NI 9229 Offset & Accuracy

[anintern]

Hello, 

I'm using an NI 9229 in a cRIO application and I'm attempting to measure voltage across a shunt resistor. The 750uV = 1A. With 24 bits the 9229 should be accurate to ~7uV (120/(2^24).

I'm putting a multimeter across the terminals on the 9229 to which the shunt is connected and reading a steady 1.15mV. I dont get this value read back from the FPGA code. Attached is an Excel logfile of the readings from the input (which have been verified at 1.15mV). As you can see it dances around between 0.00290679931640625v and 0.00153350830078125V. I should be around 0.00115000000000000V.

In the specification - http://www.ni.com/pdf/manuals/372263a.pdf - it states an offset error of ±7.66 mV - but I assumed this would stay constant and not alter between readings. 

Where should I be looking to reduce the error between the terminals on the 9229 and the values coming out as a FXP value of the FPGA code?

Cheers

Tom

Jaguar Land Rover 

 

Message Edited by anintern on 03-31-2010 01:50 PM

[Seth_B. · Accepted solution]

The calculation you show is for resolution, not accuracy.  Resolution is the smallest change that the module can digitize.  Accuracy is how close to the actual value each measured point is.  In order to calculate accuracy, you'll need to use the table of page 20 of the Operating Instructions & Specifications.  The formula for accuracy is:

Accuracy = (Gain Error * Reading) + (Offset Error * Range)

For environments that are within ±5°C of 25°C, the NI 9229 has a Gain Error of 0.03% and an offset error of 0.008%.  The reading is the value you are trying to read and the range is 10.52V.  The follow table is an example of the above calculation:

Reading		Accuracy (±)
0	mV	0.8416	mV
1	mV	0.8419	mV
10	mV	0.8446	mV
100	mV	0.8716	mV
1	V	0.0011416	V
10	V	0.0038416	V

 It looks like the values you see are varying within the expected ±0.84 mV.

Regards,

[anintern]

Cheers for the quick response on this Seth.

Much Obliged.

Tom 

[fairhairedwineboy]

How do you come up with a range of 10.54 instead of 120V?

[Seth_B.]

This was my mistake.  These calculations are for the 9239, not the 9229.  For the 9229, the calculations are the same as above, but the range is ±62.64V (per page 20 of the specifications).

[SL_Ong]

Hi Seth,

How about if the measurement is below 10mV, what is the accuracy? I saw that there is a input noise of 320uV, do we need to put that into consideration as well.

Regards,

Stanley