Explanation of the influence of switching power supply on operational amplifier
Generally, before analog signals enter the ADC chip, operational amplifiers should be used for signal conditioning to provide necessary level conversion, filtering, ADC chip driving and so on. When the operational amplifier interfaces with ADC, it is easily affected by power supply, which also affects the stability of ADC chip acquisition. Fig. 2 is a typical interface diagram of an operational amplifier and an ADC.
Most analog input terminals in ADC chips have a sampling capacitor Cin, and the resistor R1 limits the current of the output of the operational amplifier. When the switch SW is closed, the ceramic capacitor C1 several times the sampling capacitor quickly charges the sampling capacitor Cin through C1. The specific values of R1 and C1 are related to the stability of the operational amplifier, the settling time, the sampling time of ADC and the required sampling accuracy.
It should be pointed out here that the power supply of the operational amplifier will also play a great role in the above process. When the capacitor is charged by the operational amplifier, a large current is needed instantly, and the load response time of the switching power supply is not enough, which will cause a large power supply ripple and affect the output of the operational amplifier. For example, if C1=10Cin=250pF, then when SW is switched from another channel (assuming -5V) to AI0 channel (assuming +5V), Cin will switch from -5V to the voltage of +5V on C1, and C1 will charge Cin quickly. * The final voltage is (5V×10-5V)/11=4.09V, and the output of the operational amplifier will be from 5V.
Especially when the charge pump is used to provide a small negative power supply to the operational amplifier -VCC, the output voltage of the charge pump decreases with the increase of load, which makes the effect more obvious. It is found that when the operational amplifier uses DC linear regulated power supply, the acquisition results of 12-bit ADC are very stable, and the variation of the results can be below 1LSB. In contrast, when the charge pump device is used, if the output of the charge pump is not greatly filtered, the ADC acquisition result will shake up to 3LSB. If R1 is increased to 100Ω, C1=10Cin. Without considering the output resistance of the operational amplifier, the * value of the output current of the operational amplifier needs to be (5-4.09)V/100Ω=9.1mA), which is less than the * large output current of the general operational amplifier. However, if R1 is too large, it will obviously reduce the signal frequency that can be collected by ADC. During the "tracking" of this channel by ADC, the operational amplifier can't complete charging C1 and Cin, which makes the difference between this sampling and the input terminal voltage of the operational amplifier large, which will cause harmonic distortion.
