converting 0-10VDC to 4-20mA

[raidermanz]

I am currently using a BOC Edwards Wide Range Vacuum gauge which generates a 2-10VDC signal, I need to convert this signal to 4-20mA to accomodate the data logger I am using.  I am using a signal conditioner to convert the signal, but the scaling doesn't seem to be very accurate.  The signal conditioner accepts a 0-10VDC signal, outputs a 0-20mA signal, and is powered by a 24VDC power supply.  My question is the instrument excitation is being drawn from the same power supply, and everything is sharing a common ground.  The literature which is supplied with the gauge tells me to isolate the ground of the excitation and the signal.  My signal conditioner board only has one ground connection.  I originally had the ground for the power of the conditioner, the ground for the power of the gauge, and the ground for the signal all interconnected.  Could someone suggest any way around this problem.  I am really a novice at instrumentation, but have been given the task of "figuring it out".  Any help would be greatly appreciated.  The signal conditioner I am using is an Omega Om7 type conditioner.

[Marni]

Hello Raidermanz,

NI has some great signal conditioning boards with isolated inputs. On these boards the excitation voltage is already isolated from your signal. I wish I could help you with your current situation, but since I don't know anything about your current signal conditioner, I can't really say how to isolate the grounds. Try contacting Omega to see what they have to say. Have a great day!

Sincerely,

Marni S.

[Arty]

It is probably possible to build an op amp circut to provide what you need. I think a fairly simple voltage to current converter may do the trick.

[AnalogKid2DigitalMan]

I went the distance and tried to get detatiled info or a manual from Omega, just got the generic full product line details. There should not be a problem  with accuracy according to specs (+/-0.1% span acc, +/-0.02% span nonlinearity). I did note that the 3db badwidth is limited to 100Hz.

What is the frequency content of your signal? If over 100Hz, thsi coul dbe causing issues.

Without detailed info on the module (read the manual if you have one), I assume:

1.

[AnalogKid2DigitalMan]

I went the distance and tried to get detatiled info or a manual from Omega, just got the generic full product line details. There should not be a problem  with accuracy according to specs (+/-0.1% span acc, +/-0.02% span nonlinearity). I did note that the 3db badwidth is limited to 100Hz.

What is the frequency content of your signal? If over 100Hz, thsi coul dbe causing issues.

Without detailed info on the module (read the manual if you have one), I assume:

1. The

[AnalogKid2DigitalMan]

DOH- bad keystroke combo:

1. Presume module or backplane takes 24V to power it. This supply should be separate from everything else. Do not connect it's ground anywhere else.

2. The 10V signal (+hot and -ground) from xducer goes to the input pins of the module.

3. The mA output signal (+hot and -ground) goes to the data logger inputs.

Again, I could not get detailed info on the module, just going by expereience with similar isolation/conversion modules (analog devices, dataforth, etc).

[raidermanz]

I would first like to thank everyone for their help.  I am really a beginner at this stuff, so some of the advice is beyond my limited grasp of the subject.  I can explain that the backplane of the signal conditioner has a VDC+, Common,signal +, and that is all.  I think I am going to have to use a different arrangement in order to make the conversion from a floating 2-10VDC signal to a 4-20mA signal.  Does this help?

[Logan_K]

I found the user's manual for the OM7 backplanes and modules, which is what you are going to need to use because you will need to know the pinouts of those connectors to make this work.
http://omega.com/manuals/manualpdf/M3195.pdf

This backplane is actually very similar to the 5B backplane that NI sells, and given your requirements I am a bit skeptical about this setup being able to work with just one isolated current output module. The OM7 current output module expects its 0-10V input signal to be referenced to the backplane's ground, and then it will output an isolated current. This is the opposite of what you want, because you need an isolated 0-10V input which will output a referenced 4-20mA. Here is what I think might work - you will need to use two signal conditioning modules cascaded together. You should first get an isolated voltage input module, which will convert the floating 0-10V signal into a referenced 0-10V signal. Then take that voltage and send it to the input of the isolated current output module, which will convert your voltage into a 4-20mA signal.

Good Luck,
Logan Kunitz

[raidermanz]

Thanks again for all the help.  It is appreciated.

[AnalogKid2DigitalMan]

3 way galvanic isolation, 0-10V in 0-20mA out module from Dataforth

http://www.dataforth.com/catalog/doc_generator.asp?doc_id=3&attr_index_id=96

here is the figure showing isolation of power, input, and output:

http://www.dataforth.com/catalog/doc_generator.asp?doc_id=12

I bet Analog Devices makes a similar module.

Again, beware of the 3Hz BW, you did not reply to my previous post regarding this aspect.