The Influence of Switching Power Supplies on Operational Amplifiers
Before entering the ADC chip, analog signals generally require signal conditioning using operational amplifiers to provide necessary level conversion, filtering, ADC chip driving, 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's data acquisition. Figure 2 is a typical interface diagram of an operational amplifier and ADC.
Most ADC chips have a sampling capacitor Cin at the analog input end, and resistor R1 limits the current output 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, setup 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 charging process of the capacitor by the operational amplifier, a large current is required instantaneously, and the load response time of the switching power supply is insufficient, which will cause significant power ripple and affect the output of the operational amplifier. For example, if C1=10Cin=250pF, when SW switches from another channel (assuming -5V) to AI0 channel (assuming+5V), Cin switches from -5V to the voltage on C1+5V, and C1 quickly charges Cin. The final voltage is (5V × 10-5V)/11=4.09V, and the output of the operational amplifier needs to change from 5V to 4.09V. If R1 is too small, it can easily cause stability problems in the operational amplifier output and also have an impact on the operational amplifier output current, 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 that the output voltage of the charge pump decreases with increasing load makes the effect more pronounced. Comparison shows that when the operational amplifier uses a DC linear regulator power supply, the 12 bit ADC acquisition results are very stable, and the result variation can reach less than 1LSB; In contrast, when using charge pump devices, if there is no significant filtering in the output of the charge pump, the ADC acquisition result can shake up to 3LSB. If R1 is increased to 100 Ω, C1=10Cin, When not considering the output resistance of the operational amplifier, the maximum output current of the operational amplifier needs to be (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 frequency of the signal that the 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 large difference between the sampling and the input voltage of the operational amplifier, which will cause harmonic distortion.
