Hi, I have a USB-6002 DAQ card that I am trying to use to read an analog 0-10 V signal for pressure from a Vaisala PTB barometer. The barometer is sending out about 3.5 volts when read with a volt meter but the DAQ card says that it is reading about 9.6 VDC. I have tried reading both AI0 channel as well as differential to eleminate the potiential for ground differences as I am also using the AO channels for contol of other equipment. Just as soon as I touch my volt meter to the screw terminals on the DAQ card, it jumps down to the 3.5 volts that I am reading with the volt meter. The cable is not shielded but it is only about 5 feet long and is not running by any power lines which should be inducing any other signals. Do I need a resistor or cap in there to filter things out and if so, how would I go about sizing that or is there something else I should look at? I am using NIMax to read the voltages for now but when I use the Labview drivers, I get similar results.
It sounds like you need to connect your grounds.
It was giving be the high voltage (9.6) when I connected the - to AI GND and the + to AI 0 (AI 0+) as well as when I ran in DIFF mode connecting the - to AI 4 (AI 0-) and the + to AI 0 (AI 0+). In both cases, as I was running NIMax, the voltage shown in NIMax would drop from about 9.6 volts to about 3.5 volts as soon as I touched my multmeter probes to the screw terminals of the DAQ card. With the DAQ card removed from the picture, I also get about 3.5 volts as measured with the multimeter.
Also, to clarify. The reason I also tried it in diff mode is because I am controlling some other equpiment with the two AO channels so I wanted to make sure that I wasn't running into a ground loop with my - terminals but the results were still the same.
From the behavior you are describing, it definitely seems like a grounding issue. I've attached the user manual for your device below. There are wiring diagrams in that document for different terminal configurations. Another thing to check is that when you change terminal configuration, make sure to change that setting in MAX as well, or it will give you erroneous readings.