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First,I illustrate the application of this circuit,as shown in Figure 1.It is called capacitive liquid level
detection.When the needle touches the liquid level,the capacitance value will become larger.So using phase locked loop
to detect the change of capacitance.My schematic diagram is shown in Figure 2.Network named signal is connected to the
needle.Normally,the voltage of TP1 is 2.5v,if the neddle touches the liquid level,the voltage of TP1 will change,as
shown in Figure 3.However, When the needles are filled with ionized water, the voltage will become unstable,as shown
in Figure 4.The frequency of the TP1 point voltage is 50Hz.It will affect the judgement of the waveform.Now I have two
solutions,one is change the low pass filter parameter of PLL,so it can filter out the interference of 50hz, but to
consider the timeliness of change.Another is to add a notch filter.where to put the notch filter,I want to put it in
the output of OP U4,so the signal is passed through the notch filter before it is passed to MCU.
Now my question is that which will be better,and how to make it.If you have some advice,I will appreciate it very
much.Thank you very much.
detection.When the needle touches the liquid level,the capacitance value will become larger.So using phase locked loop
to detect the change of capacitance.My schematic diagram is shown in Figure 2.Network named signal is connected to the
needle.Normally,the voltage of TP1 is 2.5v,if the neddle touches the liquid level,the voltage of TP1 will change,as
shown in Figure 3.However, When the needles are filled with ionized water, the voltage will become unstable,as shown
in Figure 4.The frequency of the TP1 point voltage is 50Hz.It will affect the judgement of the waveform.Now I have two
solutions,one is change the low pass filter parameter of PLL,so it can filter out the interference of 50hz, but to
consider the timeliness of change.Another is to add a notch filter.where to put the notch filter,I want to put it in
the output of OP U4,so the signal is passed through the notch filter before it is passed to MCU.
Now my question is that which will be better,and how to make it.If you have some advice,I will appreciate it very
much.Thank you very much.
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