From July 8th to yesterday, we carried out the primary school experiment. This week we had an intense and fulfilling experience. This experiment fully combined our knowledge of analog and digital electricity, allowing us to fully understand the circuit. principle.
Experimental design tasks
1. Complete the measurement of the tiny signal with noise as required. The input small signal is DC 40mV, and the noise is a triangular wave with a frequency of 1KHz. The test result is displayed with a digital tube.
2. Complete circuit design, simulation, installation and debugging.
The content and requirements of the experiment
1. A voltage divider resistor network is used to generate a 40mV weak DC signal.
2. Use 555 circuit to generate 1KHz square wave, and send the square wave to the integrator to generate triangle wave.
3. Use LM324 to realize the addition circuit, superimpose the weak DC signal and the triangle wave as the noise signal.
4. Use LM324 to build an active low-pass filter, the passband cutoff frequency is 100Hz, the stopband cutoff frequency is 500Hz, and the stopband attenuation is -40dB.
5. Use LM324 to realize the design of 10 times amplifier, amplify the weak signal after filtering.
6. Use comparators, D flip-flops, NAND gates and other circuits to design a 2-bit parallel comparison ADC.
7. Use a resistor divider network to obtain a reference voltage of 0.5V.
8. Use CD4511 to compose a display circuit to display the final measured voltage value.
Our simulation diagram is as follows.
Starting from the lower left corner, the waveform of our 555 module at this time is on the positive half axis. 
Because we want to send the wave to the back integrator to generate a triangular wave, we must ensure the square wave positive and negative half axis distribution, so At this time we need an adder to move the waveform down, so we add an adder at the back, and input the waveform output from the adder into the integrator to get a triangle wave. Here we add a high pass to filter the low frequency part of the signal spectrum. . The picture below is the triangle wave we got
Now we look at the upper left corner and output a 40mv DC signal through the voltage division. Then we use the adder to add the triangle wave and the DC signal together, and then use a second-order low-pass filter to filter the clutter, followed by an operational amplifier to amplify 40mv ten times and input it to the ADC module behind, using a resistor divider network Obtain a 0.5V reference voltage. Then by referring to the truth table, the measured voltage value finally obtained is displayed.
When our simulation is successful, we will design our circuit board, so we cherish the opportunity to do it ourselves. I use an adjustable temperature soldering iron. The temperature of the soldering iron cannot be too high or too low. About three Baidu is fine. I’m not very confident in the ADC module I’m responsible for, but during the whole process, I also read the manual carefully and checked the truth table. Of course, I also made a lot of mistakes. I think the most fundamental reason is that we did not practice deliberately. I practiced too little, so I used a lot of jumpers to connect GND and VCC. At that time, I didn't expect to use DuPont cables. That would make the board look more beautiful. But although there are many problems and difficulties, the final result is still good.
Every elementary school we can learn what we can't learn in textbooks, or put the theory into practice, we first simulate on the computer and then we debug in reality. Generally speaking, we still need to exercise to improve our practical ability. In order to have a choice in the future.