VDC to mA conversion

[Jo-Jo]

I have a DAQ project (6 analog inputs into a PCI DAQ) that I am putting together for one of my clients that is pretty straight forward except for one thing. They are reading instruments that are on the opposite side of the lab from the computer that actually has the DAQ card in it, and the instruments output 0-5VDC signals. I don't know who wired the lab initially, but I'm sure you already see the problem. The signal that actually gets received is <100mV,in fact the 60Hz from the wires in the walls induce a larger signal on the lines (I can see it with a scope). I want to convert the signals to 4-20 mA before they get sent across the lab, and just switch the readings from volts to current, which should clean up all the problems that they are experiencing. Does any one have any recommendations for off-the-shelf hardware for this conversion? I would rather not have to fabric my own hardware if I can help it, and I don't believe NI makes any transducers that do this, or I least I couldn't find any. I have actually done this before, but I can't remember the manufacturer I used, and anyway I think it's always good to get other opinions, or find out what has worked for other poeple.

~Benjamin

[AnalogKid2DigitalMan]

Hi Benjamin:

I've used Dataforth quite a bit (usually going 4-20ma to V or 4-20ma loop isolators), you may want to check out their DSCA series.

What bandwidth do you need? Be aware some of the these units have pretty heavy 50/60 Hz CMMR's on their inputs.

There are a host of other manufacturers, I can't recall at this time withough digging through my favorites list.

http://www.dataforth.com/family.view.aspx?familyid=15

-AK2DM

[Jo-Jo]

Bandwidth?  At the risk of sounding stupid, I'm not even sure what the datasheet refering to in this instance as it's only trying to convert a DC signal to a DC signal, so what is the bandwidth measurement?

I found this one a Digikey:

http://www.crmagnetics.com/pdf/CR5300_Series.pdf

Thanks for the recommendation. The Dataforth is better all around (especially on response time, I think the Dataforth is the fastest I have seen). It's also more expensive($215 vs $151). I have to buy 12 of them, so I'll have to see how my client feels about spending the extra money.

~Benjamin

[AnalogKid2DigitalMan]

Hello Ben:

DC primarily means current flow in one direction. Even DC signals can change in time. For example a solar cell produces DC- wave your fingers back and forth over it is while connected to a scope. If your signals are changing slowly, then no problem if the converters response time is OK.

Good Luck!

-AK2DM