Only first configured analog input is correct.

[Tom_from_the_Lab]

I am trying to read 15 channels with a NI-9205 in slot 5 of a cDAQ-9172. I also tried on a cDAQ-9174 with the same results. My problem is only the first channel I configure returns the correct value, all the other channels appear to return numbers that do not relate to the inputs.  For example I have -7.5 volts on ai1 but read +3.5. If I don’t use ai0 or move ai1 to the first channel in MAX I get the correct -7.5 volt reading. I attached the VI that doesn't work. 

As you can tell I took this straight from DAQ Assist. Returns 7.5 for ai0 (correct) and 3.5 for ai1 (wrong). I tried it as a task in MAX, DAQ Assist, and with DAQmx and got the same results. I did test each channel one at a time and got the correct results. I reset all my hardware, tried several clocks, changed my cDAQ-9172 with my cDAQ-9174 and the same results. It has to be something I’m doing since it’s very consistent. What have I forgotten since I last used my NI-9205?

[RavensFan · Accepted solution]

What is the source of the voltage you are trying to read?  Perhaps it has too high of an impedance.

See Decreasing the Source Impedance of an Analog Input Signal

How Do I Eliminate Ghosting From My Measurements?

[Tom_from_the_Lab]

The first 5 channels are power supply outputs and the rest are op amp outputs except the last one is a 250 ohm resistor on a 4 to 20 mA output.

[Tom_from_the_Lab]

Ravens Fan,

I was thinking about your answer and looked back at my data. Turns out the first channel and the output across the 250 ohm current to voltage resistor were correct (it's always the assumption I make a 3 AM that comes back to bite me). I checked the circuit board and found a series of undocumented isolation resistors at all my test points. Reduced the clock rate for the MUX and scaled for the resistors and have actual real values. Even more embarrassing is the fact I saw both those documents searching for my problem!

Thanks a million for the input!