Noise of the hottest sensor and its suppression me

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Sensor noise and its suppression method

[Abstract] the noise source of the sensor circuit is analyzed in detail, and the practical solutions such as shielding, isolation, and signal processing circuits such as filtering and detection are given

Keywords: sensor, noise, signal processing

1 introduction

as the frontier sentry of the automatic control system, the sensor, like an electronic eye, receives and converts the measured information into effective electrical signals, but at the same time, some useless signals are also mixed in it. These useless signals are collectively referred to as noise

it should be said that noise exists in any circuit, but its impact on the sensor circuit is particularly prominent. This is because the output impedance of the sensor is generally very high, which makes its output signal attenuate severely. At the same time, the sensor is easily submerged by the noise signal. Therefore, the existence of noise must affect the accuracy and resolution of the sensor, and the sensor is the primary link of the automatic control system, so it is bound to affect the performance of the entire automatic control system

therefore, the research of noise is an important link that must be considered in the design of sensor circuit. Only by effectively suppressing and reducing the influence of noise can we effectively use the sensor and improve the resolution and accuracy of the system

however, there are many kinds of noise, the causes are complex, and the interference ability to the sensor is also very different, so the methods of noise suppression are also different. The following is a comprehensive study of the noise of the sensor

2 noise analysis and Countermeasures of sensor

the source of sensor noise is divided into internal noise and external noise according to the noise source

2.1 internal noise - noise from sensor devices and circuit components

2.1.1 thermal noise

the mechanism of thermal noise is that the fluctuation of potential difference occurs when the free electrons in the resistance make irregular thermal movement, which is caused by temperature and is proportional to it, and is expressed by the following Nyquist formula:

where, VN: the effective value of noise voltage; K: Boltzmann constant (1.38 × J·K-1); T: Absolute temperature (k); B: Frequency band width of the system (Hz); R: Resistance value of noise source (Ω)

noise sources include the internal resistance of the sensor itself, circuit resistance elements, etc

it can be seen from formula (1) that the thermal noise cannot be fundamentally eliminated because it comes from the device itself. It is advisable to choose a resistance with a smaller resistance value as far as possible

at the same time, the thermal noise has nothing to do with the frequency, but is proportional to the bandwidth, that is, there is a uniform power distribution corresponding to different frequencies, so it is also called white noise. Therefore, choosing narrow-band amplifier and phase sensitive detector can effectively reduce noise

2.1.2 amplifier noise

2.1.3 shot noise

the noise source of shot noise is transistor, and its mechanism is formed by the fluctuation of current caused by the fluctuation of charged particles reaching the electrode. The noise current in is directly proportional to the current IC reaching the electrode and the frequency band width b, which can be expressed as:

it can be seen that when the preamplifier of bipolar transistor is used to amplify the output signal of the sensor, IC should be selected as small as possible. At the same time, narrow-band amplifier can also be selected to reduce shot noise current

2.1.4 1/f noise

1/f noise and thermal noise are the main noise sources inside the sensor, but their generation mechanism is still controversial. It is generally believed that it is a kind of body noise, not a surface effect, which is caused by lattice scattering. Near the p-n of the transistor is the noise produced by the irregularity of electron hole recombination. The power distribution of the noise is inversely proportional to the frequency, which is named for it. The noise voltage is expressed as:

Hooge also proposed an empirical formula to explain 1/f noise in 1969:

where SRH and svh are the power noise density corresponding to resistance fluctuation and voltage fluctuation, V is the bias voltage added to R, and N is the total number of free carriers, α Called Hooge factor, it is a constant independent of device size, and it is an important parameter to judge material properties

for rectangular resistors, the total number of free carriers n = PLWH, where p is the carrier concentration and l, W and H are the length, width and thickness of the resistor

therefore, we can conclude that 1/f noise is related to the geometric parameters of the force sensitive resistor. Generally, for a certain material, expanding the resistance area can increase N and reduce 1/f noise. At the same time, the experiment shows that blindly increasing the size will reduce the sensitivity and increase the vibration amplitude of the noise spectrum, and choose l/w = 10, l at 100 μ m~200 μ M is more appropriate

at the same time, 1/f noise is also related to materials. Experiments show that monocrystalline silicon is obviously better than microcrystalline silicon, while microcrystalline silicon is slightly better than polycrystalline silicon. The main reason is that monocrystalline silicon has a relatively complete lattice structure. In addition to lattice defects, the movement of hydrogen atoms or atomic clusters in materials and the boundaries of grains are also other main reasons for 1/f noise caused by material factors

according to the above formula, the carrier concentration is inversely proportional to the 1/f noise, and different doping concentrations correspond to different carrier concentrations, so the doping concentration is also a factor affecting the 1/f noise. The experiment shows that the 1/f noise decreases by 36% - 50% when the doping concentration increases by 10 times, but the best doping concentration is generally selected as 5 × 1015cm-2。

2.1.5 noise generated by switching devices

generally, when analog multiplexers are used to make many sensor outputs use an amplifier circuit alternately (such as MOS image sensor), the opening of the switch means that "the environmental conditions around the tested product created by the experimental equipment (including the platform environment) should meet the requirements of the environmental conditions and their tolerances specified in the product experimental specifications", and the corresponding noise interference will be generated, And superimposed on the output signal

the suppression of switching noise usually uses the method of setting the corresponding pseudo sensor circuit

2.2 external noise

external noise is caused by human or natural interference outside the sensor circuit. The main reason is electromagnetic radiation. Its noise sources are very wide, including almost all electrical and electrical machinery, as well as natural phenomena such as lightning and atmospheric ionization. At the same time, when the modular part of the system has public grounding and public power supply, the frequent current changes of digital signals generate noise in the analog circuit. They exist in the sensor circuit through electrostatic coupling, electromagnetic coupling and leakage current, as shown in Figure 1

in view of the above causes, it is necessary to adopt electrostatic shielding and magnetic shielding for the sensor circuit, so as to reduce the electrostatic and magnetic coupling between the noise source and the sensor circuit, so as to achieve the purpose of suppressing external noise. The usual measures are:

2.2.1 processing of analog-to-digital hybrid circuits

it is required to separate the power supply and ground wire of analog circuit and digital circuit respectively, and make the internal resistance of DC power supply of digital circuit as small as possible, so as to reduce the impact of digital signal on analog circuit

2.2.2 anti interference of stray electromagnetic field

shielding is the main method to reduce external noise interference. Shielding is to surround components, transmission wires, circuits and assemblies with low resistance materials to isolate the mutual interference of internal and external electromagnetic or electric fields

there are generally three kinds of shielding: electric field shielding, magnetic field shielding and electromagnetic shielding

electric field shielding is mainly used to prevent interference caused by distributed capacitive coupling between components or circuits. Generally, high conductivity materials such as copper, iron and other metals are selected. The electric field shield must be reliably grounded

magnetic field shielding is mainly used to eliminate the interference caused by magnetic field parasitic coupling between components or circuits. Generally, high permeability materials such as soft iron and permalloy are selected

electromagnetic shielding is mainly used to prevent the interference of high-frequency electromagnetic field, so it is effective to use high conductivity materials such as copper, silver and other metals. They use electromagnetic field to generate eddy current in the shielding metal to absorb its energy and achieve the purpose of shielding

2.2.3 isolation

isolation is to separate the signal grounding terminals of the front and rear circuits from the circuit, because they are easy to form loop current and cause noise interference. The main method of isolation is to use transformer and optocoupler. Transformer isolation is only applicable to AC circuits. Photoelectric coupling isolation is often used in DC or ultra-low frequency measurement systems

2.2.4 requirements for output line, power line, wiring and wiring

the output lines of the sensor should be twisted with each other to reduce the influence of external magnetic lines and optimize the configuration. At the same time, the output line should be as short as possible

if the noise current flows into the power line and wiring, it will radiate the noise magnetic field and pick up the noise induced by the electromagnetic field of the noise source, that is, it is easy to send and receive the noise tear. Therefore, the wiring must not have antenna effect. The double stranded wire and stranded wire can eliminate the magnetic field, but cannot completely eliminate the electrostatic effect. Coaxial cable can eliminate electromagnetic field at the same time

when wiring in a ring, the electromotive force caused by the magnetic lines crossing the ring will produce noise. Therefore, the wiring should try to make the incoming and outgoing wires of current not close to and twisted

balanced unbalanced transformer has high impedance to common mode noise and low impedance to normal noise, so it absorbs noise in the process of balanced wiring caused by unbalanced wiring to stranded wire

2.3 signal processing circuit to reduce noise

sensor circuit first needs to amplify the sampled weak signal. But at the same time, there are many noise sources: sensor internal resistance, cable resistance, amplifier circuit and electromagnetic interference sources around the circuit. Therefore, low-pass filters and differential amplifiers are usually used to suppress differential mode noise and common mode noise (as shown in Figure 3)

set, vs is the signal voltage of the sensor; Vn1 and vn2 are the induced noise voltage of the external noise source on the cable line; VNS is the circuit noise. Therefore, the output voltage Vo of the differential amplifier is:

the relevant circuit is described in detail below

2.3.1 differential amplifier circuit

the input stage of the integrated operational amplifier can not only eliminate the zero drift phenomenon by using the symmetry of the differential circuit, but also reduce the common mode signal, improve the common mode rejection ratio, and eliminate noise interference

2.3.2 filtering electricity makes it more difficult to complete the experiment. The function of the circuit is mainly to process the input signal, usually hoping to filter out the noise. There are many kinds of filters, which can be divided into classical filters and modern filters. The classical filter can filter out the noise occupying different frequency bands from the useful signal, but it can't do anything when the spectrum of the useful signal and the noise overlap each other. Figure 4 shows the spectrum of signal and noise. In Figure 4, s (f) is a useful signal, the frequency band is F1 ~ F2, and n (f) is noise. After filtering, only noise other than F1 ~ F2 can be filtered out, and the overlapping part of noise and signal cannot be filtered out

modern filters regard signal and noise as random signals, derive a set of best estimation methods by using their statistical characteristics, and then implement them with hardware or software. Vera filter is its representative

in general, classical filters are used most. The filter circuit can be composed of passive components R, l and C, such as the line filter shown in Figure 5; It can also include active components (as shown in Figure 6), which has the advantages of certain signal amplification and load carrying capacity. Filter circuits can be divided into four categories from the function: low-pass (LP), high pass (HP), band-pass (BP) and band stop (BS) filters. Each has two forms, analog (AF) and digital (DF) filters. Since the sensor signal is generally a slowly changing signal, the low-pass filter is most used to suppress the high-frequency noise signal. Usually ruba

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