The sensor circuit is usually used to measure weak signals and has high sensitivity, but it is also easy to receive some random noise or interference signals from the outside or inside. If the magnitude of these noises and interference can be compared with the useful signals, then in The useful signal at the output of the sensor circuit may be overwhelmed, or because the useful signal component and the noise interference component are difficult to distinguish, it will inevitably hinder the measurement of the useful signal. Therefore, in the design of sensor circuits, anti-interference design is often the key to the success of sensor circuit design.
High-frequency thermal noise is caused by the random movement of electrons inside the electrical conductor. The higher the temperature, the more intense the electronic movement. The random movement of electrons inside the conductor will form a lot of tiny current fluctuations inside it. Because it is disordered movement, its average total current is zero, but when it is connected as an element (or as part of the circuit), it is amplified After the circuit, the internal current will be amplified into a noise source, especially for the high-frequency thermal noise of the circuit working in the high-frequency band.
Low-frequency noise is mainly caused by the discontinuous internal conductive particles. Especially for carbon film resistors, there are many tiny particles inside the carbonaceous material. The particles are discontinuous. When the current flows, the resistance of the resistor will change, causing the current to change, resulting in a flash explosion arc similar to poor contact . In addition, the transistor may also produce similar burst noise and flicker noise, and its generation mechanism is similar to the discontinuity of the particles in the resistor, and is also related to the doping degree of the transistor.
The change in the voltage of the barrier region across the semiconductor PN junction causes the amount of charge accumulated in this region to change, thereby exhibiting the capacitance effect. When the applied forward voltage increases, 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 decreases, it moves electrons and holes 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 the current flows through the barrier region, this change will cause slight fluctuations in the current flowing through the barrier region, thereby generating current noise. The amount of noise generated is proportional to the temperature and the bandwidth Δf. Interference of electromagnetic components on the circuit board.
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. Generally speaking, the parasitic capacitance is 0.1 ~ 0.5pF, the parasitic inductance is 5 ~ 8nH. At frequencies above 1MHz, these parasitic inductances and capacitances cannot be ignored.
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