how to achieve 8 isolated analog channels.

Guest
In my 8 channel analog input board, i would like to have
channel to channel isolation for 8 channels. in my circuit i take
analog signal (4-20ma or 0-10v) from field and convert it into digital
form, i also would like to have isolated power supply for each channel.
To use seperate power supply for each channel is not possible(space on
board,cost would matter!). can you suggest some
circuit(configuration,ICs ) which will meet my all
requirements(isolated channels,isolated power supply)?
 
Subject: how to achieve 8 isolated analog channels.
From: sanjay_td@rediffmail.com

In my 8 channel analog input board, i would like to have
channel to channel isolation for 8 channels. in my circuit i take
analog signal (4-20ma or 0-10v) from field

i also would like to have isolated power supply for each channel.
To use seperate power supply for each channel is not possible(space on
board,cost would matter!).
Click to expand...
High impedence differential amplifiers more or less give you what you need.
 
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
 
In article <252es0hoonoggdosfjctgahl0ipee68lie@4ax.com>,
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:

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

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

--
Tony Williams.
 
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?
 
In article <r3fgs0ht6cpas0cc4m0uhtk41qet1f83id@4ax.com>,
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:

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.
Click to expand...
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]
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.
Click to expand...
True.

--
Tony Williams.
 
Subject: Re: how to achieve 8 isolated analog channels.
From: Tony Williams tonyw@ledelec.demon.co.uk
Date: 12/22/04 2:00 AM Pacific Standard Time
Message-id: <4d2138b007tonyw@ledelec.demon.co.uk

In article <GXZxd.6445$Z47.845@newsread2.news.atl.earthlink.net>,
Nicholas O. Lindan <see@sig.com> 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.

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.

You could use a mos analog switch controlled through opto couplers, and then
Click to expand...
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)
 
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