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Appendix B - Noise and Resolution Tables

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The following tables provide typical noise levels of the U6 under ideal conditions. The resulting voltage resolution is then calculated based on the noise levels.

Measurements were taken with AIN0 connected to GND with a short jumper wire, or from internal ground channel #15.

All “counts” data are aligned as 24-bit values. To equate to counts at a particular resolution (Res) use the formula counts/(2^(24-Res)). For instance, with the U6 set to resolution=1 and the ±10 volt range, there are 1024 counts of noise when looking at 24-bit values. To equate this to 16-bit data, we take 1024/(2^8) which equals 4 counts of noise when looking at 16-bit values.

Noise-free data is determined by taking 128 readings and subtracting the minimum value from the maximum value.

RMS and Effective data are determined from the standard deviation of 128 readings. In other words, the RMS data represents most readings, whereas noise-free data represents all readings.

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Comments

#1

Submitted by Anonymous (not verified) on Thu, 2012-11-08 16:27.

Do you have any noise information for the DACs on the U6?

#2

Submitted by LabJack Support on Fri, 2012-11-09 11:38.

I put a scope from DAC0 to GND, and the noise I see is the same as if I put the scope from GND to GND.  That just tells us that the noise is less than the scope can detect.

I decided a good way to measure the noise was to use the U6 itself.  The analog inputs on the U6 will eliminate a lot of noise as the resolution setting is increased, but at a setting of 1 there is no noise reduction and you should see any noise that is there and above the noise of the U6 itself.

So I jumpered AIN0 to GND and jumpered AIN1 to DAC0.  I looked at AIN0 to confirm I was seeing the same noise figures from the table above, then used AIN1 to try and measure the noise on DAC0.

First I used the +/-10V range and looked at voltages across the entire range (>0 and <VS).  I could not see extra noise on AIN1 compared to AIN0, and the noise on both was roughly 1.3mV, so this tells me that the noise of the 12-bit 0-5V DAC is <1 count.

To try and detect the actual noise level I set DAC0 to 0.08V so I could use the +/-100mV AIN range, and was able to see a difference.  The noise on AIN0 was about 100uV, but the noise on AIN1 was about 240uV, so that suggests that the noise on DAC0 is about 240uV.