SCXI 1327 voltage resolution

[HNEI]

My lab has the following setup

SCXI 1000 chassis

SCXI 1121

SCXI 1327

I am currently cycling 12 volt electric vehicle batteries, but am having trouble configuring the NI hardware to provide a resolution of 1mV.

I am also having difficulty finding the limitations on the input signal value if I use 1:1 signals.  The operating voltage range would be around 10.5V to 14V.

Thanks in advance for the feedback.

[Otis]

Hi HNEI,

You stated that you have an SCXI-1121 that you are trying to read 10-14 V signals off of, but with a resolution of at least 1 mV. 

I took a look at the DataSheet for the SCXI-1121 and found that it looks like you are going to be lucky.  The gain setting you need is +/-25 V and the accuracy at that range is 728 uV.  So it looks like you can measure everything without any additional conditioning and still get the resolution that you need.

Best of luck with this,

[HNEI]

Otis-  Thank you for the feedback, but what is the best way to achieve the overall gain of 0.22 (as depicted in the specifications table for the SCXI 1121).  On reviewing the manual, it appears that you can easily set the gain in the SCXI 1121 to "1".  This gives an allowable input voltage range of +- 5V.

If I were then to set the 1327 attenuator to 1:1, the 10-14V input would exceed the +- 5V that is allowable on the 1121.  Could you please clarify how I should achieve the desired gain value to achieve the +- 25V range?

[Otis]

Hi HNEI,

I forgot that you were using an SCXI-1327.  With the SCXI-1327 you can set (on a per channel basis) of whether or not to attenuate the signal by a factor of 100.  Since we need our final measurements to be maximized over the range of +/-10V by the time it gets to the board, then we could attenuate the signal by a factor of 100 and use the following formula (signal*amplification/attenuation = max voltage) to glean the best amplification setting for the SCXI-1121.

14V * X / 100 = 10 V   ==>  X = 100*10/14 = 71.42

So in this case the closest amplification that we can use without damaging the board would be a gain of 50.

The best part about doing things this way would be that you will increase your accuracy by over 250 fold!  Prior accuracy might have been 728 uV, now it should be 2.86 uV.   Definitely an improvement.

Sorry about the oversight, but I'm glad to see that things should work even better for you now.

All the best,