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AD394TD 데이터시트(PDF) 10 Page - Analog Devices |
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AD394TD 데이터시트(HTML) 10 Page - Analog Devices |
10 / 12 page AD394 Rev. A | Page 10 of 12 8-Bit Processors The circuit of Figure 13 shows the general principles for connecting the AD394 to an 8-bit data bus. The 74LS244 buffers the data bus; its outputs are enabled when the DAC address appears on the address bus. The first byte sent to the DAC is loaded to the 74LS373 octal latch and, when the second byte is sent to the DAC, it is combined with the first byte to create a 12-bit word. The connections shown are for right-hand justified data. CS and WR inputs to the DAC are also gated, and when active, the DAC is loaded. Pull-up resistors at the output of the 74LS244 buffer ensure that the inputs to the DAC do not float at an ill-defined level when the DAC is not being addressed. This method of connecting 12-bit DACs to an 8-bit data bus is most cost effective when multiple DACs are utilized for 8-bit data bus applications. Figure 13. 8-Bit Data Bus Interface APPLICATIONS The functional density of the AD394 permits complex analog functions to be produced under digital control, where board space requirements would otherwise be prohibitive. Multiple- output plotters, multichannel displays, complex waveform generation, and multiple programmable voltage sources can all be implemented with the AD394 in a fraction of the space that would be needed if separate DACs were used. Using the AD394 for Analog-to-Digital Conversion Many systems require both analog output and analog input capability. While complete integrated circuit analog-to-digital converters (such as the AD574A) are readily available, the AD394 can be used as the precision analog section of an ADC if some external logic is available. Several types of analog-to- digital converters can be built with a DAC, comparator, and control logic, including staircase, tracking, and successive- approximation types. In systems that include a micropro-cessor, only a comparator must be added to the AD394 to accomplish the ADC function since the processor can perform the required digital operations under software control. A suitable circuit is shown in Figure 14. The AD311 comparator compares the unknown input voltage to one of the AD394 outputs for the analog-to-digital conversion, while the other three outputs are used as normal DACs. The diode clamp shown limits the voltage swing at the comparator input and improves conversion speed. With careful layout, a new compar-ison can be performed in less than 15 µs, resulting in a 12-bit successive approximation conversion in under 180 µs. The benefit of using the AD394 in this application is that one ADC and three DACs can be implemented with only two IC packages (the AD394 and the comparator). Figure 14. Using One AD394 Output for A/D Conversion Programmable Window Comparator The AD394 can be used to perform limit testing of responses to digitally controlled input signals. For example, two DACs may be used to generate software-controlled test conditions for a component or circuit. The response to these input conditions can be either completely converted from analog to digital or simply tested against high and low limits generated by the two DACs in the AD394. Figure 15. Programmable Window Comparator Used in Power-Supply Testing In the circuit shown in Figure 15, two AD311 voltage compar- ators are used within the AD394 to test the output of a 5 V power-supply regulator. The AD394 VOUT1 output (through an appropriate current booster) drives the input to the regulator to simulate variations in input voltage. The output of the regulator is applied to Comparators 1 and 2, with their outputs wire- |
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