The circuit plan is the key element of whether the sensor function is good or not, because the output end of the sensor is very small light signals. If the useful light signals are submerged due to noise, the gain is not worth the loss. Therefore, the anti-interference plan to enhance the sensor circuit is particularly tense. Before that, we must inquire about the source of the sensor circuit noise in order to find out good skills to reduce the noise.

In general, there are seven kinds of sensor circuit noise that matter:

1. Low frequency noise

Low frequency noise matters because the conductive particles inside become discontinuous. In particular, there are many fine particles in the carbon material of the carbon film resistor, and the particles are discontinuous. When the current flow expires, the conductivity of the resistor will change, causing the change of the current, producing a flash arc similar to poor contact. In addition, the transistor may also produce similar burst noise and flash noise, which is similar to the discontinuity of particles in the resistance and is also related to the doping degree of the transistor.

2. Shot noise produced by semiconductor devices

Because the change of the voltage in the barrier region at both ends of the semiconductor pn junction causes the change of the amount of charge accumulated in this region, which flashes the capacitive effect. When the applied forward voltage is raised, the electrons in the N region and the holes in the p region move to the depletion region, which is suitable for charging the capacitor. When the forward voltage decreases, it makes electrons and holes far away from the depletion region, which is suitable for capacitor discharge. When the reverse voltage is applied, the change of depletion region is opposite. When the current flows through the barrier region, this change will cause the current flowing through the barrier region to produce small bumps, resulting in current noise. The magnitude of the noise generated is directly proportional to the temperature and the bandwidth △ F.

3. High frequency thermal noise

High frequency thermal noise is caused by the irregular movement of electrons in the conductor. The higher the temperature, the more violent the electron movement. The irregular movement of electrons in the conductor will form many small current bumps in it. Because it is disordered movement, its balanced total current is zero, but when it is connected to the expansion circuit as a component (or as a side of the circuit), the current in it will be expanded into a noise source, especially for the high-frequency thermal noise of the circuit working in the high repetition period.

At ordinary times, the thermal noise of the circuit is in direct proportion to the passband. The wider the passband, the greater the impact of the thermal noise of the circuit. Taking a 1K Ω resistor as an example, if the passband of the circuit is 1MHz, the useful value of the open circuit voltage noise at both ends of the resistor is 4 μ V (set the temperature as room temperature t=290k). It seems that the EMF of the noise is not large, but if it is connected to an expansion circuit with a gain of 106 times, its output noise can reach 4V, and then the disturbance to the circuit is very large.

4. Disturbance of electromagnetic components on circuit board

Many circuit boards have electromagnetic components such as relays and coils. During the current passage period, the inductance of the coil and the diffuse capacitance of the shell radiate energy to the surrounding, which will disturb the surrounding circuits. When components such as relays work repeatedly, they will produce instantaneous reverse high voltage when switching on and off, forming instantaneous surge current. This instantaneous high voltage will have a big blow to the circuit, thus disturbing the normal work of the circuit.

5. Transistor noise

The noise of transistors mainly includes thermal noise, shot noise and flash noise.

The thermal noise is caused by the irregular thermal motion of carriers passing through the body resistance and the corresponding lead resistance of the three regions in the BJT. Among them, rbb'&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39;&# 39; The noise produced is important.

The current in BJT is only an equilibrium 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 bumps, which will produce shot noise.

The noise caused by the poor surface cleaning treatment of transistors due to the semiconductor material and production process is declared as flash noise. It is related to the recombination of a small number of carriers on the semiconductor surface, which is interpreted as the rise and fall of the emitter current. Its current noise spectral density is inversely proportional to the frequency, also known as 1/f noise. It's important to limit it at low frequencies (below kHz) for important benefits.

6. Noise of resistor

The disturbance 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 the series parallel connection of resistance R, parasitic capacitance C and parasitic inductance L. Generally speaking, the parasitic capacitance is 0.1 ~ 0.5pf and the parasitic inductance is 5 ~ 8nh. When the frequency is higher than 1MHz, these parasitic inductors and capacitors cannot be ignored.

All kinds of resistors will produce thermal noise. When a resistor with resistance value R (or the body resistance of BJT and the channel resistance of FET) is not connected to the circuit, the thermal noise voltage generated within the band width B is:K is Boltzmann constant; T is the overall temperature (unit: k). The thermal noise voltage itself is an aperiodic time function, so its frequency limitation is very wide. Therefore, the wide-band expansion circuit is more affected by noise than the narrow-band expansion circuit.

In addition, the resistance will also produce contact noise, and its contact noise voltage isI is the mean square value of the current flowing through the resistance; F is the intermediate frequency; K is a constant related to the number of patterns of the material. Because VC has the advantage of tension in the low-frequency band, it is an important noise source of the low-frequency sensor circuit.

7、Integrated circuit noise

Integrated circuit noise搅扰普通有两种：一种是辐射式，一种是传导式。这些噪声尖刺关于接在统一交换电网上的其余电子装备会产生较大影响。噪声频谱扩大至100MHz以上。在试验室中，可以用高频示波器(100MHz以上)调查普通单片机体系板上某个集成电路电源与地引脚之间的波形，会看到噪声尖刺峰-峰值可达数百毫伏乃至伏级。

Article source: magnetostrictive displacement sensorhttp://www.delysensor.com