Circuit design is a key factor in the superior performance of the sensor. Since the output of the sensor is a very small signal, if the useful signal is overwhelmed by noise, it will not be worth the loss, so it is especially important to strengthen the anti-jamming design of the sensor circuit. At first, we must understand the source of sensor circuit noise in order to find a better way to reduce noise. In general, there are seven main types of sensor circuit noise.
1. Low frequency noise
The low frequency noise is mainly caused by the discontinuity of the internal conductive particles. In particular, the carbon film resistor has many tiny particles inside the carbonaceous material, and the particles are discontinuous. When the current flows, the electrical conductivity of the resistor changes to cause a variation in current, resulting in a flash arc similar to poor contact. In addition, transistors may also produce similar burst noise and flicker noise, the mechanism of which is similar to the discontinuity of the particles in the resistor, and also to the degree of doping of the transistor.
2. Shot noise generated by semiconductor devices
The amount of charge accumulated in this region changes due to the change in the voltage of the barrier region across the semiconductor PN junction, thereby exhibiting a capacitive effect. When the applied forward voltage rises, the electrons in the N region and the holes in the P region move toward the depletion region, which is equivalent to charging the capacitor. When the forward voltage is reduced, it again causes electrons and holes to move away from the depletion region, which is equivalent to a capacitor 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 noise generated is proportional to the temperature and the bandwidth.
3. High frequency thermal noise
High frequency thermal noise is caused by the random movement of electrons inside the conductor. The higher the temperature is, the more intense the electronic motion will be. The random motion of the electrons inside the conductor creates many small current fluctuations inside it. Because it is disordered, its average total current is zero, but when it is used as a component (or as part of the circuit) it is amplified. After the circuit, its internal current is amplified into a noise source, especially for high-frequency thermal noise of circuits operating in the high-frequency band.
Usually in the power frequency, the thermal noise of the circuit is proportional to the passband, and 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 appearing across the resistor is 4μV (the temperature is room temperature T=290K). It seems that the electromotive force of the noise is not large, but if it is connected to an amplifying circuit with a gain of 106 times, the output noise can reach 4V, and the interference to the circuit is great.
4. Interference of electromagnetic components on the circuit board
Many circuit boards have electromagnetic components such as relays and coils. When the current passes, the inductance of the coil and the distributed capacitance of the outer casing radiate energy to the surroundings, and the energy of the circuit may interfere with the surrounding circuits. Components such as relays work repeatedly. When the power is turned off and off, an instantaneous reverse high voltage is generated to form an instantaneous surge current. This momentary high voltage will have a great impact on the circuit, thereby seriously disturbing the normal operation of the circuit.
5. Transistor noise
The noise of the transistor mainly includes thermal noise, shot noise, and flicker noise.
The thermal noise is caused by the irregular thermal motion of the carriers passing through the body resistance of the three regions in the BJT and the corresponding lead resistance. The noise generated by rbb is the main one.
The current in the so-called BJT is just an average. In fact, the number of carriers injected into the base region through the emitter junction is different at each instant, and thus the emitter current or the collector current has irregular fluctuations, which generate shot noise.
The noise caused by the poor cleaning of the transistor surface due to the semiconductor material and the manufacturing process level is called flicker noise. It is related to the recombination of minority carriers on the semiconductor surface, which is characterized by the fluctuation of the emitter current, and its current noise spectral density is inversely proportional to the frequency, also known as 1/f noise. It plays a major role in the low frequency (below kHz) range.
6. Noise of the resistor
The resistance of the resistor comes from the inductance in the resistor, the capacitive effect, and the thermal noise of the resistor itself. For example, a solid core resistor with a resistance value of R can be equivalent to a series-parallel connection of the resistor R, the parasitic capacitor C, and the parasitic inductance L. Generally, the parasitic capacitance is 0.1 to 0.5 pF, and the parasitic inductance is 5 to 8 nH. These parasitic inductances and capacitances cannot be ignored at frequencies above 1MHz.
All types of resistors generate thermal noise. When a resistance of R (or the body resistance of BJT or the channel resistance of the FET) is not connected to the circuit, the thermal noise voltage generated in the bandwidth B is:
Where: k is the Boltzmann constant; T is the absolute temperature (unit: K). The thermal noise voltage itself is a function of the time of a non-periodic change, so its frequency range is very broad. Therefore, the wideband amplifying circuit is affected by noise more than the narrow band.
In addition, the resistor also generates contact noise, and its contact noise voltage is:
Where: I is the mean square value of the current flowing through the resistor; f is the center frequency; k is a constant related to the geometry of the material. Since Vc plays an important role in the low frequency band, it is the main noise source of the low frequency sensor circuit.
7. The noise of the integrated circuit
There are two types of noise interference in integrated circuits: one is radiated and the other is conductive. These noise spikes can have a large impact on other electronic devices connected to the same AC grid. The noise spectrum is extended to more than 100MHz. In the laboratory, you can use a high-frequency oscilloscope (100MHz or more) to observe the waveform between an integrated circuit power supply and the ground pin on a general-purpose single-chip system board. You can see that the noise spikes and peaks can reach hundreds of millivolts or even Volt.
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