狀態(tài)標志位置低，并允許控制函數(shù)，以便外部命令可以執(zhí)行數(shù)據(jù)讀取功能。 如果總電流較大，位被關閉。%伏之間平衡，當AD574A使用15伏電源時，向雙極偏移電阻提供1mA電流外。有兩個5千歐的輸入測量電阻允許10伏或20伏的區(qū)間。要檢查信號源的輸出特性是否合適，就要在轉(zhuǎn)換進行中使用示波鏡監(jiān)控AD574的輸入端。 雖然AD574A的轉(zhuǎn)換時間最高為35秒，但為了能夠?qū)崿F(xiàn)幾個赫茲的頻率的精確12位轉(zhuǎn)換還是需要使用采樣－保持放大器（SHA）的。考慮到AD574A的轉(zhuǎn)換時間為35秒，并且擁有10Vpp的輸入信號。 AD585被推薦應用于AD574A的采樣與保持。注意： 一點點毫伏的噪聲就代表著12比特ADC（電源）的巨大誤差。單極操作模式如圖4所示。例如，如果沒有應用區(qū)標，% (10LSB)最大滿額誤差。 此器件在輸入電壓0到10伏連接時須接腳9和腳13,當輸入電壓在0到20伏之間時,應從腳14和腳9引入。如果第12腳連在第9腳上，那么單元將在規(guī)格之內(nèi)按此方