Analog Input - DAQ Card disagreeing with Multimeter

[Ramsey_Zeitoun]

Hi,

Just wondering if anyone has had an issue similar to this...

I am measuring the current across two resistors with two analog inputs.  I am expecting 2.0V and 4.0V across each resistor.  However, I am measuring (differential mode) 2.0V and 3.3V across the resistors through the DAQ card (using the standalone DAQ software).  With my multimeter I am measuing 2.0V and 4.0V.  So my DAQ card is missing 0.7V on one measurement and is perfect on another.  Also, I am not switching between measurements so ghosting shouldn't be an issue.

Does anyone have any ideas of dealing with this?  Is it a grounding issue?  If so, then why is the multimeter taking accurate measurements?

Edit:  Also, power is being supplied by a 12V switching power supply (http://www.cui.com/product/resource/vgs-75-xx.pdf). And current is around 180 uA.  I use a voltage splitting circuit to ensure the drop across my two resistors is 6V.

Thanks.

[johnsold]

Your circuit configuration is not completely clear. Can you post a .png or .jpg image of a schematic?

With 180 uA at 12 V you have fairly high impedances in your voltage divider. While the multimeter probably has ~10^7 ohm input impedance, some DAQ devices, especially the less expensive ones, often have much lower input impedances.

What DAQ device are you using? How do you have the inputs and grounds connected?

Lynn

[Ramsey_Zeitoun]

Hi Lynn,

Thanks for the response.  I have an X-series 6343 DAQ.  The datasheet says 10 Gohms analog input impedence in parallel (http://sine.ni.com/ds/app/doc/p/id/ds-153/lang/en).  Actually, the past few days I've been playing with the circuit and just got it working.  I think part of the problem was that I was measuring a differential involving 12V to 8V.  Although I am expecting 4V drop, the 12V part is outside of the range of the DAQ (which has a 10V range).  I found that in an NI whitepaper the other day, but can't find it now.  

Here is my circuit.

Does that explination sound reasonable to you?

Ramsey

[johnsold]

Ramsey,

Absolutely! The differential measurement only works when both inputs are within the input range.  This is true of any kind of device.  Glad you got it working.

Lynn