DAQ questions with the SCB-68

[wingswing]

I've got the SCB-68 board lying around, and I want to try to make it work for my application. I am trying to read resistances using a 4-wire configuration. However, I'm pretty sure this board only reads voltages. I looked around for possible solutions to this, but never found one. I think I've thought of something very simple, but I would like to run it through more experienced people first:

Since the board supposedly has a 5V output (is this going to be practically constant for any load?), I was thinking of connecting the voltage source in series with a resistor of known resistance (R1), in series with the resistor whose value I'm trying to determine (R2). I would also connect leads from the +-resistors to analog inputs in the card, to measure the voltage drops across the resistors (V1 and V2). From V1 and R1 I can get the excitation current, and from there calculate the value of R2. (R2= V2*R1/V1). 

Does this sound like a good idea? I ask because I don't quite understand all of the nuances behind circuits and electrical measurements yet. Would this kind of setup actually work? Or is it somewhat naive? I need pretty good readings for R2, preferably within a 100th or even a 1000th of an ohm if possible. I'm thinking the excitation current should be on the order of 1mA or so. 

Thank you! 

[AnalogKid2DigitalMan]

wingswing:

What DAQ card are you planning to connect to the SCB-68. The SCB-68 is only a connector block with some interface circuitry.

http://www.ni.com/pdf/manuals/372551a.pdf

What range of resistances do you plan on measuring?

-AK2DM

[wingswing]

I'm not sure; I've just arrived home so I can't check at the moment, but it is definitely connected to something.

The resistances should be small, varying between 90 ohms to around 140, so the voltage drops should be on the order of 10s of millivolts assuming the other resistor is similar. I could try to use a higher excitation current, but I would have to check the literature to see if it's ok for the elements I'm measuring. 

[wingswing]

I think it's connected to the NI-4351.

[CharlesD]

The NI-4351 is a high accuracy thermocouple module.  You should be able to measure the resistance of your unknown resistor however I don't think you'll getthe accuracy readings that you are asking for.  To get that type of accuracy I strongly suggest a high accuracy DMM (NI 4065, NI 4070, NI 4071) Depending on your range and resistance that you are actually trying to read you should be able to get 4mOhm --> 12-15mOhms of accuracy (NI 4065 on 100ohm range).  and 400uohm -->3.5mohm (NI-4070 on 100ohm range.

The DMM is actually your best bet for this type of measurement! If you look at the resistance measurementsaccuracy specifications for the NI-4351 you can see that the minimum accuracy that you can get is .2Ohm (using Iex1 =1mA) and .74 ohm (using IEx0 = 25uA).

Hope this helps Let us know if we can help you any further!

[AnalogKid2DigitalMan]

From the manual, refer to pages A-5 and 6.

http://www.ni.com/pdf/manuals/370841a.pdf

Using 625 ohm range (note table on A-5 has typo error, range should be ohms, not W) and 1mA excitation, it looks like at best your accuracy will be .04 ohms. Thermal noise and current source noise will only degrade that figure.

For your needs, it would probably be wise to go with an off the shelf dedicated instrument (like a Keithley 2400- I do not know the specs off hand, just an example). There are numerous other 4 and 6 wire micro-ohmmeters on the market.

Sounds like you have a lot of cool work going on.

-AK2DM

[CharlesD]

There is a Typo there and it has been noted and Corrective action request has been filed. Sorry for theinconvenience.