Effect of switching power supplies on operational amplifiers
General analogue signals into the ADC chip before the use of operational amplifiers for signal conditioning, in order to provide the necessary level conversion, filtering, ADC chip drive and so on. Operational amplifiers and ADC phase interface, easy to be affected by the power supply, which also affects the stability of the ADC chip acquisition.
Most of the ADC chip internal analogue input has a sampling capacitance Cin, resistor R1 on the output of the op-amp current limiting, several times the sampling capacitance of the ceramic capacitor C1 so that the switch SW closed at the moment, through the C1 quickly to the sampling capacitance Cin charging. R1, the specific value of the C1, with the stability of the op-amp, the establishment of the time, the ADC sampling time, the sampling accuracy required.
It should be noted here that the power supply of the op-amp will also play a big role in the above process. During the charging of the capacitor by the op-amp, a large current is required instantaneously, and the insufficient load response time of the switching power supply will result in a relatively large power supply ripple, affecting the output of the op-amp. For example, using C1 = 10Cin = 250pF, then when the SW from other channels (assuming -5V) cut to the AI0 channel (assuming +5V), Cin switches from -5V to the voltage on C1 +5V, C1 quickly charge Cin, * the final voltage is (5V × 10-5V)/11 = 4.09V, the op-amp output to be changed from 5V to 4.09V, R1 is too small to easily R1 is too small to easily bring the op-amp output stability problems, but also the op-amp output current will bring impact, affecting the supply voltage.
Especially when the charge pump is used to provide a small negative power supply to the op-amp-VCC, the characteristic that the output voltage of the charge pump decreases as the load increases makes the effect more obvious. Comparison found that the op-amp using DC linear regulated power supply, 12-bit ADC acquisition results are very stable, the results of the change up to 1LSB or less; in contrast, the use of charge pump device, if the charge pump output does not have a large filter, the ADC acquisition results wobble up to 3LSB. if increase R1 for 100Ω when C1 = 10Cin, do not take into account the op-amp output resistor when the op-amp output current needs to be The * large value of the current is (5-4.09)V/100Ω=9.1mA), which is smaller than the * large output current of the general op-amp. But R1 is too large, will significantly reduce the frequency of the signal that the ADC can capture, in the ADC on the channel "tracking" period, the op-amp can not be completed on the C1 and Cin charging, so that the sampling and the op-amp input voltage difference is large, which will cause harmonic distortion.
