Units of SCXI cal constants

[fsdaq]

I am wanting to adjust the EEPROM calibration constants for my SCXI 1102C (as well as 1520 card) card and am using "SCXI Cal Constants.vi" to read, write and transfer the constants. I am performing this calibration myself, i.e., I am measuring/calculating the necessary adjustments myself using appropriate calibrators. I am successful at writing to the user area, and am also successful at copying these values to the defualt load area and the NIDAQ memory locations.

But the value that I read back is double the value that I initially wrote. Right now I am just playing with the offset values and not the gain.

What are the units of "Cal Constant Out 1/2" and "Cal Constant In 1/2"? I initially thought that the units of the offset value (Cal Constant In/Out 1/2) would be millivolts, but I don't think this is correct. Do I need to change my numbers to binary (multiply by 2^16 and divide by full scale) before writing and do the opposite after I read?

Thanks much!

Matt S

[AnujD]

Here is a great KB that should help you out!

http://digital.ni.com/public.nsf/websearch/EFC5F2996627470686256DE9005E6DAA?OpenDocument

Regards,
anuj

[fsdaq]

Thank you very much for your reply, the link was indeed helpful, but I should restate my problem a bit.

The real problem that I am encountering is that the the value I read from "Cal Const Out 1" (the offset) is not the value that I write to "Cal Const In 1" (also offset). Further, if the units of these values are binary, the voltage I read with my device does not correspond to the change I made in the offset constant (I am verifying that the constants are indeed changed in NIDAQ memory as well as default load location).

The table below should illustrate my issue:

Value Written to Cal Const In 1 Write
Value Read from Cal Const Out 1 Read
Value measured through the DAQ device Voltage

Trial Write Read Voltage
1 100 -35.94349 0.0067
2 -100 35.94349 -0.00466

Some questions: Why does the number I write become changed when I read it (100 vs 35.94349)? Why does the number I write become negative when I read it (+100 to -35.94349)? My system is 16 bit and I am using SCXI gain = 1 for this excercise, so resolution it 0.305 mV (=20/2^16). Any way you cut it, the numbers don't seem to work.
total span - 2 x 35.9439 = 71.88. Times res (0.305) = 21.9mV, which is not equal to 0.0067 -(-0.00466) = 11.36 mV.

Any help would be greatly appreciated. Thank you.