·Ø¡·èÒ¹ÍÒ¨¨Ð¤Ô´ÇèÒ¡ÒÃÇÑ´à»ç¹àÃ×èͧ·Õè§èÒÂáÅÐÍÂèÒ§áÂè·ÕèÊØ´¡çäÁèÊÒÁÒöÍèÒ¹¤èÒä´é¶Ù¡µéͧ áµè¨ÃÔ§ æ áÅéǼÙ餹ÊèǹãË­è¡ÅѺÅ×Áä»ÇèÒ¡ÒÃÇÑ´¹Ñé¹ÁÕ¤ÇÒÁàÊÕ觷Õè¨Ð·ÓãËéà¤Ã×èͧÁ×ÍÃҤҹѺËÁ×蹹ѺáʹàÊÕÂËÒÂËÃ×ͺҧ¤ÃÑ駡ÒÃÇÑ´ÍÒ¨¡èÍãËéà¡Ô´»ÃСÒÂä¿«Öè§ÊÒÁÒö·ÓãËéà¡Ô´à¾ÅÔ§äËÁé¢Öé¹ä´é

You'd think taking a measurement is a simple task and the worst thing that could really happen is that you may get erroneous readings, but what most people fail to realize is the potential of a hazardous situation that could essentially damage thousands of dollars worth of equipment or create an arc-flash hazard which could lead to either a fire or serious physical burns or potentially both.

à¾×èÍãËéà¢éÒ㨧èÒ¢Öé¹ãËéÅͧ¾Ô¨ÒóҶ֧¡ÒÃãªéÁÑŵÔÁÔàµÍÃì·Ó¡ÒÃÇÑ´¤èÒáç´Ñ¹ä¿¿éÒÃÐËÇèÒ§¨Ø´µèÒ§ æ ã¹Ç§¨Ãáµè¡ÅѺ»ÃѺãËéÁÑŵÔÁÔàµÍÃì·Ó§Ò¹ã¹ÃٻẺ¡ÒÃÇÑ´¡ÃÐáÊä¿¿éÒ ¤ÇÒÁµéÒ¹·Ò¹«Ö觵èÓÁÒ¡ æ 㹡ÒÃÇÑ´¡ÃÐáÊä¿¿éҨзÓãËéà¡Ô´¡ÒÃÅѴǧ¨Ã¢Ö鹫Ö觨зÓãËéà¡Ô´¡ÃÐáÊä¿¡ÃЪҡ«Ö觷ÓãËéÁÑŵÔÁÔàµÍÃìàÊÕÂËÒÂáÅмÙéãªé§Ò¹¨Ð¶Ù¡ä¿ªê͵ áµèÊÔè§àËÅèÒ¹ÕéäÁèà¡Ô´¢Öé¹à¹×èͧ¨Ò¡ã¹ÁÑŵÔÁÔàµÍÃì¨ÐÁÕ¡ÒÃãÊè¿ÔÇÊìäÇéà¾×èͤÇÒÁ»ÅÍ´ÀÑÂ㹡ÒÃãªé§Ò¹

To explain this simply, consider a case of Digital Multimeter (DMM) using which the user wants to measure Voltage in Voltmeter mode across a circuit but instead ends up connecting it as an Ammeter across the circuit.  The minimal resistance of the Ammeter will create a short circuit condition and as a result there will be a current surge which could damage the DMM and cause an electric shock to the user but this does not happen and to avoid this possibility, the DMMs have a fuse which provides the requisite safety. 

àªè¹à´ÕÂǡѹ ÃкºÇÑ´áÅкѹ·Ö¡¢éÍÁÙÅ¡ç¤ÇèÐÁդسÊÁºÑµÔ´éÒ¹¤ÇÒÁ»ÅÍ´ÀÑÂàËÅèÒ¹Õé «Ö觤سÊÁºÑµÔ·ÕèÊӤѭ·ÕèÊØ´¡ç¤×Í äÍâ«àŪÑè¹ ã¹º·¤ÇÒÁ¹Õé àÃÒ¨ÐÁÒ·Ó¤ÇÒÁà¢éÒ㨶֧ÃкºäÍâ«àŪÑè¹ã¹ÃٻẺµèÒ§ æ áÅÐÇÔ¸Õ¡ÒÃàÅ×Í¡ÃкºäÍâ«àŪÑè¹·ÕèãËé¡Ñº¡ÒÃãªé§Ò¹

Similarly, for any measurement system or data acquisition system it is critical to have some safety features, the most needed of them being the Isolation. In this article, the focus lies on understanding different types of isolation and ultimately choosing the measurement system with optimum isolation levels as needed for the application.

äÍâ«àŪÑè¹
Isolation
äÍâ«àŪÑè¹ ¤×Í ¡ÒÃᡨҡ¡Ñ¹·Ò§¡ÒÂÀÒ¾áÅзҧ俿éҢͧͧ¤ì»ÃСͺÊͧÊèǹã¹ÍØ»¡Ã³ì äÍâ«àŪÑ蹪èÇ»éͧ¡Ñ¹à¤Ã×èͧÁ×ÍÇÑ´ ǧ¨Ã¤ÍÁ¾ÔÇàµÍÃì áÅмÙéãªé§Ò¹ ¹Í¡¨Ò¡¹ÕéÂѧ»éͧ¡Ñ¹¡ÒÃà¡Ô´¡ÃÒǹ´ìÅÙ» (ground loop) áÅлÃѺ»Ãاáç´Ñ¹ä¿¿éÒÃèÇÁ (common-mode voltage) áÅСӨѴÊÑ­­Ò³Ãº¡Ç¹ã¹ÊÀÒÇÐáÇ´ÅéÍÁ·ÕèàµçÁä»´éÇÂÊÑ­­Ò³Ãº¡Ç¹·Ò§ä¿¿éÒ¨Ò¡à¤Ã×èͧ¨Ñ¡ÃáÅТ´ÅÇ´à˹ÕèÂǹÓ

Isolation is defined as the means of physically and electrically separating two parts of a device. It protects the measurement device, computer circuitry and human operators, breaks ground loops and improves common-mode voltage and noise rejection in electrically noisy environments containing machinery and inductive loads.

Åͧ¾Ô¨ÒóҶ֧ÃкºÇѴ㹡Ò÷´Êͺà¤Ã×èͧ¹µìÊѹ´Ò»ÀÒÂã¹ «Öè§ÁÕáç´Ñ¹ä¿¿éÒÃèÇÁÁÒ¡ÁÒÂËÅÒ¨شã¹à¤Ã×èͧ¹µìáÅÐÂѧÁÕâÍ¡ÒÊ·Õè¨Ðà¡Ô´ä¿¡ÃЪҡÊÙ§ ¡µÑÇÍÂèÒ§àªè¹ àÃÒãªéà·ÍÃìâÁ¤Ñ»à»ÔéÅ㹡ÒÃÇÑ´ÍسËÀÙÁÔ¡Ãк͡ÊÙº«Öè§áç´Ñ¹ä¿¿éÒ·ÕèÍèÒ¹ä´é¨ÐÍÂÙèã¹ÃдѺÁÔÅÅÔâÇÅ·ì 㹵͹ááàÃÒÍÒ¨¨Ð¤Ô´ÇèÒäÁè¹èÒ¨ÐÁÕÍÐä÷ÕèÁռšÃзº¡Ñº¡ÒÃÇÑ´áµèá¹è¹Í¹ÇèÒä¿¡ÃЪҡáÅÐáç´Ñ¹ä¿¿éÒÊÙ§ÂèÍÁÁռšѺ¤ÇÒÁáÁè¹ÂÓ㹡ÒÃÇÑ´ÍسËÀÙÁÔ ÊÔ觷ÕèàÃÒµéͧ·Ó¤×Í¡ÒÃᡪèͧÊÑ­­Ò³·ÕèÇÑ´ÍسËÀÙÁÔÍÍ¡¨Ò¡ªèͧÊÑ­­Ò³·ÕèÇÑ´áç´Ñ¹ä¿¿éÒÊÙ§

Consider the measurement system for an IC Engine testing. There are varying common mode voltages across the engine and there is a high possibility of transients during the operation. For example, if a TC is used for Cylinder temperature measurement, the output will be few mVs and in the first perspective, it might seem nothing could go wrong with the measurements but the presence of transients and higher voltages could negatively affect the accuracy of the temperature measurements. What we need in this case is an isolated channel(s) for temperature measurement from those used for higher voltages.

¡ÒÃãªé§Ò¹ÃٻẺÍ×è¹ æ ·ÕèµéͧÃÐÁÑ´ÃÐÇѧã¹àÃ×èͧà´ÕÂǡѹ ¤×Í ¡ÒÃÇѴẵàµÍÃÕè·Õèµè;èǧ¡Ñ¹ËÅÒÂ æ µÑÇã¹Ã¶ä¿¿éÒËÃ×Í¡Ò÷´Êͺà«ÅÅìàª×éÍà¾Åԧ㹴ÒÇà·ÕÂÁ
Other applications which have similar concerns are Battery Stack testing for Electric Vehicles and Fuel Cell Testing for Satellites.
àÃÒÅͧÁÒ´ÙµÑÇÍÂèÒ§áÅзӤÇÒÁà¢éÒã¨ÇèÒäÍâ«àŪÑ蹨ЪèÇÂÅ´»Ñ­ËÒáç´Ñ¹ä¿¿éÒÃèÇÁáÅСÃÒǹ´ìÅÙ»ä´éÍÂèÒ§äÃ?
Let us now take an example and understand how isolation helps in reducing common mode voltages and ground loops.

 
ÃÙ»´éÒ¹º¹áÊ´§ãËéàËç¹ÃкºÇÑ´ÊͧÃкº Ãкº´éÒ¹¢ÇÒÁÕäÍâ«àŪÑè¹ã¹µÑÇ
Figure above shows two measurement systems, the right one used with the inbuilt isolation.

à¹×èͧ¨Ò¡ÃкºááäÁèÁÕäÍâ«àŪÑè¹ÃÐËÇèÒ§¡ÃÒǹ´ì¢Í§áËÅ觨èÒÂáç´Ñ¹ä¿¿éÒ ¡ÃÒǹ´ì¢Í§ÃкºáÅСÃÒǹ´ì¢Í§ÍØ»¡Ã³ì «Öè§ÍÒ¨·ÓãËéà¡Ô´¡ÃÐáÊä¿¿éÒäËÅ㹡ÃÒǹ´ìÅÙ»¢Í§ÃкºÇÑ´«Öè§ÁռšѺ¤ÇÒÁáÁè¹ÂӢͧ¤èÒ·ÕèÇÑ´ä´é

Because in the first system, the source ground, system ground and equipment ground are non-isolated and are at different potentials, this causes a ground loop current to flow within the measurement system and thereby affecting the accuracy of measured voltage.

ã¹ÃкºÇÑ´·ÕèÁÕäÍâ«àŪÑè¹ ¡ÃÒǹ´ìÅÙ»¨ÐäÁèÊÒÁÒöà¡Ô´¢Öé¹ä´éà¹×èͧ¨Ò¡¡ÃÒǹ´ì¢Í§áËÅ觨èÒÂáç´Ñ¹ä¿¿éÒ ¡ÃÒǹ´ì¢Í§ÃкºáÅСÃÒǹ´ì¢Í§ÍØ»¡Ã³ì¶Ù¡á¡ÍÍ¡¨Ò¡¡Ñ¹ «Ö觷ÓãËéÃкºÇѴẺ¹ÕéÁÕ¤ÇÒÁáÁè¹ÂÓÁÒ¡¢Öé¹

In the measurement system with isolation, the formation of ground loop is prevented by isolating Equipment ground from System ground and Source ground and thus the measurement system with isolation provide more accurate readings.

àÃÒÅͧÁÒ·Ó¤ÇÒÁà¢éÒã¨ÇèÒà¡ÕèÂǡѺáç´Ñ¹ä¿¿éÒÃèÇÁáÅÐäÍâ«àŪÑ蹨ЪèÇ»ÃѺ»Ãاã¹àÃ×èͧ¹Õéä´éÍÂèÒ§äÃ?

Using the same diagram, let us understand Common mode voltage and how isolation is needed to improve the common mode voltage range.

áç´Ñ¹ä¿¿éÒÃèÇÁ ¤×Í ªèǧáç´Ñ¹ä¿¿éÒ·ÕèÊÒÁÒöà»ÅÕè¹á»Å§ä´éàÁ×èÍà·Õº¡Ñº¡ÃÒǹ´ì¢Í§ÃкºÇÑ´ ã¹ÃкºÇѴẺ´Ô¿à¿ÍàùàªÕèÂÅ (differential) ·ÕèäÁèÁÕäÍâ«àŪÑ蹫Öè§ÁÕ¡ÒÃàª×èÍÁµèͷҧ俿éÒÃÐËÇèÒ§ÍÔ¹¾Ø·áÅÐàÍÒ·ì¾Ø· ´Ñ§¹Ñ鹤سÊÁºÑµÔ·Ò§ä¿¿éҢͧǧ¨Ã¢ÂÒ¨Ðà»ç¹µÑÇ¡Ó˹´¤èÒáç´Ñ¹ä¿¿éÒÃèÇÁ¢Í§ÍÔ¹¾Ø· áµè¶éÒàÃÒãªéǧ¨Ã¢ÂÒÂẺ·ÕèÁÕäÍâ«àŪÑ蹫Öè§äÁèÁÕ¡ÒÃàª×èÍÁµèͷҧ俿éÒÃÐËÇèÒ§ÍÔ¹¾Ø·áÅÐàÍÒ·ì¾Ø· ¤Ø³ÊÁºÑµÔ·Ò§´éÒ¹áç´Ñ¹ä¿¿éÒÃèÇÁ¨Ð´Õ¢Öé¹à»ç¹ÍÂèÒ§ÁÒ¡

The common-mode voltage range is defined as the maximum allowable voltage swing on each input with respect to the measurement system ground. In a non-isolated differential measurement system, an electrical path still exists in the circuit between input and output. Therefore, the electrical characteristics of the amplifier limit the common-mode signal level that you can apply to the input. With the use of isolation amplifiers, the conductive electrical path is eliminated and the common-mode rejection ratio is dramatically increased.

ÊÓËÃѺǧ¨Ã·Ò§´éÒ¹«éÒ«Öè§äÁèÁÕäÍâ«àŪÑè¹ÃÐËÇèÒ§ÍÔ¹¾Ø·áÅÐàÍÒ·ì¾Ø·«Öè§à»ç¹àÃ×èͧ»¡µÔã¹ÃкºÇÑ´·ÑèÇ æ ä» áç´Ñ¹ä¿¿éÒÃèÇÁµéͧÁÕ¤èÒà»ç¹ 0 âÇÅ·ìà¾×èÍãËéÍÔ¹¾Ø·ÊÒÁÒö»é͹ä´éÃÐËÇèÒ§ -10 ¶Ö§ 10 âÇÅ·ìã¹¢³Ð·Õèǧ¨Ã·Ò§´éÒ¹¢ÇÒ«Öè§ÁÕäÍâ«àŪÑè¹ÃÐËÇèÒ§ÍÔ¹¾Ø·áÅÐàÍÒ·ì¾Ø·¢Í§Ç§¨Ã¢ÂÒÂÊÒÁÒö»é͹ÍÔ¹¾Ø·áÅÐàÍÒ·ì¾Ø·ä´éÃÐËÇèÒ§ -10 ¶Ö§ 10 âÇÅ·ìáÁé¨ÐÁÕáç´Ñ¹ä¿¿éÒÃèÇÁÊÙ§¶Ö§ 300 âÇÅ·ìâ´ÂäÁèÁռšѺ¤ÇÒÁáÁè¹ÂÓáÅÐÂѧÁÕ¤ÇÒÁ»ÅÍ´ÀÑÂ㹡ÒÃÇÑ´ÍÕ¡´éÇÂ

For the circuit on the left, there is no isolation between the inputs and output of the amplifier which is typically a component in the measurement system. Because of zero isolation, the common mode voltage, Vcm is 0V and thus the source voltage Vs needs to be within +/-10V, whereas in the circuit on the right, sufficient isolation is provided between the inputs and output of the amplifier which ensures that even though Vcm is 300V, the source Voltage, Vs which is in +/- 10V range of Vcm can be measured accurately and with no safety compromise.

¡ÒÃÍ͡ẺäÍâ«àŪÑè¹
Design for Isolation
¡ÒÃãªé§Ò¹ã¹ÃٻẺ·ÕèᵡµèÒ§¡Ñ¹ÂèÍÁµéͧ¡ÒÃäÍâ«àŪÑè¹·ÕèᵡµèÒ§¡Ñ¹ «Öè§äÍâ«àŪÑè¹ã¹à¤Ã×èͧÁ×ÍÇÑ´¹Ñé¹ÁÕËÅÒ¡ÅËÅÒÂÃٻẺ
Different applications need different types of isolation and there are multiple ways of implementing isolation in measurement devices.
äÍâ«àŪÑé¹ 3 Ẻ¨Ð¶Ù¡¡ÅèÒǶ֧㹷Õè¹Õéâ´ÂàÃÔèÁ¨Ò¡áºº¾×é¹°Ò¹ (¤ÇÒÁ»ÅÍ´ÀѵèÓ) 仨¹¶Ö§áºº·ÕèÊÁºÙóìẺ (¤ÇÒÁ»ÅÍ´ÀÑÂÊÙ§)
a. äÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³¡ÑºàÍÔÃì¸ (Channel to Earth Ground)
b. äÍâ«àŪÑè¹ÃÐËÇèÒ§¡ÅØèÁ¢Í§ªèͧÊÑ­­Ò³ (Bank to Bank)
c. äÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³ (Channel to Channel isolation)

ÅͧÁÒ·Ó¤ÇÒÁà¢éÒã¨äÍâ«àŪÑè¹áººµèÒ§ æ ¡Ñ¹´Ù

The 3 types of isolation techniques can be listed from basic (low level protection) to complete (high level protection) in the order of
d. Channel to Earth Ground
e. Bank to Bank
f. Channel to Channel isolation

Let’s now understand the different isolation techniques.

1. äÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³¡ÑºàÍÔÃì¸ (Channel to Earth Ground)
äÍâ«àŪÑè¹áºº¹ÕéªèͧÊÑ­­Ò³¡ÑºàÍÔÃ츨Ðᡨҡ¡Ñ¹·Ò§ä¿¿éҴѧáÊ´§ã¹ÃÙ»´éÒ¹ÅèÒ§ áç´Ñ¹ä¿¿éÒ´éÒ¹·Õè»é͹ÊÑ­­Ò³à¢éÒÁÒ¨ÐäÁèÁÕ¡ÒÃäÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³áµè¨ÐÁÕ¡ÒÃᡨҡ¡Ñ¹·Ò§ä¿¿éÒÃÐËÇèÒ§ªèͧÊÑ­­Ò³¡ÑºàÍÔÃì¸ ¹Õèà»ç¹äÍâ«àŪÑè¹¢Ñé¹¾×é¹°Ò¹·ÕèÊØ´áÅжéÒÍØ»¡Ã³ìÁÕ¡ÒÃäÍâ«àŪÑè¹ÃÐËÇèÒ§¡ÅØèÁ¢Í§ªèͧÊÑ­­Ò³ËÃ×ÍÃÐËÇèÒ§ªèͧÊÑ­­Ò³¡ç¨ÐÁÕ¡ÒÃäÍâ«àŪÑè¹áºº¹Õé´éÇÂ

Note: ¡ÒÃáÃà§Òã¹ÀÒ¾´éÒ¹ÅèÒ§áÊ´§ãËéàË繶֧ºÃÔàdz·ÕèÁÕ¡ÒÃäÍâ«àŪÑ蹫Ö觷ÓãËéà¡Ô´¡ÒÃᡨҡ¡Ñ¹·Ò§ä¿¿éÒ¢Öé¹ ÊÑ­ÅѡɳìËÁéÍá»Å§áÊ´§ãËéàË繶֧¡ÒÃᡨҡ¡Ñ¹·Ò§ä¿¿éÒàªÔ§¡Åà¾×èÍãªé㹡ÒÃÊè§ÊÑ­­Ò³¢éÒÁäÍâ«àŪÑè¹â´ÂÊÃéҧʹÒÁáÁèàËÅç¡ä¿¿éÒ·Õèá»Ã¼Ñ¹µÒÁÊÑ­­Ò³ä¿¿éÒ

1. Channel-to-Earth Ground Isolation
Channels of the device and the device's Earth ground are electrically isolated from one another.  Channel-to-Earth isolation is represented in the Figure below. Voltages of the isolated front end (Va-c ) are on the same bus; these voltages are not isolated from one another. Ve,d are on a separate bus and are isolated from the front end.  This is the most fundamental type of isolation and this protection is covered by bank and channel-to-channel isolation. 

Note: For the following diagrams, the diagonal hash marks indicate the isolation barrier, this separates circuitry. The transformer symbols represent electromagnetic isolation used to couple a signal across the isolation barrier by generating an electromagnetic field proportional to the electrical signal.
 

2. äÍâ«àŪÑè¹ÃÐËÇèÒ§¡ÅØèÁ¢Í§ªèͧÊÑ­­Ò³ (Bank to Bank)
äÍâ«àŪÑè¹áºº¹ÕéªèͧÊÑ­­Ò³¢Í§à¤Ã×èͧÁ×ÍÇÑ´¨Ð¶Ù¡¨Ñ´à»ç¹¡ÅØèÁà¾×èÍãªé§Ò¹Ç§¨Ã¢ÂÒÂẺ·ÕèÁÕäÍâ«àŪÑè¹ÃèÇÁ¡Ñ¹´Ñ§áÊ´§ã¹ÀÒ¾´éÒ¹ÅèÒ§ 㹡ÒÃäÍâ«àŪÑè¹áºº¹Õé¨ÐÁÕ¢éͨӡѴã¹àÃ×èͧáç´Ñ¹ä¿¿éÒÃèÇÁÃÐËÇèÒ§ªèͧÊÑ­­Ò³·ÕèÍÂÙè㹡ÅØèÁà´ÕÂǡѹ áµèªèͧÊÑ­­Ò³·ÕèÍÂÙ褹ÅСÅØèÁ¨ÐäÁèÁջѭËÒàÃ×èͧ¹Õé  ¡ÅèÒǤ×Í äÁèÁÕ¡ÒÃᡨҡ¡Ñ¹·Ò§ä¿¿éÒÃÐËÇèÒ§ªèͧÊÑ­­Ò³áµè¨ÐÁÕ¡ÒÃᡨҡ¡Ñ¹·Ò§ä¿¿éÒÃÐËÇèÒ§¡ÅØèÁ¢Í§ªèͧÊÑ­­Ò³áÅÐàÍÔÃ츹Ñè¹àͧ äÍâ«àŪÑè¹áºº¹ÕéÁÕãªé·ÑèÇä»ã¹ÃкºÇÑ´ÊÑ­­Ò³´Ô¨ÔµÍÅ

2. Bank (Channel-to-Bus) Isolation
Channels of a device are banked (grouped) together to share a single isolation amplifier. Figure below represents Bank isolation. In this topology, the common-mode voltage difference between channels is limited, but the common-mode voltage between the bank of channels and the non-isolated part of the measurement system can be large. In other words, individual channels are not isolated, but  the channel groups are isolated from one another and earth ground. Bank1, Bank2, Vi, and Vj,k are on separate buses and isolated from one another. Bank isolation is most commonly used in Digital I/O measurement systems.

 
3. äÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³ (Channel to Channel isolation)
äÍâ«àŪÑè¹áºº¹ÕéáµèÅЪèͧÊÑ­­Ò³áÅЪÔé¹Êèǹ·ÕèäÁèÁÕäÍâ«àŪÑ蹨ж١ᡨҡ¡Ñ¹·Ò§ä¿¿éҴѧáÊ´§ã¹ÀÒ¾´éÒ¹ÅèÒ§ ËÁéÍá»Å§ã¹ÀÒ¾à»ç¹ÍØ»¡Ã³ì·Õèãªé㹡ÒÃÊÃéÒ§äÍâ«àŪÑ蹫Öè§ÊÒÁÒö·´á·¹´éÇÂÍͻⵤѻà»ÅÍÃìËÃ×ÍäÍ«ÕẺ´Ô¨ÔµÍÅ

3. Channel-to-Channel Isolation
Each channel is isolated from every other channel and other non-isolated components.  Figure below represents channel-to-channel isolation. Va,b, Vc,d, Ve,f, and Vg,h are all on separate buses and are isolated from one another.

In the figures above, the transformer represents the isolator implementation, could also be replaced by an Opto-coupler or a Digital IC.

·Ó¤ÇÒÁà¢éÒ㨢éÍ¡Ó˹´·Ò§à·¤¹Ô¤¢Í§äÍâ«àŪÑè¹ àªè¹à´ÕÂǡѺẹÇÔ¸áÅФÇÒÁ¶Õè㹡ÒÃÊØèÁÊÑ­­Ò³ äÍâ«àŪÑè¹ÊÓËÃѺà¤Ã×èͧÁ×ÍÇÑ´¡çÁÕ¢éÍ¡Ó˹´·Ò§à·¤¹Ô¤ «Ö觡ÒÃàÅ×Í¡à¤Ã×èͧÁ×ÍÇÑ´ãËé¶Ù¡µéͧ¡ç¨Óà»ç¹µéͧà¢éÒ㨢éÍ¡Ó˹´·Ò§à·¤¹Ô¤àËÅèÒ¹Õé

Understanding Isolation Specifications
Just like bandwidth, sampling rate etc Isolation is also specified for the Measurement System and to choose a right system for the application it is important to understand the specifications related to Isolation.

1.áç´Ñ¹ä¿¿éÒÃèÇÁ
áç´Ñ¹ä¿¿éÒÃèÇÁ ¤×Í ¤èÒ¤ÇÒÁµèÒ§ÈÑ¡ÂìÃÐËÇèÒ§¨Ø´ÍéÒ§ÍÔ§¢Í§ÃкºÇÑ´¡Ñº¡ÃÒǹ´ì¢Í§àÍÔÃì¸ ¶éÒ¢éÍÃкطҧ෤¹Ô¤¡Ó˹´ÇèÒ ¤ÇÒÁµèÒ§ÈÑ¡ÂìÃÐËÇèÒ§¨Ø´ÍéÒ§ÍÔ§áÅÐàÍÔÃ측ÃÒǹ´ìäÁèà¡Ô¹ +/-300 âÇÅ·ìáÅÐÃÐËÇèÒ§ªèͧÊÑ­­Ò³äÁèà¡Ô¹ +/-10 âÇÅ·ì àÃÒÊÒÁÒöÇÑ´ÊÑ­­Ò³ +/-10 âÇÅ·ìàÁ×èÍà·Õº¡Ñº¨Ø´ÍéÒ§ÍÔ§ä´éâ´Â·Õè¤èÒ¤ÇÒÁµèÒ§ÈÑ¡ÂìàÁ×èÍà·Õº¡Ñº¡ÃÒǹ´ìäÁèà¡Ô¹ +/-300 âÇÅ·ì
1. Common Mode Voltage
 
Common mode voltage is spec’ed as COM to Earth Ground which basically is the potential difference between the COM of the measurement system and the Earth Ground.

Let’s say COM to Earth Ground is spec’ed at +/-300V and Channel to COM, Channel to Channel is spec’ed at +/-10V. This Measurement System can successfully measure a signal of +/-10V with reference to COM and since the maximum isolation of COM with respect to Earth Ground is +/-300V, one can measure signals of +/-10V with reference to +/-300V using this Measurement System.

2.áç´Ñ¹ä¿¿éÒµèÍà¹×èͧ
¤èÒ RMS ÊÙ§ÊØ´·Ñ駢ͧ俿éÒ¡ÃÐáÊÊÅѺáÅÐä¿¿éÒ¡ÃÐáʵç·ÕèäÍâ«àŪÑè¹ÊÒÁÒö·¹ä´é

2. Continuous Voltage
The highest possible root-mean-square value of the ac and dc voltage across insulation.

3.áç´Ñ¹ä¿¿éÒ·ÕèÊÒÁÒö·¹ä´é
áç´Ñ¹ä¿¿éÒ·Õèà¡Ô´¨Ò¡ä¿¡ÃЪҡ·ÕèÍØ»¡Ã³ìÊÒÁÒö·¹ä´éã¹ÃÐÂÐàÇÅÒÊÑé¹ æ â´ÂäÁèÁÕ¤ÇÒÁàÊÕÂËÒÂà¡Ô´¢Ö鹡ѺÃкºÇÑ´ ÁÑ¡¨Ð¶Ù¡ÃкØã¹ÃٻẺ x âÇÅ·ìà»ç¹àÇÅÒ t ÇÔ¹Ò·Õ

3. Withstand Voltage
The transient voltage the device can withstand for a brief period of time without damaging the measurement system. Withstand Voltage is specified like X Volts for t seconds.

4.ÃٻẺ¡ÒõԴµÑé§
ÃٻẺ¡ÒõԴµÑ駹Ñé¹¢Öé¹ÍÂÙè¡Ñºáç´Ñ¹ä¿¿éÒ·Õèà¡Ô´¨Ò¡ä¿¡ÃЪҡ«Ö觤ҴÇèÒ¨Ðà¡Ô´¢Öé¹ ÃкºÇÑ´ÊèǹãË­è¨ÐãªéẺ CAT I áÅÐ CAT II

4. Installation Category
Depending upon the expected magnitude of transients, Isolation category of the measurement system is chosen. Most of the measurement systems fall in Category I and Category II.
 
5.Áҵðҹ´éÒ¹¤ÇÒÁ»ÅÍ´ÀÑÂ
Áҵðҹ´éÒ¹¤ÇÒÁ»ÅÍ´ÀÑ¢ͧà¤Ã×èͧÁ×ÍÇÑ´ÁÕËÅÒ¡ËÅÒÂÃٻẺ Ẻ·Õèá¾ÃèËÅÒ·ÕèÊØ´ ¤×Í IEC 61010-01  «Öè§ÁÕ¡ÒÃãªé§Ò¹¡Ñ¹·ÑèÇâÅ¡ ÁҵðҹÍ×è¹ æ «Öè§ÁÕ¡ÒÃãªé§Ò¹ã¹ÃдѺÀÙÁÔÀÒ¤¶Ù¡áÊ´§äÇé´éÒ¹ÅèÒ§
1. ÍàÁÃÔ¡Òà˹×Í UL 61010 -01
2. á¤¹Ò´Ò CAS 61010-01
3. ÂØâû EN 61010-01

5. Safety Standard
There are multiple safety standards specified for measurement systems. The most common is IEC 61010-01 which is followed globally. Some geo-specific standards are also followed, listed below
4. N. America UL 61010 -01
5. Canada CAS 61010-01
6. Europe EN 61010-01

àÃÒµéͧ¡ÒÃäÍâ«àŪÑè¹áººä˹?
·Õè¼èÒ¹ÁÒàÃÒ¾Ù´¤ØÂà¡ÕèÂǡѺäÍâ«àŪÑè¹áººµèÒ§ æ áÅТéÍ¡Ó˹´·Ò§à·¤¹Ô¤·Õèà¡ÕèÂÇ¢éͧ áµèàÃҨзÃÒºä´éÍÂèÒ§äÃÇèÒàÃÒµéͧãªéäÍâ«àŪÑè¹áººä˹ã¹ÃкºÇÑ´¢Í§àÃÒ?

á¼¹¼Ñ§´éÒ¹ÅèÒ§ªèÇÂãËéàÃÒÊÒÁÒöàÅ×Í¡äÍâ«àŪÑè¹ä´é¶Ù¡µéͧµÒÁ¤ÇÒÁµéͧ¡ÒÃ

Which Isolation do I need?
So far, we have discussed different types of isolation and the associated specifications but how does one as a user choose the right isolation in the measurement system?
The flowchart below will help select the right type of isolation.   

 àÃÒÊÒÁÒöàÅ×Í¡ÃкºÇÑ´·Õè¶Ù¡µéͧä´é¢Öé¹ÍÂÙè¡Ñºª¹Ô´¢Í§äÍâ«àŪÑè¹·Õèµéͧ¡ÒÃãªé§Ò¹ «Öè§äÍâ«àŪÑè¹áµèÅÐẺ¡çÁÕ¤èÒãªé¨èÒ·ÕèᵡµèÒ§¡Ñ¹ä» «Öè§á¹è¹Í¹ÇèÒäÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³¹Ñé¹ÁÕ¤èÒãªé¨èÒÂÊÙ§ÊØ´·ÇèÒÁÕ¤ÇÒÁ»ÅÍ´ÀÑÂÊÙ§ÊØ´

Depending on the type of isolation one needs for the industrial application, the right measurement system can be selected. There is a tradeoff between the type of isolation and the cost associated with it. As expected, the channel to channel isolation is the costliest and the safest.

๪Ñè¹á¹ÅÍԹʷÃÙàÁé¹Êì ¼Ùé¹Ó´éÒ¹ÃкºÇÑ´«Öè§ÁÕ·Ñ駫Ϳ·ìáÇÃìÍ͡ẺÃкºÇÑ´´éÇ¡ÃÒ¿¿Ô¤áÅÐà¤Ã×èͧÁ×ÍÇѴẺâÁ´ÙÅèÒÃìã¹ÃٻẺ PXI USB áÅÐ Ethernet «Ö觪èÇÂãËéÁÕ¤èÒãªé¨èÒ·ÕèàËÁÒÐÊÁ¡ÑºÊÔ觷Õè¨Ðä´éÃѺ

National Instruments, a pioneer in Data Acquisition (DAQ) systems provides Graphical Software and Modular DAQ cards in PXI, USB and Ethernet form factor which offers the right cost Vs benefit portfolio.

¡µÑÇÍÂèÒ§àªè¹ PXIe SC Express «Öè§ÁÕäÍâ«àŪÑè¹ÃÐËÇèÒ§¡ÅØèÁªèͧÊÑ­­Ò³ÊÓËÃѺ¡ÒÃÇÑ´´éÇÂà·ÍÃìâÁ¤Ñ»à»ÔéÅáÅÐÍÒÃì·Õ´Õ áÅÐäÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³ÊÓËÃѺáç´Ñ¹ä¿¿éÒ·Ñé§ÍÔ¹¾Ø·áÅÐàÍÒ·ì¾Ø· NI 9244 «Öè§ÊÒÁÒöÇÑ´áç´Ñ¹ä¿¿éÒÃÐËÇèÒ§ÊÒÂÊÑ­­Ò³ä´éÊÙ§¶Ö§ 800 âÇÅ·ìáÅÐÁÕäÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³áºº CAT-III ÊÙ§¶Ö§ 400 âÇÅ·ì ÍÔ¹¾Ø·áÅÐàÍÒ·ì¾Ø·áºº´Ô¨ÔµÍÅáÅÐà¤Ò¹ìàµÍÃì¢Í§à¹ªÑè¹á¹ÅÍԹʷÃÙàÁé¹Êìà¡×ͺ·Ñé§ËÁ´ÁÕäÍâ«àŪÑè¹ÃÐËÇèÒ§¡ÅØèÁ¢Í§ªèͧÊÑ­­Ò³à¾×èÍ·Õè¼Ùéãªé§Ò¹¨ÐÊÒÁÒöÊÃéÒ§ÃкºÇÑ´·ÕèÁÕ¤ÇÒÁ·¹·Ò¹áÅФÇÒÁ»ÅÍ´ÀÑ ¹Í¡¨Ò¡¹ÕéâÁ´ÙÅ C series ·Ø¡ÃØ蹢ͧ๪Ñè¹á¹ÅÍԹʷÃÙàÁé¹ÊìÁÕäÍâ«àŪÑè¹ÃÐËÇèÒ§ªèͧÊÑ­­Ò³¡ÑºàÍÔÃì¸

For instance, the NI PXIe SC Express series provides bank isolation for Thermocouple and RTD measurements and Channel –Channel isolation for Voltage input and output. The NI C Series NI 9244 which can measure 800Vrms Line-Line provides 400Vrms Cat -III Channel-Channel Isolation. Almost all the NI Digital I/O and Counter I/O cards have bank isolation so that the users can build robust and safe measurement systems for their applications. All the NI C series cards have Channel to Earth Ground isolation which is the bare minimum isolation.