Thermocouple Channel influenced by other Voltage Channel

[CaswellBerry]

Hello,

I am using the BNC-2120 and a K-type Thermocouple (TC). On the software side I use C/C++ and DAQmx to acquire the temperature and some analog input signals (AI). While I was setting up the input channels, I noticed some odd behaviour.

If I create the TC-channel before the AI-channels, everything seems to be fine as the TC shows a reasonable temperature trend. But if I do it the other way round, the last AI-channel, that I created right before the TC-channel, is influencing my temperature measurement. That means, if I record some voltage increase on the AI-channel, my temperature will go up significantly.

Can someone explain to me why this is happening?

PS: All channels are part of the same task.

[Tony_Dean · Accepted solution]

I have experienced similar issues in the past, though not with this card. The problem seems to be a 'carry over' of charge in the internal multiplexer/sample & hold when changing from a relatively high signal level to the low signal level from a thermocouple. I got around it by reading the thermocouple signals twice and discarding the first reading but I don't know if it is possible to put the same channel twice in the same scan. Alternatively, try increasing the scan interval so the measure value has longer to settle out to the new value.

[CaswellBerry]

Thank you for your reply. I found a similar statement somewhere hidden in a manual. My workaround at the moment is to just declare it as the first channel I want to read. But for me this is somehow confusing as it seemed to be just a software related problem at first. I mean, why would exchanging two lines of code change something in the hardware...!?

[HeinzG.]

Hi all,

to bring more clarity to this. It sounds like this is some kind of carryover effect - a.k.a. gohsting. You can find an explaination here:

How Do I Eliminate Ghosting from My Measurements? - National Instruments
http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00028DDF

When you change the order of the cannels, you most probaly also change the seen effect.

Hope it helps.

Heinz