The Influence of Switching Power Supply on Operational Amplifiers
Before the analog signal enters the ADC chip, it is necessary to use an operational amplifier for signal conditioning to provide necessary level conversion, filtering, ADC chip driver, and so on. When the operational amplifier interfaces with the ADC, it is easily affected by the power supply, which also affects the stability of the ADC chip acquisition. Figure 2 is a typical interface diagram between an operational amplifier and an ADC.
Most ADC chips have a sampling capacitor Cin at the analog input end, and the resistor R1 limits the output current of the operational amplifier. The ceramic capacitor C1, which is several times larger than the sampling capacitor, quickly charges the sampling capacitor Cin through C1 when the switch SW is closed. The specific values of R1 and C1 are related to the stability of the operational amplifier, establishment time, ADC sampling time, and required sampling accuracy.
It should be pointed out that the power supply of the operational amplifier also plays a significant role in the above process. During the capacitor charging period of the operational amplifier, a large amount of current is required instantly, and the insufficient load response time of the switching power supply will cause significant power ripple, affecting the output of the operational amplifier. For example, if C1=10Cin=250pF is used, when SW switches from another channel (assuming -5V) to AI0 channel (assuming+5V), Cin switches from -5V to the voltage+5V on C1. C1 quickly charges Cin, and the final voltage is (5V × 10-5V)/11=4.09V, the output of the operational amplifier needs to be changed from 5V to 4.09V. If R1 is too small, it can easily cause stability issues in the output of the operational amplifier, and it can also impact the output current of the operational amplifier, affecting the power supply voltage.
Especially when using a charge pump to provide a small negative power supply to the operational amplifier VCC, the characteristic of the charge pump output voltage decreasing with increasing load makes the effect more obvious. By comparison, it was found that when the operational amplifier uses a DC linear regulated power supply, the 12 bit ADC acquisition results are very stable, and the results can vary by less than 1 LSB; In contrast, when using charge pump devices, if the charge pump output does not have a large filter, the ADC acquisition results can shake up to 3LSB. If R1 is increased to 100 Ω and C1=10Cin, without considering the output resistance of the operational amplifier, the * maximum output current of the operational amplifier is (5-4.09) V/100 Ω=9.1mA), which is smaller than the * maximum output current of a typical operational amplifier. But if R1 is too large, it will significantly reduce the signal frequency that ADC can collect. During the ADC's "tracking" of this channel, the operational amplifier cannot complete the charging of C1 and Cin, resulting in a significant difference in voltage between the sampling and the input of the operational amplifier, which can cause harmonic distortion.
