模拟数字转换器.docx

1、模拟数字转换器 An analog-to-digital converterAn analog-to-digital converter(ADC) is used to convert a continuously variable signal to a corresponding digital form which can take any one of a fixed number of possible binary values. If the output of the transducer does not vary continuously, no ADC is necess

2、ary. In this case the signal conditioning section must convert the incoming signal to a form which can be connected directly to the next part of the interface, the input/output section of the microcomputer itself.The I/O section converts digital “on/off” voltage signals to a form which can be presen

3、ted to the processor via the system buses. Here the state of each input line, whether it is “on” or “off”, is indicated by a corresponding “1” or “0”. In the analog inputs which have been converted to digital form, the patterns of ones and zeros in the internal representation will form binary number

4、s corresponding to the quantity being converted. Feedback ControlThe class of control problems to be examined here is one of considerable engineering interest. We shall consider systems with several inputs, some known as controls because they may be manipulated and others called external disturbance

5、s, which are quite unpredictable. For example, in an industrial furnace we may consider the fuel flow, the ambient temperature, and the loading of material into the furnace to be inputs. Of these, the fuel flow is accessible and can readily be controlled, while the latter two are usually unpredictab

6、le disturbances.In such situations, one aspect of the control problems is to determine how the controls should be manipulated so as to counteract the effects of the external disturbances on the state of the system. One possible approach to the solution of this problem is to use a continuous measurem

7、ent of the disturbances, and from this and the known system equations to determine what the control inputs should be as functions of time to give appropriate control of the system state.Digital Interface CircuitsThe signals used within microcomputer circuits are almost always too small to be connect

8、ed directly to the “outside world” and some kind of interface must be used to translate them to a more appropriate form. The design of section of interface circuits is one of the most important tasks facing the engineer wishing to apply microcomputers. We have seen that in microcomputers information

9、 is represented as discrete patterns of bits; this digital form is most useful when the microcomputer is to be connected to equipment which can only be switched on or off, where each bit might represent the state of a switch or actuator.Care must be taken when connecting logic circuits to ensure tha

10、t their logic levels and current ratings are compatible. The output voltages produced by a logic circuit are normally specified in terms of worst case values when sourcing or sinking the maximum rated currents. Thus Voh is the guaranteed minimum “high” voltage when sourcing the maximum rated “high”

11、output current Ioh, while Vol is the guaranteed minimum “low” output voltage when sinking the maximum rated “low” output current Iol. There are corresponding specification for logic inputs which specify the minimum input voltage which will be recognized as a logic “high” state Vih, and the maximum i

12、nput voltage which will be regarded as a logic “low” state Vil.For input interface, perhaps the main problem facing the designer is that of electrical noise. Small noises signals may cause the system to malfunction, while larger amounts of noise can permanently damage it. The designer must be aware

13、of these dangers from the outset. There are many methods to protect interface circuits and microcomputer from various kinds of noise. Following are some example:1.Input and output electrical isolation between the microcomputer system and external devices using an opto-isolator or a transformer. 2.Re

14、moving high frequency noise pulses by a low-pass filter and Schmitt-trigger.3.Protecting against excessive input voltages using a pair of diodes to power supply reversibly biased in normal direction.For output interface, parameters Voh,Vol,Ioh and Iol of a logic device are usually much to low to all

15、ow loads to be connected directly, and in practice an external circuit must be connected to amplify the current and voltage to drive a load. Although several types of semiconductor devices are now available for controlling DC and AC powers up to many kilowatts, there are two basic ways in which a sw

16、itch can be connected to a load to control it.With series connection, the switch allows current to flow through the load when closed, while with shunt connection closing the switch allows current to bypass the load. Both connections are useful in low-power circuits, but only the series connection can be used in high-power circuits because of the power wasted in the series resistor R.AT89C51Compatible with MCS-51? Products 8K B