Seven Sources of Sensor Circuit Noise

Circuit design is a key factor in whether the sensor performance is superior, because the sensor output is a very small signal, if the noise caused by the useful signal is submerged, it is not worth the loss, so strengthen the anti-interference design of the sensor circuit is particularly important. Before that, we must understand the source of sensor circuit noise in order to find better ways to reduce noise.

There are seven main types of sensor circuit noise

　　low frequency noise

The low frequency noise is mainly caused by the discontinuity of the conductive particles inside. In particular, the carbon film resistor, there are many tiny particles inside the carbonaceous material, and the particles are discontinuous. When the current flows through, the conductivity of the resistor will change and cause the current to change, resulting in a flash arc similar to poor contact. In addition, the transistor may also produce similar burst noise and flicker noise, the mechanism of which is similar to the discontinuity of particles in the resistor, and also related to the doping degree of the transistor.

　　Shock noise generated by semiconductor devices

Since the change of the voltage of the barrier region at both ends of the semiconductor PN junction causes the change of the amount of charge accumulated in this region, the capacitance effect appears. When the applied forward voltage rises, the electrons in the N region and the holes in the P region move to the depletion region, which is equivalent to charging the capacitor. When the forward voltage decreases, it again moves the electrons and holes away from the depletion region, equivalent to a capacitive discharge. When a reverse voltage is applied, the change in the depletion region is reversed. When a current flows through the barrier region, this change causes a small fluctuation in the current flowing through the barrier region, thereby generating current noise. The magnitude of the generated noise is proportional to the temperature and the frequency bandwidth Δf.

　　high frequency thermal noise

High frequency thermal noise is generated by the irregular movement of electrons inside the conductor. The higher the temperature, the more intense the electron movement. The irregular movement of electrons inside the conductor forms many tiny current fluctuations inside it. Because it is disorderly movement, its average total current is zero. However, when it is connected to the amplifier circuit as a component (or as a part of the circuit), its internal current will be amplified and become a noise source, especially for high-frequency thermal noise of the circuit working in the high frequency band.

Usually within the power frequency, the thermal noise of the circuit is proportional to the passband, and the wider the passband, the greater the impact of the circuit thermal noise. Taking a 1kΩ resistor as an example, if the passband of the circuit is 1MHz, the effective value of the open circuit voltage noise presented at both ends of the resistor is 4 μV (set the temperature at room temperature T = 290K). It seems that the electromotive force of the noise is not large, but assuming that it is connected to an amplification circuit with a gain of 106 times, its output noise can reach 4V, and then the interference to the circuit is very large.

　　Interference of electromagnetic components on the circuit board

Many circuit boards have electromagnetic components such as relays and coils. When the current passes through, the inductance of the coil and the distributed capacitance of the housing radiate energy to the surrounding area, and the energy will interfere with the surrounding circuits. When components such as relays work repeatedly, instantaneous reverse high voltage will be generated when power is turned on and off, forming 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.

　　noise of transistor

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

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

The current in the 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 that there is no regular fluctuation in the emitter current or the collector current, and shot noise is generated.

The noise caused by poor cleaning of the transistor surface 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.

　　Noise of resistor

The interference of the resistance comes from the inductance in the resistance, the capacitance effect and the thermal noise of the resistance itself. For example, a solid-core resistor with a resistance value of R can be equivalent to a series and parallel connection of a resistor R, a parasitic capacitance C, and a parasitic inductance L. The parasitic capacitance is 0.1~0.5pF, and the parasitic inductance is 5~8nH. At frequencies above 1MHz, these parasitic inductances are not negligible.

Resistors all generate thermal noise. When a resistor with a resistance value of R (or BJT body resistance, FET channel resistance) is not connected to the circuit, the thermal noise voltage generated in frequency band B is in the formula: K is Boltzmann constant; T is temperature (unit: k). The thermal noise voltage itself is a non-periodic time function, and its frequency range is very wide. Therefore, the wide band amplifier circuit is more affected by noise than the narrow band.

The resistance produces contact noise, and the contact noise voltage formula is: I is the mean square value of the current flowing through the resistance; F is the frequency; K is a constant related to the geometric shape of the data. Because Vc plays an important role in the low frequency band, it is the main noise source of the low frequency sensor.

　　noise of integrated circuit

There are generally two kinds of noise interference in integrated circuits: one is radial and the other is conductive. These noise spikes can have a significant impact on other electronic devices connected to the same AC network. 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 supply and the ground pin of an integrated circuit on the general microcontroller system board, and you can see the noise spike peak-peak value can reach hundreds of millivolts or even volts.