Analysis of the seven major noises inside the sensor circuit in the circuit design

“Circuit design is a key factor for the superior performance of the sensor. Since the output of the sensor is very small signal, if the useful signal is flooded due to noise, it will not be worth the loss. Therefore, it is particularly important to strengthen the anti-interference design of the sensor circuit. Before that, we must understand the source of the sensor circuit noise in order to find a better way to reduce the noise.

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Circuit design is a key factor for the superior performance of the sensor. Since the output of the sensor is very small signal, if the useful signal is flooded due to noise, it will not be worth the loss. Therefore, it is particularly important to strengthen the anti-interference design of the sensor circuit. Before that, we must understand the source of the sensor circuit noise in order to find a better way to reduce the noise. In general, there are seven main types of sensor circuit noise:

Low frequency noise

Low-frequency noise is mainly caused by the discontinuity of internal conductive particles. Especially for carbon film resistors, there are many tiny particles inside the carbonaceous material, and the particles are discontinuous. When current flows, the conductivity of the resistor will change and the current will change, resulting in a flash arc similar to poor contact. . In addition, transistors may also produce similar popping noise and flicker noise, the mechanism of which is similar to the discontinuity of particles in the resistor, and is also related to the degree of doping of the transistor.

Shot noise generated by semiconductor devices

The change in the voltage of the barrier region at both ends of the semiconductor PN junction causes the amount of charge accumulated in this region to change, thereby exhibiting a capacitance effect. When the applied forward voltage increases, the electrons in the N zone and the holes in the P zone move to the depletion zone, which is equivalent to charging the capacitor. When the forward voltage decreases, it keeps electrons and holes away from the depletion zone, which is equivalent to capacitor discharge. When a reverse voltage is applied, the depletion zone changes in the opposite direction. When the current flows through the barrier area, this change will cause small fluctuations in the current flowing through the barrier area, thereby generating current noise. The magnitude of the noise produced is proportional to the temperature and the frequency bandwidth △f.

High frequency thermal noise

High-frequency thermal noise is caused by the irregular movement of electrons inside the conductor. The higher the temperature, the more intense the movement of electrons. The irregular movement of electrons inside the conductor will form a lot of small current fluctuations inside it. Because it is a disorderly movement, its average total current is zero, but when it is used as a component (or as a part of a circuit) it is connected and amplified After the circuit is completed, the internal current will be amplified and become a noise source, especially the high-frequency thermal noise of the circuit working in the high-frequency band.

Usually in the power frequency, the thermal noise of the circuit is proportional to the passband. The wider the passband, the greater the influence of the thermal noise of the circuit. Taking a 1kΩ resistor as an example, if the passband of the circuit is 1MHz, the effective value of the open-circuit voltage noise present at both ends of the resistor is 4μV (set temperature as room temperature T=290K). It seems that the electromotive force of the noise is not large, but assuming that it is connected to an amplifier circuit with a gain of 106 times, its output noise can reach 4V, and the interference to the circuit will be great at this time.

Interference of electromagnetic components on the circuit board

Many circuit boards have electromagnetic components such as relays and coils. When current passes through, the inductance of the coil and the distributed capacitance of the shell radiate energy to the surroundings, and the energy will interfere with the surrounding Circuits. Components such as relays work repeatedly, and when the power is turned on and off, they will generate instantaneous reverse high voltage, forming an instantaneous surge current. This instantaneous high voltage will have a great impact on the circuit, which will seriously interfere with the normal operation of the circuit.

Transistor noise

The noise of the transistor mainly includes thermal noise, shot noise, and flicker noise.

Thermal noise is generated when the irregular thermal movement of carriers passes through the bulk resistance of the three regions in the BJT and the corresponding lead resistance. Among them, the noise generated by rbb is the main one.

Generally speaking, the current in BJT is just an average value. In fact, the number of carriers injected into the base region through the emitter junction is different at each instant, so the emitter current or collector current has irregular fluctuations, which will produce shot noise.

The noise caused by the poor cleaning of the surface of the transistor due to the semiconductor material and manufacturing process level is called flicker noise. It is related to the recombination of minority carriers on the semiconductor surface, which is manifested as the fluctuation of the emitter current, and its current noise spectral density is approximately inversely proportional to the frequency, also known as 1/f noise. It mainly plays a major role in the low frequency (below kHz) range.

Resistor noise

The interference of resistance comes from the inductance and capacitance effect in the resistance and the thermal noise of the resistance itself. For example, a solid core resistance with a resistance value of R can be equivalent to a series-parallel connection of resistance R, parasitic capacitance C, and parasitic inductance L. Generally speaking, the parasitic capacitance is 0.1 to 0.5 pF, and the parasitic inductance is 5 to 8 nH. When the frequency is higher than 1MHz, these parasitic inductances and capacitances cannot be ignored.

All kinds of resistors will produce thermal noise. When a resistor with a resistance value of R (or BJT body resistance or FET channel resistance) is not connected to the circuit, the thermal noise voltage generated in the bandwidth B is:

In the formula: k is Boltzmann’s constant; T is the absolute temperature (unit: K). The thermal noise voltage itself is a non-periodic time function, so its frequency range is very wide. Therefore, the wide-band amplifier circuit is more affected by noise than the narrow-band.

In addition, the resistance will also produce contact noise, and its contact noise voltage is:

In the formula: I is the mean square value of the current flowing through the resistance; f is the center frequency; k is a constant related to the geometry of the material. Because Vc plays an important role in the low frequency band, it is the main noise source of the low frequency sensor circuit.

Integrated circuit noise

There are generally two types of noise interference from integrated circuits: one is radiation type and the other is conduction type. These noise spikes will have a greater impact on other Electronic devices connected to the same AC power grid. The noise spectrum extends above 100MHz. In the laboratory, you can use a high-frequency oscilloscope (above 100MHz) to observe the waveform between the power and ground pins of an integrated circuit on a general single-chip system board, and you will see noise spikes up to hundreds of millivolts or even peaks. Volt level.