Simultaneous temperature and voltage measurement

[Coolest]

We have a NI-4351 to measure both the temperature and a voltage across a resistor. We have utilized two successive flat sequence structure frames, each containing an AI-Single Scan SubVI, to first scan the voltage off the thermocouple, and the second to scan the voltage off the resistor. Our voltage readings across the resistor are roughly 3V.

When we run our VI without scanning the resistor voltage, our temperature readings are correct. However, once we connect the voltage supply to a channel on the terminal board, we notice two things:

1) A constant error of 2500
2) The voltage readings across the thermocouples shoot up three magnitudes of error to approximately 0.08 volts

Would we need to separate the two scans into two separate buffers? If so, how would we do this?

Much Appreciated!!

[Alan_L]

Coolest,

At first glance, this particular issue may be related to the fact that the 4351 can only be configured for one gain setting across all of its channels. I recommend that you connect both signals and run a program which only performs one measurement at a time, reconfiguring the board between each measurement. If you still have the problem, disconnect both signals and then measure each one at a time. Make sure that you are configuring the channels to be ground referenced. If you've tried all of this and are still experiencing the same problem, there might be some cross-talk occurring between the channels -- potentially due to a grounding issue.

I hope that this is helpful to you.


Thanks,
Alan L
Applications Engineer
National Instruments