Multiple channel AI DAQ issue

[Jason2010]

Hi,

I am use PCI-6251 to have a 8-channel analog signal acquisition. 8 channels are configed to Differential. Data from 8 channels are acquired at the same time. All the channel data seems OK except channel 2. It is always low. At first I found output impedance is high than the other channel, I made a voltage follower and connect between channel 2 and DAQ card. The problem is still same. Finally I found I need to adjust the property AIConv.Rate, acquisition data will be low if the property is set too high or too low. There is an optimum value to get the maximum channel 2 data. I don't understand why and which value should I believe. If there is any cable connection change etc. will I adjust the property again? Channel 2 is not a high frequency signal, is a kind of DC signal after it is settled down. 

Jason

[Simran_K]

Hi Jason,

Do you only see low voltage on channel 2 when measuring multiple channels? Can you test your setup using the Test Panels in Measurement & Automation Explorer? To access Test Panels, just right-click on your device under NI-DAQmx Devices and choose Test Panels. Please let me know if everything works properly in the Test Panels.

Also, below is some information that I found related to the AIConv.Rate property you are using.

How is the Convert (Channel) Clock Rate Determined in NI-DAQmx and Traditional NI-DAQ?

How do I Increase Interchannel Delay Using NI-DAQmx or Traditional NI-DAQ (Legacy)?

Thank you,

[Jason2010]

Hi Simran K,

When I use test panels in MAX, channel 2 reading is fine and I confirm with a multi-meter.

[Simran_K]

Hi Jason,

Do you see low voltage when you are running multiple channels and channel 2 alone? Since everything worked in MAX, there is nothing wrong with the hardware. Have you looked at the KnowledgeBases I posted earlier? What happened when you tired the solutions mentioned in those articles.

Thank you,

[Jason2010]

Hi Simran,

I have tried the KnowledgeBases articles. I tried to change AIConv.Rate property manually. Running multiple channels (I need to run them anyway). Like I said, there seems a AIConv.Rate value when channel 2 data is maximum. When AIConv.Rate is higher or lower than the value, channel 2 data will be lower than what I measured by digital multi-meter. That's what I don't understand. Even I tried to connect a voltage follower behind channel 2.

Jason

[Simran_K]

Hi Jason,

What sampling rate and version of NI-DAQmx are you using? 

When you tested channel 2 using the test panels, the problem went away. I found a forum on a similar issue and ghosting was the problem. 

Thank you,