The chip used in this project is STM8005, ADC 10-bit precision
When doing an AC voltage sampling project recently, I found that the data read by the adc and its input voltage have a large deviation, but the adc data of other channels is normal, and only the voltage sampling data is caused by the problem. Finally, after searching, it was found that the adc input impedance was too small to cause the problem.
table of Contents
1. The circuit before the change
3. Possible causes of the problem
1. The circuit before the change
2. The modified circuit
Change 10M, 510K to 100K, 5.1k, the voltage that adc outputs at this moment is normal.
3. Possible causes of the problem
Looking up the manual for the description of this section, I found that there is no requirement similar to the input impedance, only the limit of the input current. It is guessed that the problem may be caused by the input current being too small. I found some opinions on the Internet, the input impedance of the chip adc is about 10k, more than this value may cause problems. Checking the datasheet of stm8 did not find the description of RAIN, and found the relevant description in stm32, the input impedance of its adc is 50K, so it is inferred that the input impedance of stm8 should also be about 50K.
Fourth, the final solution
Subsequently, because of the ratio of 100K to 5.1K, the voltage deviation will be too large and the accuracy of voltage sampling will be too low. A voltage follower is added to the output terminal to reduce the output impedance of the signal, and the voltage sampling is normal.
