Circuit design is the key factor in whether the sensor performance is superior, because of the sensor output is very small signal, if because of the noise lead to useful signal submerged, that thed loss outweights the gain, so strengthening the sensor circuit anti-interference design is particularly important. Before this, we must understand the sensor circuit noise sources, in order to find better ways to reduce the noise. In general, the sensor circuit noise mainly include seven:

Low frequency noise,

Low-frequency noise is mainly caused by internal of conductive particle discontinuous. Carbon film resistor, in particular, there are many tiny particles, the carbonaceous material internal between particles is discontinuous, when current flows through, can make the conductivity of the resistance change caused the change of the current, produce similar poor contact this arc. In addition, the transistor will have a similar burst noise and flicker noise, its mechanism and the resistance of particles in near discontinuity, can also be related to the doping level of the transistor.

Semiconductor devices have shot noise

Due to both ends of the semiconductor PN junction barrier area voltage changes the amount of charge accumulated in the area change, thus showing the capacitance effect. When plus forward voltage rise, N the electron and the hole to the depletion region motion P, equivalent to the capacitance charging. When the positive electricity reduce hours, it make the electrons and holes from depletion region, equivalent to a capacitor discharge. When coupled with reverse voltage, exhausted area change instead. When current flows through the barrier zone, this change will cause the electric current flows through the potential barrier to produce small fluctuations, resulting in a current noise. The size of its noise is proportional to the temperature, the band width delta f.

The high frequency thermal noise

The high frequency thermal noise is due to the process by which a conductive body internal electronic. The higher the temperature, the electronic sport becomes more intense. Internal electronic conductor random games in its internal form many tiny electric current wave, because of its movement is chaos, therefore, its average total current is zero, but when it as a component (or) as part of a circuit is connected to the amplifier circuit, its internal current can be amplified as noise source, especially to work in high frequency circuit in high frequency band are thermal noise influence.

Usually within the power frequency, thermal noise of the circuit is proportional to the passband, passband is wide, the greater is the impact on the thermal noise of the circuit. In a 1 k Ω resistance, for example, if the circuit passband is 1 MHZ, are open circuit voltage RMS noise on opposite sides of the resistance of 4 mu V set temperature to room temperature (T = 290 k). Looks like noise electromotive force is not big, but suppose to wear it while plugged into a gain of 106 times the amplifying circuit, the output noise can be up to 4 v, then it will make a great disturbance to the circuit.

The interference of electromagnetic components of circuit boards

Many circuit boards have relay, the coil electromagnetic components, such as when the current through the coil inductance and the shell of the distributed capacitance to the surrounding radiation energy, the energy can produce interference on the surrounding circuits. Components such as relay the repeated work, reverse voltage of power would produce instant form the instantaneous surge current, the instantaneous high pressure will produce a great impact on circuit, thus seriously interfere with the normal work of the circuit.

The noise of the transistor

Transistors are mainly thermal noise, shot noise and flicker noise.

Thermal noise is due to the carrier within the irregular thermal motion through the BJT three OuDeTi resistance and the corresponding lead resistance from time to time. The RBB "' ' ' ' ' ' ' ' ' ' ' ' ' 'noise is generated by the major.

As the BJT current, just an average. Actually the carrier number, launch and injected into the base area is not the same in every instantaneous, which have any rules or emitter current collector current fluctuations, shot noise is produced.

Because of semiconductor materials and manufacturing technology level of causing the noise of the transistor surface cleaning is bad and is called the flicker noise. It is associated with minority carrier on the surface of the semiconductor compound, show the emitter current of ups and downs, its current noise spectral density is inversely proportional to the frequency approximation, also known as 1 / f noise. It mainly plays a main role in low frequency (below kHz).

The noise of the resistor

Interference resistance comes from the resistance, inductance, capacitance effect and thermal noise of resistance itself. For example a solid core resistance, resistance to R can be equivalent to the resistance R, parasitic capacitance C, parasitic inductance L series parallel. Generally speaking, the parasitic capacitance is 0.1 ~ 0.5 pF, parasitic inductance is 5 ~ 8 nh. When the frequency is higher than 1 MHZ, these parasitic inductance capacitance can not be ignored.

All kinds of resistance thermal noise can be generated a resistance R resistance (or body resistance of BJT and FET channel resistance) is not connected to the circuit, the bandwidth of the B thermal noise voltage is:

Type: k is the boltzmann constant; T is the absolute temperature (unit: K). Thermal noise voltage is itself a non-periodic change time function, therefore, its frequency range is very wide. So the wideband amplifier circuit is affected by noise is bigger than narrow band.

In addition, the resistance will also produce contact noise, noise voltage of the contact:

Current through the resistance in the type: (I) for the mean square value; F center frequency; K is associated with the geometric shape of material constant. Due to the Vc play an important role in low frequency band, so it is the main noise source of low frequency sensor circuit.

The noise of the integrated circuit

Integrated circuit noise generally has two kinds: one kind is radiant, one is the conduction type. These noise spikes for pick up in the same ac online other electronic equipment can produce great influence. Noise spectrum extended to more than 100 MHZ. In the lab, you can use the high frequency oscilloscope (more than 100 MHZ) to observe the general single chip microcomputer system on an integrated circuit board waveform between the power and ground pins, will see the peak - peak noise spike up to hundreds of millivolt and even v level.