Single chip microcomputer: main index of a/d (analog-to-digital conversion)

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A/D( A / D conversion ) The main indicators of

We are choosing and using A/D When , It is important to rely on what indicators to judge . because AD There are many kinds of , It is divided into integral type 、 Successive approximation type 、 parallel / Serial comparison type 、Σ-Δ Type and other types . At the same time, there are many indicators , And some indicators are slightly different , Here I explain it in a way that is easy for students to understand , If with a certain type A/D Concept and principle are different , It will not affect the practical application .

ADC Number of digits

One n Bit ADC Express this ADC share 2 Of n To the power of a scale .8 Bit ADC, The output is from 0～255 altogether 256 A numerical quantity , That is to say 2 Of 8 To the power of a data scale .

Reference source

Reference source , Also called reference voltage , yes ADC An important indicator of , To enter ADC The signal measurement is accurate , Then the reference source must be accurate first , The deviation of the reference source will directly lead to the deviation of the conversion result . Like a meter ruler , The total length should have been 1 rice , Suppose this meter ruler is baked by fire , It turns out to be 1.2 rice , If you use this meter ruler to measure the length of the object, there will be a large deviation . If our benchmark should be 5.10 V, But what it actually offers is 4.5 V, By mistake 4.5 V Take it for granted. 5.10 V To deal with it , The deviation will also be relatively large .

The resolution of the

Resolution is when the digital quantity changes by a minimum scale , The amount of change in the analog signal , Defined as full scale and span 2n-1 The ratio of the . Assume 5.10 V Voltage system , Use 8 Bit ADC Take measurements , So it's equivalent to 0～255 altogether 256 A scale handle 5.10 V Divided equally into 255 Share , So the resolution is 5.10/255 = 0.02 V.

INL（ Integral nonlinearity ） and DNL（ Differential nonlinearity ）

The two concepts that beginners are most likely to confuse “ The resolution of the ” and “ precision ”, Think the higher the resolution , The higher the accuracy , But in fact , There is no necessary connection between the two . Resolution is used to describe scale division , Accuracy is used to describe the degree of accuracy . The same meter ruler , The number of scales is the same , The resolution is quite , But the accuracy can vary greatly , Pictured 17-2 Shown .

The accuracy shown in the figure above is clear at a glance , Don't say more . and ADC The two indicators of great importance to accuracy are INL(Integral NonLiner) and DNL(Differencial NonLiner).

INL refer to ADC The analog value corresponding to the device in all values , The error value of the point with the largest error between and the real value , yes ADC The most important accuracy index , The unit is LSB.LSB（Least Significant Bit） LSB means least significant bit , So it actually corresponds to ADC The resolution of the . One benchmark is 5.10 V Of 8 position ADC, Its resolution is 0.02 V, Use it to measure a voltage signal , And what you get is 100, It means that the measured voltage value is 100*0.02 V=2 V, Suppose its INL yes 1LSB, It means that the true and accurate value of this voltage signal is 1.98 V～2.02 V Between , Ideally, the corresponding number should be 99～101, The measurement error is a least significant bit , namely 1LSB.

DNL It means ADC The biggest difference between two adjacent scales , The unit is also LSB. A resolution is 1 Mm ruler , Not all adjacent scales are just 1 mm , And there will always be big or small errors . Empathy , One ADC The resolution between the two scale lines is not always exactly equal to the resolution , There are also errors , The error is DNL.

One benchmark is 5.10 V Of 8 position ADC, Suppose its DNL yes 0.5LSB, So when its conversion results from 100 Add to 101 when , Ideally, the actual voltage should increase 0.02 V, but DNL by 0.5LSB In this case, the increase value of the actual voltage is 0.01～0.03 V Between . It is worth mentioning that DNL Not necessarily less than 1LSB, Many times it will be equal to or greater than 1LSB, This is equivalent to a certain degree of scale disorder , When the actual voltage remains constant ,ADC The result may jump between several values , It is largely for this reason （ But not exactly , Because there is always the influence of ubiquitous interference ）.

Conversion rate
Conversion rate , Refer to ADC The maximum number of sample conversions per second , The unit is sps（ or s/s、sa/s, namely samples per second）, It is associated with ADC The time required to complete a conversion from analog to digital is reciprocal to each other .ADC There are many kinds , Among them, the integral type ADC The conversion time is on the order of milliseconds , It belongs to low speed ADC; Successive approximation type ADC The transition time is subtle , Medium speed ADC; parallel / Serial ADC The conversion time can reach nanosecond , It's high speed ADC.

ADC Let's get familiar with these main indicators , For others , As an entry-level player , Don't worry about understanding . In the process of later use, I encountered , Then find relevant materials for in-depth study , The current focus is on building a ADC Basic concepts of .

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