The sampling here is actually divided into current sampling, voltage sampling, DC sampling and AC sampling. The sampling type is determined by the load, and sampling can be divided into high-voltage side sampling and low-voltage side sampling.
Let's take a look at this common circuit: This circuit is a synchronous signal generating circuit, which is often used in grid voltage sampling circuits.
sampling circuit
The circuit consists of three parts:
1. Resistor, slide wire rheostat and voltage comparator LF353 form a buffer link.
2. The voltage comparator LF353 performs zero-crossing comparison.
3. The output part of the circuit, that is, the input circuit of the clamp protection I/O port of the MCU system.
According to the virtual short and virtual break, we can calculate the voltage of the second part of the circuit, and the formula is the same as the calculation method of the same-phase proportional operation. R5 belongs to the feedback resistor of the second part, which compensates for the stability of the sampling signal of this part.
Let's move on to the circuit below.
sampling circuit
This circuit is also a synchronous signal generating circuit.
The first part: a filter circuit composed of a first-order RC, that is, a differential circuit. This connection of lm124 acts as a voltage follower, which constitutes the function of a voltage follower.
The second part: voltage comparator, used to realize zero-crossing comparison. It can be noted that this comparator uses a hysteresis link to suppress disturbances and signal oscillations.
When designing an op amp, the following points should be paid attention to:
1) Input bias current: When designing the op amp, keep in mind the existence of virtual shorts and virtual breaks. If a bias current is input, it will cause a large post-stage error.
2) PSRR: When selecting an op amp, choose an op amp with a higher PSRR on the basis of cost control.
3) The op amp compensation capacitor cannot be added indiscriminately, otherwise the phase will be revers. We know that the current and voltage of the capacitor are out of phase.
4) Pay attention to slew rate and adding feedback resistors, but also pay attention to op amp sampling time and power dissipation.
The above analysis of the three components of the sampling circuit hopes to provide you with some references!
