VI Logger data offset

[owensigurdson]

I am using vi logger with a PXI/SCXI combination chassis with an 8175
controller, 6030E 100KS/s DAQ board, 2 1120B SCXI cards, 1 1120 card
and an 1126 frequency module. After I set up all of the channels in
MAX, I can inject a signal and view it on the test panel and every
thing is fine. When I start vi logger and view the data in the real
time screen, I notice a significant offset on both the frequency and
thermcouple channels.

Has anyone seen anything like this with this kind of hardware setup
and/or vi logger. I have two identical systems with both displaying
the same problem.

Thanks in advance for any help

[Ben]

Quick question:

Does the "Offset" change when you reduce the sample rate?

Ben

[aasenjo]

I agree with Ben, the test panels in MAX scan only one channel at a time while VI logger is scanning all selected channels. You might be seeing the effect of residual charge in the multiplexer as it switches through the different signals, specially affecting the lower voltage signals like the thermocouples and, depending on the measured frequency, the frequency inputs. Logging at slower speed may vary the results, pointing to this possible cause.

Try logging the data from the thermocouples only to see if there's still an offset. Try slower scan rates, and logging each channel individually to try to see if the problem is related to speed or just the scan order. This will help find the problem and then we can suggest some solutions.

Alejandro Asenjo
Nat
ional Instruments