Dear experts, what is the function of R3 and NET point on the fourth stage op amp in the ultrasonic receiving circuit?

拯救小p孩

Dear experts, what is the function of R3 and NET point on the fourth stage op amp in the ultrasonic receiving circuit? [Copy link]

 

Ultrasonic receiving circuit

maychang

『What are the functions of R3 and NET points? 』

The two resistors R3 and R10 form a negative feedback loop for the amplifier circuit of U3A and Q1.

maychang

『What are the functions of R3 and NET points? 』

The figure does not show where the NET point is connected to, so there is no way to know what the NET point is used for.

拯救小p孩

maychang posted on 2021-2-26 07:20 『What is the function of R3 and NET point? 』 The figure does not show where the NET point is connected to, so there is no way to know what the NET point is for...

The NET point is connected to the microcontroller, and you said that the two resistors R3 and R10 form the negative feedback loop of the amplifier circuit of U3A and Q1. I don't quite understand this sentence. Isn't U3A a comparator?

maychang

Save the little p child published on 2021-2-26 09:12 The NET point is connected to the microcontroller, and you said that the two resistors R3 and R10 form the negative feedback loop of the amplifier circuit of U3A and Q1. I don't quite understand this sentence. U ...

"You said that the two resistors R3 and R10 form the negative feedback loop of the amplifier circuit of U3A and Q1. I don't quite understand this sentence."

Two amplifiers cascaded (the output of the previous amplifier serves as the input of the next amplifier) is still one amplifier.

U3A is an amplifier, and Q1 is also an amplifier. These two amplifiers are in cascade connection. However, Q1 is an inverting amplifier (the collector signal is inverted to the base signal), so U3A and Q1 are amplifiers, and their output (Q1 collector) and input (U3A non-inverting input) signals are inverted. Then R3 and R10 are obviously the voltage divider resistors from the output to the inverting input (U3A non-inverting input), forming the negative feedback circuit of this cascaded amplifier.

maychang

Save the little p child published on 2021-2-26 09:12 The NET point is connected to the microcontroller, and you said that the two resistors R3 and R10 form the negative feedback loop of the amplifier circuit of U3A and Q1. I don't quite understand this sentence. U ...

"The NET point is connected to the microcontroller"

Based on this sentence alone, it is still impossible to judge what the role of NET points is.

chunyang

From the circuit point of view, R3 is a negative feedback resistor. Although the final stage of the common ultrasonic ranging receiving amplifier is a comparator, it is not necessary to design it that way. The introduction of Q1 can extend the output level to the vicinity of the power rail, playing the role of comparator setting level, and even closer to the power rail than the direct output of TL074.

The NET terminal should be connected to the output of the MCU, and the level of the NET terminal is controlled by software, which determines the bias level of U3A, and then affects the conduction threshold of Q1, so as to control the detection distance. That is, the ultrasonic ranging circuit has two range gears: "far" and "near".

拯救小p孩

maychang posted on 2021-2-26 10:12 "You said that the two resistors R3 and R10 form the negative feedback loop of the amplifier circuit of U3A and Q1. I don't quite understand this sentence. 』 Two amplifiers cascade (front...

Thanks

拯救小p孩

chunyang posted on 2021-2-26 15:29 From the circuit point of view, R3 is a negative feedback resistor. Although the final stage of the common ultrasonic ranging receiving amplifier is a comparator, it is not necessary to design it that way. The introduction of Q1...

I still don't understand what the purpose of this design is.

Jacktang

First, the ultrasonic signal is amplified by -R1/R6 times, and then other interference sources are filtered out through a bandpass filter, and then amplified again by -R2/R8 times, and finally a falling edge is generated through comparison for the next stage to capture.

U3D, U3B are amplifier circuits, U3C is a bandpass filter, and U3A is a comparison

拯救小p孩

Jacktang published on 2021-2-27 14:51 First, amplify the ultrasonic signal by -R1/R6 times, filter out other interference sources through a bandpass filter, amplify it again by -R2/R8 times, and finally generate a falling edge through comparison...

Why does the comparator need an R3?

Gen_X

R3 is negative feedback, which is used to improve the output waveform and prevent oscillation, because the amplification factor of the transistor is also very high (generally 300).

NET can use simple PWM (high frequency) to adjust the threshold of the 4th level comparator.

For example, if the duty cycle is zero, it means that the comparison benchmark is 1/3. By changing the duty cycle as needed, the threshold can be gradually increased to improve the anti-interference ability.

It can also be considered that in order to prevent false triggering in static state, the power supply voltage output is used to shut down the fourth stage of operation...