Circuit design is a key element of sensor performance. Since the output of the temperature and humidity sensor is a tiny signal, if the useful signal is engulfed due to noise, the anti-interference of the enhanced sensor circuit is outstanding. We need to understand the source of sensor circuit noise in order to find a good way to reduce noise. Sensor circuit noise generally has the following seven types:
1, low frequency noise,
Low frequency noise is formed by the discontinuous motion of internal conductive particles. It is a carbon film resistor. There are many tiny particles in the carbonaceous data. The particles are discontinuous. When the current flows, the electrical conductivity of the resistor changes and the current changes, resulting in a flashover arc with similar contact failure. Transistors have also produced similar burst noise and flicker noise, and their vibrancy is similar to the discontinuity of the particles in the resistor and also to the doping level of the transistor.
2. Shot noise generated by semiconductor devices
Due to the change in the voltage of the barrier region across the semiconductor PN junction, the amount of charge accumulated in this region is altered, resulting in a capacitive effect. When the applied forward voltage rises, the electrons and P regions of the N region move toward the depletion region, which is equivalent to charging the capacitor. When the forward voltage is small, it keeps electrons and holes away from the depletion region, which is equivalent to a capacitor discharge. When a reverse voltage is applied, the depletion region changes inversely. When current flows through the barrier region, this change causes the current flowing through the barrier region to cause a slight shake and generate current noise. The amount of noise generated is proportional to the temperature and the bandwidth Δf.
3, high frequency thermal noise
High frequency thermal noise is caused by the random movement of electrons inside the conductor. The higher the temperature, the more intense the electronic motion. The random motion of the electrons inside the conductor will constitute a lot of tiny current fluctuations inside it, because it is disorderly motion, so its uniform total current is zero, but when it is connected as a component (or as a part of the circuit) After the amplifier circuit is amplified, the internal current is amplified into a noise source, which is especially affected by the high-frequency thermal noise of the circuit 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 at both ends of the current resistor is 4μV (the temperature is room temperature T=290K). It seems that the electromotive force of the noise is not large, but it is assumed that when 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 large.
4, the noise of the transistor
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 the temperature and humidity transmitter is the main one.
The current in the BJT is usually a uniform value. In practice, the number of carriers injected into the base region through the emitter junction is different at each instant, so that the emission current or the current collector has irregular shaking, which generates shot noise.
Due to the semiconductor data and the degree of manufacturing process, the noise caused by the poor cleaning of the transistor surface is called flicker noise. It is related to the recombination of minority carriers on the surface of the semiconductor. It is characterized by the fluctuation of the emission current, and its current noise spectral density is inversely proportional to the frequency, also known as 1/f noise. It plays a major role mainly in the low frequency (below kHz) range.
5, the noise of the resistor
The interference 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. The parasitic capacitance is 0.1 to 0.5 pF, and the parasitic inductance is 5 to 8 nH. These parasitic inductances and capacitances must not be ignored at frequencies above 1MHz.
The resistors generate thermal noise. The resistance of a resistor R (or the body resistance of BJT, the channel resistance of the FET) is not connected to the circuit. In the thermal noise voltage generated in the frequency band B: k is Boltz Mann constant; T is temperature (unit: K). The thermal noise voltage itself is a time function of a non-periodic change, and its frequency range is very broad. Therefore, the wideband amplifying circuit is affected by noise more than the narrow band.
The resistor generates contact noise, and the contact noise voltage is: where I is the mean square value of the current flowing through the resistor; 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 for low frequency sensors.
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