[electronic circuit] how to select ADC chip

1. Analog input range

1. Acquisition of analog quantity , The input range must be just needed , Non negligible
2. Confirm the size of the analog signal , You can choose the analog input range of different chips , If more than , Analog signals can be collected by partial voltage and op amp following .

2. Sampling rate

1. The sampling rate refers to ADC How many times per second will the clock convert analog quantity to digital quantity , Such as 10K/s That is to say ADC Every second , Just collect 10K Analog quantity , And convert analog quantity into digital quantity .
2. When the sampling frequency is greater than the highest frequency in the signal 2 Times , The sampled digital signal completely retains the information in the original signal , In general practical applications, the sampling frequency is guaranteed to be the highest frequency of the signal 2.56～4 times ; Sampling theorem is also called Nyquist theorem .
3. for example , collection 50K The signal of , The sampling frequency is at least greater than 2.56*50K

3. The channel number

1. Is multichannel an independent channel or a differential channel ？
2. Whether synchronous sampling can be carried out ？
3. Whether the differential channels can be interchanged ？
4. Whether other channels can be grounded ？

Be careful ： The above points are multi-channel considerations

4. The resolution of the

1. ADC Resolution refers to the maximum number that the analog-to-digital converter can represent , namely ADC Number of digits , If ADC yes 10 position ADC, So the resolution is 2 Of 10 Power , namely 1024 The resolution of the , If the analog quantity is temperature , The measurement range is 0~100 degree , So you can take 100 Divide into 1024 Share , Every copy you can feel , When the temperature is 100/1024 When the degree changes , Can measure .
2. The higher the number of bits, the more accurate the sampling accuracy . However, it is not the time to select the type. The higher the accuracy, the better , The higher the accuracy, the higher the cost , According to the actual needs of the project , Choose the right cost accuracy .
3. 8 Less than A/D The converter is called low resolution ADC,9~12 This bit is called medium resolution ADC,13 Bits above are high resolution .A/D The higher the number of bits of the device , The higher the resolution , The smaller the quantization error , The higher the accuracy can be achieved .
4. Quantization error , because AD The error caused by the limited resolution of , Finite resolution AD Stepwise transfer characteristic curve and infinite resolution AD（ Ideal AD） Transfer characteristic curve of （ A straight line ） The maximum deviation between . Usually 1 Analog variation of one or half of the minimum digital quantity , Expressed as 1LSB、1/2LSB. In the process of transformation , Due to quantization error and systematic error , Accuracy will be lost . The influence of quantization error on accuracy is computable , It mainly depends on A/D The number of bits of the conversion device .
   

5. Interface type

1. It is divided into ： A serial port , In parallel ; The serial port is generally SPI
2. According to the resources of single chip microcomputer

6. ADC Input impedance

ADC The impedance matching problem in a particular architecture ADC It's very important in the world , It will affect the accuracy of data conversion . When stringing into a specific interface ADC When , It is equivalent to a parallel impedance of ADC Input impedance element , Therefore, it will have a certain impact on the partial voltage of the circuit . When the internal resistance of the signal source is ADC When the input impedance is similar , Would be right ADC Accuracy has a great impact . The common solution is to ensure that the source side is compared with ADC Low input impedance , Or use input buffer ( commonly Σ-Δ type ADC Built in internal meeting ) To improve the input impedance .