输出缓存区的损坏。
AD5474A单机操作
AD5474A可以“独立”模式使用，它是系统里很好用的、可用的和专用的端口，以这种方式不需要用总线连接。按这方式，CE和12/8置成高位，CS和AO置成低位，而转化由RC控制。 当RC置成高位时，三态缓冲器启动，当RC置成低位时开始转换。其允许两种控制信号一种高电位脉冲，低高电位脉冲。由如图11所示的低脉冲操作。在这种情况下R/C下降沿的输出响应被强制为高阻状态，在一个转换周期结束后置回有效逻辑。STS线在R/C变为低电平600ns后变为高电平，当数据有效300ns后恢复低电平。
图7
如果转换是由如图12所示的高电平脉冲所初始化的，那么在R/C为高电平时，数据链是被允许的。R/C的下降沿启动下一个转换，并且数据链返回到三态（并一直保持三态），知道下一个R/C高电平脉冲出现。
图8
通常应用R/C单机模式下的低脉冲。图13阐明了典型的8086型处理机的单机构造。额外的74F/S374 插销提高了总线的访问/放行次数并协助简化转炉数-模部分的连接线。
图9
AD574与单片机接口
AD574A的控制逻辑使得绝大多数情况下和微处理器系统总线直接连线变成可能。然而它不可能描述出每一种微处理器类型的接口连接的所有细节，下面将举几个具有代表性的例子。
典型的数模转换器接口程序序列涉及以下几步：首先, 在初始化会话的时候，地址被写进数模转换。处理器必须等待会话周期的结束，因为多数数模转换器需要一个以上的指令周期来完成会话操作。当然，有效数据只有在会话结束后才能被读取。AD574A 提供信号端输出(STS) ，它能指示会话过程。这个信号可以由处理器通过读取外部三态缓冲（或其它输入端口）获得。如果系统的计时要求非常严格（请记住AD574A的最大转换时间只有35毫秒）并且处理器在ADC转换周期中有其它任务要做的话，这个STS信号同样可以用于产生一个中断信号传递给转换过程。另一种可行的延时方法是，先假设模数转换器会消耗35微秒来进行转换，然后插入足够多的空指令来保证处理器消耗掉35微秒的时间。
一旦建立,即完成转换,可以读取数据.在8位(或数位更少)ADC的情况下,单次读数运行即已足够.在转换器数位多于总线可使用数位的情况下,须选择数据格式,需进行多重读数运行.AD574A含有内部逻辑(器),允许通过选择连接12/8输入而直接到8位或多或16位数据总线界面上。在采用16位数据总线时,(12/8 高)数据总线(DB11 通过 DB0)既可以连接到数据总线的12位有效位或12位无效位。剩余4位应用软件将其掩蔽.到8位数据总线的界面是采用左优格式来实现的。在数位的上半部偶数地址(A0 低)包含 8 MSBs (DB11 通过 DB4).。奇数地址(A0 高)包含 4 LSBs (DB3 through DB0),后面跟有4个零,从而消除数位掩蔽指令.
AD574A可以在输入/输出或者储存映像结构中被接线到Z-80 处理机上。图15阐明了一个输入/输出或者映像结构。Z-80使用A0–A7 地址线来解码输入/输出端口地址。
Z-80的一个有趣的特性就是当进行I/O操作时会自动插入一个等待状态，允许AD574A和时钟频率高达4MHz的Z-80处理器一起使用。对于高于4MHz的实际应用，可
以使用图16所展示的等待状态发生器。在配置内存内部，AD574A可以和时钟频率高达2.5MHz的Z-80处理器通过接口相连接。
12-Bit A/D Converter
【Abstract:】AD574 is a 12 bit A/D converter Analog produced by Device company America. It uses successive approximation type A/D converter, the maximum conversion time is 25us, the conversion precision of 0.05%, fast conversion so suitable for high precision sampling system. The chip contains a microprocessor excuse logic (three state output buffer), so it can be directly connected with the various types of 8 bit or 16 bit microprocessors, without the need for additional logic interface circuit, cutting can be compatible with CMOS and TTL circuit, can work in a variety of complex environment.
Keywords high precision successive approximation converter
CIRCUIT OPERATION
The AD574A is a complete 12-bit A/D converter which requires no external components to provide the complete successive approximation analog-to-digital conversion function. A block diagram of the AD574A is shown in Figure 1.
Figure 1. Block Diagram of AD574A 12-Bit A-to-D Converter
When the control section is commanded 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 end-of-convert 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 command.
During the conversion cycle, the internal 12-bit 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 comparator determines whether the addition of each successively-weighted 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 12-bit binary code which accurately represents the input signal to within 1/2 LSB.
The temperature-compensated buried Zener reference provides the primary voltage reference to the DAC and guarantees excellent stability with both time and temperature. The
addition to the requirements of the reference input resistor (0.5 mA) and bipolar offset resistor
V supplies, or if external current must be supplied over the full temperature range, an external buffer amplifier is recommended. Any external load on the AD574A reference must remain constant during conversion. The thin-film application resistors are trimmed to match the full-
and connected to the 10 volt reference for bipolar operation.
DRIVING THE AD574 ANALOG INPUT
Figure 2. Op Amp – AD574A Interface