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what is isolation amplifier
Add date:9/25/2012  Hits:1826

                        what is isolation amplifier

    Isolation amplifier treatment inputting power signal or physical signals through a technique means , isolated outputs a set of standards analog signals 4-20mA/0-20mA/0-10mA/0-10V/0-5V/1-5V/2-10V/0-2.5V/0-±20mA/0-±10mA/0-±10V/0-± 100mV/0-±5V and other signals to control systems and instrumentation equipment sampling acquisition, what is isolation amplifier here only say so much, and more detailed technical data, please visit the "Shenzhen Jie ShengDa Technology Co., Ltd.," official website: http://www.isolation-transmitter.com/ , The website of the "news section" there is an article called "isolation amplifier/transmitter isolation definition and presentation"(click on the green Font can directly go into the
article), the article stresses more detailed and clear that we can refer,inadequacies please forgive me.

What is two-wire isolation amplifier?
Two-wire isolation amplifiers only one Isolation amplifiers of transmission type , which is  in series the power supply and load resistance with  isolation amplifiers ,two wires simultaneously transmit the transmitter required  power and output current signal, most current isolation transmitters are two-wire isolation amplifier;  Many of them are three-wire and four-wire isolation amplifier

First, what is two-wire current isolation amplifier ?
1、what is  two-wire ?
2、what are Two-wire system features:
    (1)、two-wire means field  isolation amplifier or isolation transmitter only use two wires to achieve signal transmission and drive a load when isolation amplifier signal transmittion、signal acquisition and transform  to the control room instrumentation , the two wire  both a power supply line and the signal line .
    (2)、two-wire isolators advantages:
    a、Less susceptible to affect by  parasitic the rmocouples, resistance pressure drop  from along the wire loop and temperature drift , can use very cheap and finer wire, you can save a lot of cables and installation costs;
    b、When the current source output resistance is large enough, the magnetic field sensor coupled to the wire loop internal voltage,which does not have a significant impact, because the current is very small caused by interference source ,if there is interference generally using twisted pair can reduce the interference, two-wire isolation amplifier have to use shielded cable, shielded cable shield should be properly grounded;
    c、Campacitive interference can cause the error from receiver resistance, for 4-20mA two-wire loop isolators , the receiver resistance typically precision 250Ω ( sample Vout=1-5V) that resistance small enough to can't produce significant error, therefore,allowed wire length farther and longer, However, the specific length according to the actual situation;
    d、Each single display device or recording device can access between the different cable lengths and different channels, not due to wiring length ranging caused different accuracy to achieve decentralized acquisition,which advantage is : decentralized acquisition centralized control ......
    e、4-20mA use in zero level,you can determine the open circuit,short circuit,sensor damage (0mA state )it is very convenient ;
    f、add one or two lightning protection and surge device two at the two-wire transmitter isolation/isolation amplifier output port for facilitating safe mine explosion;
What is a three-wire system ?
    Three-wire system is on-site isolation amplifier power supply positive terminal and the signal output positive terminal separation, but they share a COM terminal, with three lines transmit signal to the control room instrumentation and industrial equipment (PC, PLC, DCS)  of industrial site ,Achive signal acquisition and isolation , which  three-line are power supply  positive terminal, the signal output positive terminal and the common terminal.

What is a four-wire system?
     Four-wire is the isolation amplifier completely separated by power supply and signal , each is relatively independent work, no shared terminal (COM terminal) ,
so the four-wire line is distribution into:  working power is two lines, signal transmission is two lines . But in two-wire, three-wire system, Most of weak  electric to provide power supply(5-24VDC) .Four-wire  power supply may be weak electric(5-24VDC) or may be strong electric 220VAC.
    Three-wire and four-wire transmitters are not available  the advantages of two-wire amplifier,so it is about to be replaced by two-wire isolation amplifiers, from the abroad industry dynamics and isolation chip supply and demand to know about it, current isolation amplifier  should be used with equipment installed in the field power line,  SCM as the core of the monitoring system is located in the further  from the equipment at the scene of control room , they are generally distance from tens to hundreds even farther. Equipment field environment is more severe, a variety of industrial equipment and strong electricity signal will produce a variety of electromagnetic interference, Lightning sensor will generates a strong surge pulse, in this case, the microcomputer application system encountered a thorny question is how to long distances reliably transport  tiny signals  in harsh environments.

     Two-wire transmitter devices solved this problem . It isolates and converts the field device power lines current  into  4-20mA linear proportional change in the standard current signal output, and then delivering to the monitoring system  input interface via a pair of UTP,which will also transport 24V power supply in the monitoring system to the current transmitter. Measurement signal and power supply transport via twisted at the same time, thus not only eliminates the expensive
transmission cables, anti-jamming capability  greatly strengthened because the signal is transmitted in the form of current
Second, the current 4-20mA output isolation amplifier is how to convert?
    Two-wire current  isolation amplifier output is 4-20mA, through 250Ω precision resistor to convert 1-5V or 2-10V analog voltage signal. Converted into digital signal has a variety of methods, if the system is long-term use in a harsh industrial environment,it need to consider the hardware system security and reliability. The system input module adopts the  voltage to frequency converter device (LM231) to convert analog voltage signal into a frequency signal, with the optical coupling device (TL117) for  isolating and transforming analog signals and the digital signal.while the analog signal  processing circuit  and digital signal processing circuits respectively using two groups separated power supply, analog ground and digital ground separated from each other, rather than common ground connection, this can improve system  security. using voltage convert  into frequency device (LM231) also have some functions of anti-high-frequency interference.Third, comparing current output type and voltage output type isolation amplifiers strengths and weaknesses

     Many industrial control applications field have to use the isolator to convert the PLC、PC、 DCS、 instrumentation can not  be directly measured signal into analog signals which PLC、PC、DCS, instrumentation can process, such as the current transmitter , voltage transmitter,temperature transmitters , flow transmitters, etc. .
     Early isolators , transmitters mostly voltage output  type, by which converting the measure signal  into 0-5V voltage output ,this is the op amp direct output, its signal power <0.05W,Via analog/digital converter circuit converts the digital signal for the micro controller to read, control and signal acquisition.But in long-distance transmission signal requirement environment or in the grid disturbance larger occasions , the using voltage output type sensor  suffer great limit,exposing the weakness such as poor anti-interference ability, lower isolation amplifiers  accuracy class because of the broken line loss .thus causing larger errors when reading the parameters , while two -wire current output type isolation amplifier/transmitter has a very high anti-interference ability and  long-distance transmission signal without distortion  ensuring the isolation accuracy level, improving the industrial field measurement accurate  acquisition value,So current type isolation amplifiers has been widely applied in industrial control field or industrial field signal acquisition.

    Voltage output type isolation amplifier has poor anti-interference ability, great line losses and could not achieve long-distance transmission,or causing great line pressure drop,reduce the level of accuracy and in transit suffer all sorts of interference,sometimes output DC voltage also superimposed AC component, so that micro controller or other control devices (PLC、 DCS、 PC) generate false judgments, control errors, even damage the equipment , many of the ADC、 PLC、 DCS input signal ports are two-wire current output type 4-20mA isolation amplifier,  which proved that voltage output isolation amplifier will be eliminated inevitable trend.

Forth, 4-20mA current output to the interface approach?
    Current output type transmitter output range commonly are 0-20mA and 4-20mA two kinds, when the current isolation transmitter output  minimum current and the maximum current,respectively representative current isolation transmitter calibrated minimum and maximum rated output values the next ,with 0-100A current isolation amplifier as the example to describe the measurement range . output 0-20mA isolation amplifier 0mA current corresponding input 0A value,and output 4-20mA  isolator 4-20mA current corresponding input 0A value , two types of sensors 20mA current both correspond 100A current value.

    For 0-20mA output isolator , in the circuit design simply select the appropriate dropping resistor,in  the A/D converter input interfaces directly convert  the resistor 0-5V or 0-10V voltage to a digital signal, circuit debugging and data processing are relatively simple.But the disadvantage is that the isolator damage can not be determined , and unable to distinguish the state between output open circuit and short circuit .
    For output 4-20mA isolators, circuit debugging and data processing are relatively complicated. However,when the circuit nowhere , short circuit or damage,this isolator  can detect normal range within current the(normally  minimum is 4mA)to determine the current loop is fault,isolators for damage, so be more widely used.

    Since when 4-20mA isolator  output 4mA, the sampling resistor voltage is not equal to 0 , directly through the analog digital conversion circuit converted  digital is not zero , the SCM can not directly use it , calculated by the formula is too complex. Therefore, the general approach is via hardware circuit eliminate sampling resistor voltage drop caused by 4mA , then  convert A/D . Such hardware circuit devaluation RCV420,it's a sophisticated I/V convert circuit , which also can apply LM258 self-supply I/V conversion circuit, which consists of two -wire current  isolator generated 4-20mA current with 24V and sampling resistor form a current loop , resulting  a 1-5V voltage drop in the sampling resistor , and input  this voltage  to the amplifier (LM258) ,Through the I/V conversion circuit Provides the signal to the micro controller CPU or industrial control systems (DCS、PC、PLC, instrumentation) to read,through the signal acquisition , isolation, data transformation processing method to make two-wire current isolation amplifier 4-20mA current display as 0-100A value  in LCD, LED screen , instrumentation , industrial control systems , the principle don't describe in detail .

Fifth, two-wire current isolation amplifier comprehensive protection functions:
      1、Input overload protection;
      2、Output over-current limit protection;
      3、The output current short circuit protection for a long time ;
      4、Two-wire port transient lightning and surge current  TVS  suppression protection;
      5、Power supply over voltage limit protection ≤35V;
      6、The power supply reverse polarity protection.  

Sixth,designing  two-wire isolation amplifier conditions:
To realize the two-wire ,isolation amplifier/transmitter must meet the following conditions
     1、V≤ Emin-ImaxRLmax
     Transmitter output port voltage V is equal to the specified minimum power supply voltage minus the  voltage drop from
the load resistance and transmission wire resistance
     2、I≤ Imin
     Transmitter normal operating current I must be less or equal to the transmitter output current.
     3、P<Imin (Emin-IminRLmax)
     the minimum Isolated transmitter power must not exceed  above formula, usually<90mW.
     in the formula: Emin = minimum power supply voltage ,for the most instrumentation  Emin=24(1-5%)=22.8V, 5% is the 24V power supply allows negative variation;
      Imin= 4mA;
      RLmax=250Ω+transmission wire resistance.

     If the isolation transmitter/isolation amplifier is designed to meet the above three conditions, it can achieve two-wire transmission. so-called two-wire is  power supply in series with the load, there is a common point, the signal  contact and power supply  between  on-site  transmitter and the control room instrument just use two wires,which are both the power supply line and signal line, Two-wire transmitter  due to signal start point is 4mA.DC, provides a quiescent working
current for the transmitter, while the instrument electrical zero point is 4mA.DC, does not coincide with the mechanical zero,This "living zero" Favor of identifying outages and disconnection faults. also  two-wire system easy to use isolation barrier, conducive to safe explosion.
Isolation amplifier Datasheet Download: http://www.isolation-transmitter.com/download.asp?BigClassName=Isolation+Amplifier
Isolation-transmitter Datasheet Downloadhttp://www.isolation-transmitter.com/download.asp?BigClassName=Isolation+Transmitter

Isolation amplifier (isolated transmitter) picture:



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