residual gain error and residual offset error

[Tedy]

why the NI DAQ board still have the Residual gain error and Residual offset error after calibration? how to caculate the residual gain error and residual offset error?

[dousley]

Hi Tedy,

I can give a somewhat general answer to this question. I hope it helps.

Not all errors can be removed through adjustment or autozero. So while an offset and gain adjustment during a calibration can remove much (or nearly all of the uncertainty) various factors still cause errors that cannot be compensated for. This is true of all instrumentation.

Some of those un-removable errors include things like Integral Non Linearity (INL), thermal voltage offsets that change depending on usage of the instrument (usually in the few microvolts to sub microvolt levels), or bias current induced offsets that change depending on the source impedance of the sensor being measured.

In devices that employ muxing there are shared errors that can be calibrated/adjusted out and there are independent errors that either can't be adjusted out or can't be adjusted out without using independent calibration constants for each channel. In some cases, sophisticated calibration constants and scaling mechanisms can help address non-linear error sources or system specific error sources, but even sophisticated scaling is not able to completely remove all error.

Generally, NI provides specifications for usage of its instruments at a system level including uncertainty that comes from all the permutations and flexibility inherent to modular platforms. Those published specs include the residual gain and offset errors that follow calibration/adjustment.

I hope that helps.

DOusley

[rafach]

Hi my name is Rafael,

I am working in a proyect of heat transfer and I need to measure temperatures with high accuracy. As you mention, when we use an instrument to measure any physical paremeter always there are errors. In my case I was able to quantify the offset error of each the thermocouple. I developed a program in LabVIEW which graphs the behaviour of the temperature in a CHART and SAVE the data; besides with this program, I want to eliminate the offset error, adding or substracting this from the temperature data, but when I modify the data these are not graphed or saved. Previously, I developed a program program without eliminating the offset error and it works perfectly. I attached an image of my program I hope you could help me. 

Thanks in advance.