1. ADC analog to digital converterWhat is it?

ADC refers to analog/digital converter or analog-to-digital converter. Refers to a device that converts a continuously changing analog signal into a discrete digital signal. Real-world analog signals, such as temperature, pressure, sound or images, need to be converted into digital forms that are easier to store, process, and transmit.

2. Classification of ADC

There are many types of analog-to-digital converters, which can be divided into indirect ADCs and direct ADCs according to different working principles. The indirect ADC converts the input analog voltage into time or frequency first, and then converts these intermediate quantities into digital quantities. A double integral ADC is commonly used. Direct ADC is directly converted into digital quantity. Commonly used are parallel comparison ADC and successive approximation ADC. What I want to discuss here isSuccessive approximation ADC。

3. Successive approximation ADC

3.1 Principle: Successive approximation adc is composed of comparators, D/A converters, buffer registers and several control logic circuits. The principle is to compare bit by bit from high bit to low bit, first clear the bits of the buffer register to zero; after the conversion starts, first set the highest bit of the register to 1, send the value to the D/A converter, and send the analog after the D/A conversion The comparator, called Vo, is compared with the analog quantity Vi to be converted by the comparator. If Vo<Vi, the highest bit 1 of the buffer register is reserved, otherwise it is cleared to 0. Then, set the next highest bit of the register to 1, send the new digital value in the register to the D/A converter, and compare the output Vo with Vi. If Vo<Vi, this bit is reserved, otherwise it is cleared to 0. Loop this process until the lowest bit of the register to get the digital output.

3.2 Resolution

Resolution is the ability of the ADC to recognize the smallest signal change. The resolution of a 12-bit ADC is 12 bits, or the resolution is 1/(2^12) of full scale.
Taking Vref as 5V, the minimum input voltage change that 12-bit ADC can distinguish is 5V/(2^12 )=0.0012V.

3.3 accuracy

The accuracy of the adc module is generally 8 bits, 10 bits, 12 bits, 16 bits, and 24 bits.

Take the 5V ADC module as an example to introduce:

8-bit precision: divide 0~5V into 256 parts, and each part represents 5/256=0.02V;

10-bit precision: divide 0~5V into 1024 parts, and each part represents 5/1024=0.005V;

12-bit precision: divide 0~5V into 4096 parts, each representing 5/4096=0.0012V;

16-bit precision: divide 0~5V into 65536 parts, each representing 5/65536=0.000076V;

24-bit precision: divide 0~5V into 16777215 parts, each representing 5/16777215=0.00000023V;

Actual application:

The reference voltage is also called the reference voltage. If there is no reference voltage, the accurate amplitude of the measured signal cannot be determined. For example, when the reference voltage is 2.5V, when the measured signal reaches 2.5V, the ADC outputs a full-scale reading, and the user will know that the full-scale output of the ADC is equal to 2.5V. For different ADCs, some have an external reference, and some have built-in references that do not need to be connected. Other ADCs can use both external and built-in references, but the external reference takes precedence over the built-in reference.

Use an external reference voltage as the ADC reference voltage, because VDD and GND are generally connected to the external reference voltage pins VREF+ and VREF- of the ADC. So the voltage of VDD is the reference voltage of ADC, because the voltage that LDO supplies MCU has certain deviation. Therefore, you need to know the voltage of VDD before using the ADC. For a 12-bit ADC, the converted value corresponding to the VDD voltage is 4096 (2^12)