Reading accuracy with inaccurate USB voltage | LabJack
 

Reading accuracy with inaccurate USB voltage

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MikeKulls
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Reading accuracy with inaccurate USB voltage

I setup my labjack to take a few measurements. I noticed the USB voltage was a little low at 4.85 volts but didn't think much of it. I did some calibrations in software to get the correct readings out. I was assuming there were inaccuracies due to some components external to the labjack. Anyway, I changed to a new USB adapter and it was at 5.10 volts. All of my readings are now different to what I had before. Is this to be expected? Do I need to supply it with a more accurate input voltage? I was going to cut the USB cable and use a 7805 to drop 12V down to 5V.

LabJack Support
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No, all our devices are

No, all our devices are reference-based so the analog input readings vary little to none with supply voltage.  Where do your signals come from, as perhaps they changed?  Are you using high voltage analog inputs (AIN0-AIN3 on the U3-HV only) or low voltage (FIO/EIO)?

MikeKulls
MikeKulls's picture
Thanks for the reply. It is a

Thanks for the reply. It is a HV using AIN0,1 and 2. One input is reading 12V divided in half with a pair of resistors (so max about 7V). The other is measuring a 0 to 5V signal. The third input is using the cheaper labjack temp sensor which looks accurate. The labjack is quite old (10+ years I think) if that makes a difference. There could well be another reason for the change. Voltage is out by about 0.1 volt.

LabJack Support
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The range on those channels

The range on those channels is +/-10V, so 0.0 volts is an easy place to test.  Remove all your connections and instead just jumper AIN0 to GND.  Power up with your lower voltage and see what AIN0 reads, then power up with the higher voltage and see what AIN0 reads.  I would not expect a 100 mV shift.