Power supply ground connected to AIGND

[Jamie Fraser]

I am using a 6052E DAQCard and am powering my laptop (Toshiba Satellite Pro 4360) from a 24V battery via a linear regulator. I have noticed that the DAQCard appears to dislike AIGND being tied to the 0V of the battery. The card still works but the measurements are slightly out.

With AIGND isolated from the battery 0V, AIGND is approx. 147mV above the battery 0V. Can anyone advise on this issue, e.g. DAQCard internal circuitry that will explain why AIGND and the PC power gnd should be isolated?

With thanks

Jamie Fraser

[Brent_R]

Okay, I'm going to play the semantics game here. Are you using the DAQPad-6052E (1394) or the DAQCard-6062E (PCMCIA)? I'm guessing the latter; let me know if this is not correct.

This sounds like a ground loop issue caused by the laptop's circuitry and/or the arrangement of your sensors. It's entirely possible that the "ground" that is the negative supply from the battery and the ground going to the PCMCIA card are not at the same voltage, due to a ground loop between the two.

Is there a reason that you are referencing your measurements to the the negative termanal of the DC input? Are your sensors referenced to it? If not, don't use it. If so, treat them as grounded sources and configure them as differential or NRSE as shown in Figure 4-5 in the use
r manual (Jan 2002)

http://digital.ni.com/manuals.nsf/websearch/552436964E347DAF86256BDE00670E9B?OpenDocument&node=132090_US

Regards,

Brent R.
Applications Engineer
National Instruments

[Jamie Fraser]

Brent,

Thanks for your response. It is a DAQCard, not a DAQPad. I was originally using a DAQPad but the first byte latency was killing me (plus 9 minutes to self-calibrate!!!) but that's another story.

I think your ground loop idea is probably right, I just thought the battery 0V would float to the AIGND level, but clearly not. I have broken the ground loop now, which was caused semi-accidentally via a link between the case of the linear regulator (at the battery 0V potential) and some custom signal conditioning electronics whose enclosure was at AIGND, but I would really like to know why the 0V of the battery would not just float to the AIGND level (I'm probably being dumb).

Cheers

Jamie Fraser

[Filipe A.]

Jamie;

You are probably seeing a ground loop in between the DAQ card and the LapTop GND.

That happens when both GNDs have different voltages.
The best way to avoid that to happen is to configure the DAQ Card to work as diffential input mode.

Hope this helps.
Filipe A.
Applications Engineer
National Instruments

[Brent_R]

Jamie,

If the the negative terminal of the DC input to the laptop were completely isolated from the DAQCard (isolated switching regulator in the laptop for example), then yes, the two terminals could be tied together. This entirely depends on the circuitry in the laptop, which sounds like the two terminals are not isolated. Probably this is a cost issue; maybe it's for safety. I'm really not very familiar with laptop power supply standards.

Try contacting the laptop manufacturer if this is still concerning. You may have to push a little to get to someone who can help, since they probaly don't get many questions about the internal circuitry.

Regards,

Brent R.