Noise floor on T7Pro | LabJack
 

Noise floor on T7Pro

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Clarkman
cdunson's picture
Noise floor on T7Pro

Hello!

We received our IR board yesterday, which means lots of LabJack use in the coming month.

I started by investigating noise, and found an interesting thing.  I know I am not the first down this road.

Attached are three spectra.  Sampling 1Ch at 50,000 S/s ethernet.  The input to AINO is shorted to ground, and in every case I computed from the histogram of values that I am getting 16 bits of resolution.

What the peak is at ~3kHz is a mystery, but the quality of power between the WallWart supplied with the LabJack, my Dell, and my Mac is mildly astonishing.  Dell quietest, Lj WallWart next, Mac bad.  The time series I inspected on my Tek scope by measuring Vs-Gnd and saw similar noise variances.  Is that measurement point correct?

I want to investigate it compared to a battery source.  Is one 3.7 Li-Ion OK?  Two in series?

Thanks!

Clark

LabJack Support
labjack support's picture
Do you have a particular

Do you have a particular reason to be concerned about power supply (VS) noise?  We take a lot of measures to make the analog system reject power supply noise, so most likely you would find that analog inputs perform the same with any of those supplies.  Try collecting data from AIN15 (internally shorted to GND) or from AIN0 with a jumper to GND, and see if the noise level is different with those different power supplies.  You should be able to replicate the noise figures we provide:

https://labjack.com/support/datasheets/t7/appendix-a-3-1

In all of your plots we see a small to large component at 3kHz.  Since this is present with all supplies, at least slightly, it suggests the T7 is doing something every 1/3000th of a second.  When that happens you get a change in current draw which causes the supply voltage to change.  Does the frequency of this component change if you stream at a different sampling rate or stop stream totally?

In your Mac plot only there is a strong and sharp 20kHz component.  Could be that is the switching frequency of the power supply on your Mac.

The power supply requirement for the T7 (Appendix A-5) is 4.75 to 5.25 volts.  Don't go over 5.25 volts.  The T7 will run at lower voltages, but when VS gets too low you risk bad readings from analog inputs.  Typical voltages for that with current T7 hardware rev 1.30 are <4.2 at room temperature and <4.7 at 70 deg C.

One way to use a battery is to power from a USB battery like is typically used to charge a cell phone, but these are going to have some sort of switching converter between the battery and the 5V output so you are not getting a pure battery supply:

https://labjack.com/support/datasheets/t7/vs