Can anybody tell me how the assumed TempChangeFromLastExternalCal and TempChangeFromLastInternalCal values are derived in the Absolute Accuracy Formula shown in the Ni-9205 Operating Instructions manual on page 24?
The assumed value for TempChangeFromLastExternalCal appears to be simply the maximum temperature specified for the module.
The 70 degrees from last external calibration simply comes from the stated assumption that the device is operating within 70 degrees of the last external calibration (this assumption is indicated in the specs). External calibration is what happens when you send your device in for calibration (or follow the calibration procedure using the necessary hardware).
The 70 degree assumption seems a bit extreme for most use-cases and makes the 9205 look worse than reality (for more "typical" applications). It might have been picked because this is the max operating temperature (though it doesn't really make sense since the device isn't calibrated at 0 degrees) or perhaps it was assumed that the device might have been calibrated at 30 degrees and is operating at the minimum temperature of -40. I can't say for sure.
The 1 degree from last internal calibration comes from the assumption "immediately following internal calibration". Internal calibration is what happens when you "self calibrate" your device (more info here if you're interested).
So the bottom line is you should substitute your own numbers into the equation if you are looking for accuracy calculations more specific to your use-case. I believe the cal certificate for the 9205 includes the calibration temperature (you can try here if you don't have it handy), so take the difference between that number and the ambient temperature to determine what number you should use. There isn't much of a reason to not self-calibrate immediately before using the 9205 if you care about accuracy, but if the ambient temperature is going to vary during use you should account for this as well.
Thanks for the detailed explanation. I'll use the 30 degree value that you recommended in place of the 70 degree temperature limit for my system accuracy calculations.
For the absolute accuracy formula used for the 9205, does it matter what mode it is in (Single ended, differential)? Will Differential mode be double what is shown in the 0-10V range since it uses 2 op amps?
I guess I am not 100% clear on how the 9205 hardware is working. From the document there is a mux and then the PGIA to the ADC. All channels (32) share the PGIA and ADC. The input to the PGIA is ether an AI channel and Com (Single ended) or 2 AI channels (differential). Now the question is how does the MUX work? Can it switch in more than 1 AI channel at a time? Is there two 16 channel MUX? Or does it take 1 AI channel at a time, convert the reading in the ADC and do the differential in the FPGA with math? I would hope that 2 AI channels are MUX to the PGIA and the Differential done on hardware.
In ether case since we have 2 AI (AI+ and AI-) channels going through the circut do we have to double the accuracy (uncertainty)?
Please reference Figures 6, 7, and 8 on pages 13, 14, and 16 to see how the internal hardware functions for different sensor configurations. These Figures show that the PGIA is the only OP Amp between the sensor and the ADC regardless of the connection type, so the absolute accuracy equation applies to all sensor configurations.
In regards to the multiplexor (MUX): The 9205 posses one ADC and the multiplexor rotates through the connected input channels, allowing the ADC to convert each one in turn. This device can’t read and convert multiple inputs simultaneously because of the single ADC limitation, but they do happen in quick succession.
In regards to the PGIA Op Amp: When in differential mode, the PGIA references the positive and negative sensor channels allowing it to cancel out common signal noise in hardware. When connected in a referenced single-ended mode the PGIA references one analog input channel and ground. The PGIA references the AISENSE line and one analog input line when connected in non-referenced single ended mode.
I hope this clarifies how the 9205 functions and answers your questions!