Measure 8 channel 4-20mA; isolated

[Vindhyachal.takniki]

1. Need to measure 8 channel 4-20mA. Ckt is 3.3V powered.
2. One option is to measure by adding a resistor of 165ohms (or 150ohm for error margin) & get 3.3V.
But problem is I have to ground all the resistor connection to that of ADC.I want to keep all of them isolated.
3. I have read abt receiver RCV420, but its minimum voltage requirement is higher.
4. Is there any way to do it isolated.

[commie]

Hi,

I don't think there is an solution to your measurement problem as it is.Granted they all should be isolated but isolated at the instrument end or output. If you have a DCS then all  the process instruments in the field must be isolated such that they maybe read in at the DCS input using a non isolated multichannel adc plugin card or pcb with a common ground.There is another alternative, DCS manufactures can supply isolated inputs to their input pcb plugin multi channel adc's but then they want big big bucks for them.

Note also, if you have 8 instruments feeding a non isolated pcb adc plugin, 7 might be okay but one is causing ground loop problems then all 8 will not work.



4..20mA ground loop comes about because electronic instrument designers use the full +-15V supply for headroom giving maximum resistance compliance, great but it causes ground loop problems and the poor teckie has to deploy a passive isolator which isn't ideal I might add.   Ideally instruments should be designed with a single positive supply exclusively to avoid ground loop problems, ie., single 24V only.

[PSR B1257]

http://sine.ni.com/nips/cds/view/p/lang/en/nid/209075 should do it 
It's only 4k$ - bargain 

Since you are notoriously not replying  that's about all I have to say

[krish2487]

To add to what others have already said,
1. Grammar. Please type complete, succinct, unambiguous sentences.
2. NO Messaging "shorthand". An otherwise helpful contributor might not be help if he/she feels you are not professional enough in asking and responding.
3.Clarity Of Thought before posting. The most important of all. Please formulate, completely, what you would like post before posting.
Do not assume that others might have faced a similar situation. They might or might have not. Either way, even for them to help you they need to understand what you done. Mention explicitly what, where, how, why of your predicament.
4. Respond back. So that those who spent their time trying to help you, wont feel they have done it pointlessly.