Low Cost analog input (4-20mA) measurement with DAQ

[splee]

Hi,

I would like to have a very low cost current loop measurement system which I can plug into my laptop:

I have a force sensor which is connected to a signal conditioning/amplifier circuit, which output a 4-20mA. I would like to measure this current loop signal.

Any idea for lowest cost system? I think most NI DAQ are too expensive and too overkill.

I have LabVIEW.

[SnowMule · Accepted solution]

Use a low-value current sensing resistor, then measure the voltage drop across it using a 6008/6009 DAQ?  They're about $150.

You'll probably need to amplify the current-sense resistor with a MAX4372 or similar to get an output that allows you to use a reasonable scale on the DAQ.  I measure current through our products in almost all of our test equipment this way.  Size the sense resistor accordingly.  The 6008 is pretty accurate, but it's not the fastest nor is it well-featured.  But at $150, they're tough to beat.

[splee]

Hi,

what would you recommend a 4-20mA current sensor? I found that NI SCC-CI20 not a good choice as it has to be used with a SC-2345

[SnowMule]

V=I*R, a 250-ohm resistor's going to give you 5V across it with 20mA through it. (Waffle1.png)

Might have some problems with the low impedance of the 6008 using this method.

If that's too noisy or for whatever reason that doesn't work, you can drive the current through a smaller resistor and amplify the difference to get the 0-5V output. (Waffle2.png)