how to achieve 8 isolated analog channels.

any other ways to do it....

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How far do you want the isolation to go?

That is, if you have eight floating outputs from the field feeding
eight isolated analog inputs do you also need the analog outputs from
your circuit to be isolated from each other and from the supply(ies)?
Will you be using one ADC and multiplexing the inputs to it or will
you be using eight isolated ADC's?

thanks for replying!!! if i use iso-amp at input end then, preferably i'll use one ADC(multiplexing inputs) or 8 channel 16 bit ADC, as eight seperate ADC would be costly matter.

__ sanjay

High impedence differential amplifiers more or less give you what you need.

If you need 8 isolated input and 8 isolated (12~24V/~30mA)? supplies to meet the requirements, that's what you need.

Isolated-output DC-DC converters and isolation amplifiers are in order, or maybe you can design them yourself. Yes, it will be costly per channel and it will take up some board area. The reliable solution sometimes is like that.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
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ISO-AMPs - run search...

Thanks for suggesting IL300, In application notes of IL300, +15 V suppy is used, but you say they could be self powered. how is that ? can you give more details?

The 4-20mA inputs don't need DC-DC converters, they could be self-powered off the 4-20mA. Something simple maybe, such as an opamp and an IL300 optoisolator.

Perhaps the OP could consider an isolated 8-channel 4-20mA design, and deal with 0-10V inputs separately (as an optional extra, and for more money).

The Analog Devices and Burr-Brown prices for analogue voltage isolator modules do give some indication of the technical problems.

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Tony Williams.

My assumption is that he *needs* the power supplies to power individual loop-powered transmitters on each input. This is the most general (and negative, and costly) assumption but will be the most reliable solution. Often a single isolated supply is used for several channels, but that's not as good, especially if the option is there to mix them up with voltage inputs at times.

Which, of course, is nothing compared to the cost of field problems in this kind of system, as reflected in the prices of typical modules for the process control market.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

I was thinking of something along these lines.

---------------------------------------------- One transmitter channel. | ~~~~~~~~~~~~~~~~~~~~~~~~ |/4-20mA | +-------------------+-----------+ | | _|_ | | _ /_ IL300 LED _|_ | T (2-10mA) /_ 5v | [R4] | _ | | | +----+---|- | |--+ | _|_ | |Op >-[R3]--||nmos | D1__/_ | +-|+ /| |--+ | | | | | | +------+------+-----+-----------+ | | [R1] [R2] |100 ohms | +-----------+ | |/4-20mA | ---------------------------------------------- One receiver channel. ~~~~~~~~~~~~~~~~~~~~~ -+-5v | +---[R5]---------+--Vout | _ | | +--------|- | | D2_|_ |Op >---+ / +-|+ /| T | | +------+-----+----0v

D1 and D2 are the IL300's optos. D1 and the opamp servo the IL300's LED to keep the D1 current equal to the R2 current, so measuring the voltage across the current sense resistor, R1.

Run the LED over the range of about 2-10mA, making sure that the 5v voltage reference and opamp take less than 2mA.

The isolated receiver simply converts the current from the second opto into a voltage.

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Tony Williams.

Yes that is a consideration. ITSTM that on the last 4-20mA job I did, everyone else assumed that someone else would provide the 4-20mA power..... me.

[High cost of analogue isolator modules]

True.

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Tony Williams.

"Tony Williams" wrote

There is usually one bulk 24VDC power supply for each unit of

4-20 process inputs. All the loops are powered by the same supply. The input card may or may not provide isolation. Isolation is normally only needed if the loop supply is remote to the AI card/ unit.

For digital contact inputs there is again one bulk supply, usually of 100VDC, the input card supplies the dropping resistor and optos are used to pass the signal. 100 VDC is used to blow the contacts clean. 5v @ 2ma will not keep relay contacts in working condition.

If the application is digital->digital over some distance then just plain 'ole optos are the safe way to go. The output of the driving gate supplies the power.

Another method involves pulse transformers. The transformer secondary is across the contact. Closure is detected by monitoring primary current.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer:  Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
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Problems arise in systems where there can be several independant 4-20mA receivers in series on the same loop. This could arise where there is a control receiver plus independant receivers for safety monitoring or display.

One receiver (usually the PLC) will demand that it occupies the easy place, (down on 0v, little or no isolation req'd), so all other receivers in series on that loop have to have a common mode capability.

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Tony Williams.

You could use a mos analog switch controlled through opto couplers, and then use one isolation amp for all 8, if you don't need them that fast. You could use 8 indivdual iso amps, that is not so impractical if you choose the CP Clare ones cause they are pretty cheap. Sorry if this has been said before, I missed it.

Rocky

Thanks for suggesting clare chips! I give analog signals to 8 input channels then i convert it to digital form using ADC then i give this count to microcontroller.

i.e....|iso amp|--->|?mux?|-->|ADC|-->|opto isolation|-->|uController|

can i get good accuracy? If i want channel to channel isolation then... upto what extent should i give isolation? What should i use for my scheme a)mux & 1 ADC or b)8 seperate ADC's to achieve channel-channel isolation???

if i use a)mux & 1 ADC then... can i call it as channel channel isolation?? if I use b)8 ADC (12/16 bit) then...which is cheap & good accuracy ADC

how to get isolated power supply for each channel from single source(of

5V)