Circuit design is the crucial factor with sensor whether superior performance, because sensor output end is very tiny signal, if bring about useful signal to be flooded because of noise, that thed loss outweights the gain, the interference rejection design that strengthens sensor circuit so is attached most importance to especially should. Before this, we must understand the origin of sensor circuit noise, so that find out better method,will reduce noise. As a whole, sensor circuit noise basically has 7 kinds: Noise of low frequency of low frequency noise basically is caused. Especially carbolic film resistor, its carbon pledges material interior is put in a lot of little grain, it is discontinuous between grain, outdated flows in electric current, can make the conductivity of resistor produces change to cause voltaic change, produce similar contact to shine undesirably explode electric arc. Additional, the dissilient noise that transistor produces likeness possibly also and twinkle noise, its produce the discontinuous sex close of the particle in mechanism and resistor, also concern with the impure degree of transistor. Semiconductor device arises medicinal powder bead noise writtens guarantee as a result of semiconductor PN the change of voltage of area of base of two end power is caused accumulate here the charge amount of area is changed, show capacitance effect thereby. When adscititious when lifting to voltage, the cavity of the electron of N area and P area moves to extinct area, be equivalent to charging to capacitance. When be being reduced to voltage, it makes electron and cavity are far from extinct area again, be equivalent to capacitance discharge. When adscititious and retrorse voltage, the change of extinct area is adverse. Shed zone time of classics power base when electric current, this kind of change can cause the electric current that has shed power base division to produce tiny wave motion, produce voltaic noise thereby. Its produce F of △ of width of the size of noise and temperature, frequency band to become direct ratio. High frequency thermal noise is high frequency because the random of electron of electric system interior moves,thermal noise is of generation. Temperature is higher, electronic campaign is more intense. Of conductor interior electron in its without regular athletic meeting interior forms a lot of small electric current to fluctuate, because its are the motion that do not have foreword, its average total electric current is reason 0, but serve as a component when it (or the one part that serves as circuit) be received after enlarge circuit, its in-house electric current can be magnified become noise source, be in to the job especially high frequency frequency paragraph high frequency thermal noise affects the circuit inside blame very. It is normally inside labour frequency, the thermal noise of circuit and transmission bands become direct ratio, transmission bands is wider, the impact of circuit thermal noise is bigger. It is with the resistor of Ω of a 1k exemple, if the transmission bands of circuit is 1MHz, appear in resistor the virtual value of noise of open circuit voltage of two end sets temperature to be room temperature T=290K) for 4 μ V(. Look the electromotive force of noise is not large, but when receiving its a gain to be the enlarge circuit of 106 times suppose, its output noise to be able to amount to 4V, be opposite at this moment the interference of circuit is very big. The interference of the electromagnetism component on circuit board a lot of circuit board go up to have the electromagnetism element such as relay, coil, the distributinging capacitance that sheds the inductance of its coil when passing and crust in report to all round radiation energy, its energy can generate interference to the circuit all round. Like the component such as relay its work repeatedly, connect the meeting when cutting off the power to generate retrorse maximum pressure of the instant, form instantaneous surge electric current, the high pressure of this kind of instant will produce huge impact to circuit, disturb the regular job of circuit badly thereby. The noise of the noise transistor of transistor basically has thermal noise, medicinal powder bead noise, twinkle noise. Because carry,thermal noise is stream child the irregular body resistor that heats up motion to carry 3 districts inside BJT and corresponding down-lead resistor from time to time arise. Among them the noise that Rbb place produces is main. The electric current in the BJT that says normally, it is an average only. Arrive through launching knot infuse actually base of the area carry stream child amount, in each instantaneous not identical, blast off consequently extremely electric current or collector electric current have ruleless wave motion, meeting generation comes loose a noise. Because semiconductor material and workmanship level make transistor surface clean,handle bad and the noise that cause is called twinkle noise. It and semiconductor surface minority carry stream child compound about, expression is blast off extremely of electric current rise and fall, spectrum density of noise of its electric current and frequency become inverse ratio approximately, call 1/f noise again. It basically is in low frequency (KHz is the following) the main effect since limits. The interference of the noise resistor of resistor comes from the thermal noise of effect of the inductance in resistor, capacitance and resistor itself. Exemple the solid core resistor that is R like value of a block, but the series-parallel connection that equivalent is L of inductance of C of capacitance of resistor R, parasitism, parasitism. Generally speaking, parasitism capacitance is 0. 1 ～ 0.
5pF, parasitism inductance is 5 ～ 8nH. When 1MHz of frequency prep above, these parasitism inductance capacitance cannot be ignored. Of all kinds resistor can produce thermal noise, the resistor that value of a block is R (or the body resistor of BJT, raceway groove resistor of FET) when receiving circuit, the thermal noise tension that produces inside B of frequency band width is: In type: K thises for Bo Er graceful constant; T is absolutely temperature (unit: K) . Itself of thermal noise voltage is aperiodic time function of change, accordingly, its frequency limits is very broad. The influence that so broadband belt magnifies circuit suffers noise is compared narrow frequency band is big. Additional, resistor still can produce osculatory noise, its contact noise voltage to be: In type: I is the cost of voltaic mean square that has spread resistor; F is central frequency; K is the constant that concerns with the geometrical appearance of material. Because Vc is in low frequency paragraph a main effect, so it is the main noise source of circuit of low frequency sensor. The noise interference of the noise integrated circuit of integrated circuit has two kinds commonly: One kind is radiation type, one kind is to conduct pattern. These noise are spinal bigger to receiving the other electron equipment on same communication electrified wire netting to be able to arise impact. Noise spectrum is patulous to 100MHz above. In the lab, can use high frequency oscilloscope (100MHz above) observation is common odd power source of on systematic board a certain integrated circuit and ground make a machine the weaveform between the foot, can see noise spinal summit - peak value can amount to hundreds millivolt even volt class. CNC Milling
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