The reference voltage of the single-chip microcomputer is generally 3.3V. If the external signal exceeds the AD measurement range, the resistor divider method can be used, but the impedance matching problem should be paid attention to. For example, the analog-to-digital input impedance of SMT32 is about 10K. If the external voltage divider resistance cannot be much smaller than this value, the output impedance of the signal source is relatively large, and the input impedance of AD is relatively small. The voltage causes voltage division, which eventually leads to a large error in voltage reading.
Therefore, for the use of a single-chip microcomputer to read the external signal voltage, a smaller resistor must be selected for the external voltage divider, or if power consumption is required, a voltage divider with a large resistance can be selected, and then a voltage follower can be used for impedance matching (The input impedance of the voltage follower can reach several megaohms, and the output impedance can be several ohms or less). If the output impedance of the signal source is large, a voltage follower can be used to match and then a resistor divider can be connected. 
For the external ADC chip, when selecting the type, pay attention to its type (SAR type, switched capacitor type, FLASH type, double integral type, Sigma-Delta type). Different types of ADC chips have different input impedances——
1. SAR type: The internal resistance of this ADC is very large, generally above 500K. Even if the impedance of the ADC is small, the impedance is fixed. Therefore, even if the internal resistance of the source under test is stable, it is only equivalent to the resistance divider and can be corrected;
2. Switched capacitor type: such as TLC2543, which requires very low input impedance to quickly charge the internal sampling capacitor. At this time, it is best to have a low resistance source, otherwise it will cause errors. If it is not possible, a large capacitor can be connected in parallel externally. After each sample, the voltage of the large capacitor does not drop much. Therefore, after the external large capacitor is connected in parallel, the switched capacitor input can be equivalent to a pure resistive impedance, which can be corrected;
3. FLASH type (direct comparison type): Most high-speed ADCs are direct comparison type, also known as flash type (FLASH), generally low impedance. A low-impedance source is required. It is purely resistive to the outside and can be directly connected to the op amp;
4. Double integral type: Most of this type has extremely high input impedance, almost no need to consider impedance;
5. Sigma-Delta type: This is the ADC type with the highest precision at present, and the following issues need to be paid attention to:
Measurement range problem: SigmaDelta ADC is a switched capacitor type input and must have a low impedance source. So in order to simplify the external design, most of the internal buffers are integrated. When the buffer is opened, it presents high resistance to the outside and is easy to use. But note that the buffer is actually an op amp. Then there must be restrictions on the upper and lower rails. Most buffers are 50mV on the lower rail and AVCC-1.5V on the upper rail. In this kind of application, the total input range is greatly reduced, and 0V cannot be measured. Be very careful! It is generally used in bridge measurement because the common mode range is around 1/2VCC. Don't worry too much about the zero votes of the buffer, it is easy to correct through the internal zeroing register;
The problem with the RC filter at the input: SigmaDelta ADC is a switched capacitor input and works well on low-impedance sources. But sometimes in order to suppress the common mode or suppress the signal outside the Nyquist frequency, it is necessary to add an RC filter at the input. Generally, a table of the relationship between the maximum allowable input impedance and C and Gain is given on DATASHEET. A strange feature at this time is that the larger C is, the maximum input impedance must be reduced accordingly! Many people may be puzzled at first, but in fact, it's easy to understand just thinking about the capacitor charging characteristics for a long time. Another compromise is to take a large C, much larger than several million times the sampling capacitor Cs (generally 4-20PF), then input the equivalent pure resistance, and the voltage division error can be corrected with the GainOffset register.
The op amp must not be directly connected to the SigmaDelta ADC! As mentioned earlier, the switched capacitor input circuit periodically uses sampling capacitors to sample from the input, and each time it is connected in parallel with the op amp, it will exhibit low resistance and divide the output impedance of the op amp, causing the voltage to drop, and the negative feedback will immediately begin to correct. But the op amp slew rate (SlewRate) is limited and cannot respond immediately. As a result, it causes an instantaneous voltage drop. When the sampling is almost complete, it is equivalent to high resistance, and the output voltage of the op amp rises, but the slew rate makes the op amp too late to correct, and the result is overshoot. And this is the most critical sampling end time. Therefore, the op amp and SD ADC must be connected through a resistor and capacitor (connected as a low pass). The RC relationship must obey the rules stated in the datasheet.
Differential input and bipolar problems: SD ADCs can all be differential input, and both support bipolar input. But the bipolarity here does not mean that negative pressure can be measured, but the voltage between the Vi+ Vi- pins. Assuming that Vi- is connected to AGND, the negative pressure measurement range will not exceed -0.3V. The correct connection method is that the common mode of Vi+ Vi- is differential input between -0.3~VCC. A typical example is the electric bridge. Another example is connecting Vi- to Vref, and the voltage of Vi+ to Vi- allows bipolar input
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