【正文】 英文原文12Bit A/D ConverterCIRCUIT OPERATIONThe AD574A is a plete 12bit A/D converter which requires no external ponents to provide the plete successive approximation analogtodigital conversion function. A block diagram of the AD574A is shown in Figure 1.Figure 1. Block Diagram of AD574A 12Bit AtoD ConverterWhen the control section is manded to initiate a conversion (as described later), it enables the clock and resets the successiveapproximation register (SAR) to all zeros. Once a conversion cycle has begun, it cannot be stopped or restarted and data is not available from the output buffers. The SAR, timed by the clock, will sequence through the conversion cycle and return an endofconvert flag to the control section. The control section will then disable the clock, bring the output status flag low, and enable control functions to allow data read functions by external mand. During the conversion cycle, the internal 12bit current output DAC is sequenced by the SAR from the most significant bit (MSB) to least significant bit (LSB) to provide an output current which accurately balances the input signal current through the 5kΩ(or10kΩ) input resistor. The parator determines whether the addition of each successivelyweighted bit current causes the DAC current sum to be greater or less than the input current。 if the sum is less, the bit is left on。 if more, the bit is turned off. After testing all the bits, the SAR contains a 12bit binary code which accurately represents the input signal to within 1/2 LSB. The temperaturepensated buried Zener reference provides the primary voltage reference to the DAC and guarantees excellent stability with both time and temperature. The reference is trimmed to volts 177。%。 it can supply up to mA to an external load in addition to the requirements of the reference input resistor ( mA) and bipolar offset resistor (1 mA) when the AD574A is powered from 177。15 V supplies. If the AD574A is used with 177。12 V supplies, or if external current must be supplied over the full temperature range, an external buffer amplifier is remended. Any external load on the AD574A reference must remain constant during conversion. The thinfilm application resistors are trimmed to match the fullscale output current of the DAC. There are two 5 kW input scaling resistors to allow either a 10 volt or 20 volt span. The 10 kW bipolar offset resistor is grounded 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, maki