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how to achieve 8 isolated analog channels.

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)?
 
C

CBarn24050

Jan 1, 1970
0
Subject: how to achieve 8 isolated analog channels.
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!).

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

John Fields

Jan 1, 1970
0
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)?

---
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
 
S

Spehro Pefhany

Jan 1, 1970
0
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)?

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
 
T

Tony Williams

Jan 1, 1970
0
Spehro Pefhany said:
If you need 8 isolated input and 8 isolated (12~24V/~30mA)?
supplies to meet the requirements, that's what you need.

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.

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

Spehro Pefhany

Jan 1, 1970
0
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.

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.
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.

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
 
T

Tony Williams

Jan 1, 1970
0
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?

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.
 
T

Tony Williams

Jan 1, 1970
0
Spehro Pefhany said:
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.

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.

True.
 
N

Nicholas O. Lindan

Jan 1, 1970
0
Tony Williams said:
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.

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.
 
T

Tony Williams

Jan 1, 1970
0
Nicholas O. Lindan said:
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.
 
R

Rolavine

Jan 1, 1970
0
Subject: Re: how to achieve 8 isolated analog channels.
From: Tony Williams [email protected]
Date: 12/22/04 2:00 AM Pacific Standard Time
Message-id: <[email protected]>



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
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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