Spurious 2.1V offset on analogue input

[andrewol]

Hi,

I wonder if anyone would know why this problem occurred in my labview application.

I am using a PCI6229 and CB-68LP breakout board to measure the voltage drop across some diodes. This morning labview added a 2.1V offset to the voltage I was measuring on two channels. Measuring the voltage across the diodes themselves gave 0.43V, measuring across the pins on the breakout board gave 0.43V but the value returned after sampling the channel in my labview application was 2.53V.

Rebooting my computer and labview etc did not solve the problem.

However, after switching off the power supply to the diodes, then switching it back on again, the problem was solved and labview returned the correct value of 0.43V (the voltage across the pins on the breakout board).

I am completely puzzled about why this problem would occur. If anyone can tell me why so I can avoid it again in future I would be grateful.

Thanks,

Andy

[LeeM]

Hello Andy,

    This does seem strange. I was wondering if you had looked at the test panels in Measurement and Automation Explorer to ensure that the device itself (6229) was measuring the correct value? If this had appeared to be the correct value then it would look like there was something in your code that was offsetting the value. As you describe the resolve to the situation to be when you turn off the power supply across the diode then i think that it is unlikely that the LabVIEW coding is placing an offset in there and it would be more down to the fact that there was some intereference on the signal somewhere.

What type of measurment configuration do you have? I.e. differential or referenced or non-referenced?

Has this occurred again since you originally posted this forum?

Kind Regards,

LeeM
AE NIUK

[johnsold]

Andy,

Do you have all the unused analog inputs tied to AI Gnd or some other low impedance source near the common mode voltage of the signals? If not, the multiplexers in some boards can "charge up" to a voltage defined by leakage currents and stray impedances. This often causes the appearance of drifting readings. I cannot say about your particular device, but check the manual for connection information.

Lynn

[andrewol]

LeeM - I don't think that it was a problem with my code, as the same code was working fine for a week before the problem manifested itself and worked okay again after cycling the power to the diodes.Thanks for your suggestion though.

johnsold - Thinking about it, yes I did have 3 inputs connected which would effectively have been floating when this problem occurred, so this may well have been the cause of the problem. I'll have a read of the manual for any information about this.

Thanks for both of your responses.