ADC basic concepts

1、 Concept 、 Application scenarios 、 Basic knowledge of

(1) ADC clock frequency

ADC The clock frequency is the number of clock pulses per second , It is ADC The basis of module operation .

It is obtained by dividing the frequency of the system clock through many links , It depends on the external clock input and the frequency doubling or frequency division coefficient of each link .

(2) Sampling conversion time = Sampling time +12.5 individual Clock cycle （12 position AD Fixed value ）--- According to the chip manual

The sampling time is ADC Record the time of input voltage analog quantity , The input voltage shall be stable enough within this time , The recorded value will not be inaccurate due to voltage fluctuation .

Conversion time refers to ADC Time to convert recorded analog quantity into digital quantity .

(3) ADC sampling frequency 、 Sampling period

Sampling frequency means ADC How many samples can the module complete per second , The sampling frequency depends on the startup ADC The frequency of . start-up ADC There are many ways , For example, the software starts directly , Use the event manager to write events from memory , Or use an external pin to start , start-up ADC The frequency of ADC Sampling frequency of , for example : every other 1ms Start once ADC, that ADC The sampling frequency of is in place 1KHZ. The period is the reciprocal of the frequency , The sampling period is 1ms

(4) The conversion time cannot be longer than the sampling period , To sample correctly

For a specific hardware environment and conversion bits , The transition time is certain , That is to say, at a certain dominant frequency 、 Frequency division coefficient and conversion resolution , The hardware conversion time is fixed , The specific value of this time can be found through AD transformation The data book of the chip is obtained .

in general , For fast changing measured objects, a higher sampling frequency , Low sampling frequency can be adopted for slow variables to save processing resources of single chip microcomputer . However, the sampling frequency should not exceed the limit of conversion time ！

(5) Sampling theorem ,

sampling frequency Greater than or equal to the maximum frequency of the target signal 2 times , Will not be distorted . namely fs=2fm, Suppose the target signal is a single frequency signal , The frequency is f, Then the cycle T=1/f, therefore fs=2f=2/T, Again because fs=1/Ts, therefore Ts=T/2

（6） Reference voltage

The reference voltage is the maximum voltage that the analog-to-digital converter can convert . for example 3.3V,3V

(7) Conversion accuracy

Reference voltage /AD Maximum conversion value , The conversion progress is AD Minimum step value , for example ：3.3/4096

(8) Cumulative accuracy

Consider targeting , Each system , Set the compensation value 、 Calibration value （ Storage EE or Flash）

2、 Key knowledge 、 How to use 、 Debugging method

2.1 How to use

（1）ADC Take points

The DC voltage is taken as 14 A little bit , median filter .

AC voltage sampling

2.2 How to use

(1) Determine the sampling target frequency ;

(2) Determine the sampling frequency >= Target frequency *2

(3) Choose the right ADC clock frequency , Calculated conversion time , The conversion time cannot be longer than the sampling period , To sample correctly

(4) Select trigger ADC The way , for example ： Timer ： A signal （ such as ADC collection ） Conduct timing sampling （ That is to say, every once in a while , for instance 2ms）, There are three ways - HXJ_521 The blog of - CSDN Blog

（5） Choose the right algorithm .

（6） Configuration chip ADC, Write trigger 、 Algorithm code .

3、 verification 、 test 、 exception handling

（1） calibration , Point tracking .

For DC voltage sampling , It can be obtained from the formula of the corresponding relationship between the reference voltage and the actual voltage AD The value is related to the actual voltage value （ Provided by hardware ）, Or go to two points to get the linear relation calculation .

Then compare the actual measured value with the sampled value , Calculate the sampling accuracy , To determine if calibration is required .

4、 Existing problems

（1） Calibration circuit ？

（2）DMA Use ？