Grounding & Internal Power Supply OLD story I know .... CUSTOM breakout

[Prefrontal]

Dear Forum,

Grounding is an old and long story I know but I need a little help with my problem.

I'd like to use the +5V power out of my PCI-6040e to power a small analog front end. The manuals says the +5V is referenced to DGND. OK ...

I would use two non isolated DC/DC converters and subsequent linear regulators to end up with a +/-5V power supply still with 0V at DGND. Now all my filter stages would have their reference at DGND ....

1) Is this approach wise? Has anybody experience with the internal power supply and its stability/usefullness to power analog measuremnt equipment?

I already played a bit with these cards and realized that there is a strong feed through at the DGND whenever the PC has to calculate something - with a frequency at 1 kHz e.g. having a hidden task bar , go with you pointer over it and i can measure 'voltage spikes'!

2) Where would I ground the Chassis of the Analog front end? also to DGND? that wouldn't be wise at all, or would it?

3) Is any output of a DAQ card usefull to actively GROUND something? Should I use a water pipe (or similar realy earth) connection instead?

The alternative approach for me would be to use the AI SENSE as a reference for my analog frontend, use a battery split volate supply, tie the 0V to AI SENSE, tie the AI SENSE via some resistance to AI GND and the Chassis to the water pipe.

Sorry for this evetually stupid qiestion,

yours,

Prefrontal

[johnsold]

Prefrontal,

Why not use isolated DC/DC converters? Low power devices which cost only a few dollars (US$) are readily available.  Then tie the 0 V common of their outputs to AI GND.

As you are discovering, Digital Ground on many DAQ devices is a rather noisy place which you do not want connected to your analog circuits.

How to ground your system cannot be defined without knowing what you are trying to measure and what else is connected to the system. In general you want to try to have a "star" grounding system with only one grounding conductor connecting each component to the common reference potential point (which may or may not be connected to earth).  When you can acheive this, you avoid ground currents flowing in a shared conductor which causes volatge drops due to the impedance of the conductor.  Unfortunately, many devices (including computers) are not designed with best measurement practices in mind.

Lynn

[Prefrontal]

Dear Lynn,

Thank you for reply. I actually have considered an isolated DC/DC converter in an already existing but didn't think about it to tie it to AIGND but left it isolated and floating but this generated more problems tha it would have solved. But this is really a greatz idea THANK YOU!

I've attached a scheme of what I want do do so you could give me some advice.

I have a small unity gain preamplifer wich is connected to the object to measure which is a muscle in this case. The signal source gives me 100 µV referenced to the reference input. The subject is grounded to keep it's potential somewhere close to the voltage supply of the differential amp. After the differential amp there are certain filter stages which add a Gain of 1000x and passes a band ov 0.8-5kHz. So - no critical, crazy high frequency strories.

My real problems are to interface with the DAQcard and where to ground the chassis.

I hope this will somehow elucidate my project,

Yours,

prefrontal

[johnsold]

Prefrontal,

You should be able to make this system work.

I suggest connecting the isolated power supply common to AI GND. From the appearance of your diagram it seems that the chassis you are concerned about is the one containing the analog front end circuits. Probably the best connection for that is to AI GND also.  The connection should be made via a separate conductor - not the same wire connecting the power supply or signal common.  Make the connections at the 6040 connector.

It the muscle you are measuring is attached to an animal (including human animals) which could possibly touch an grounded object (or energized object) while connected to your system, you need to be completely isolated to avoid the possiblity of electrocution.  Simply isolating the power supply to the analog front end does nothing to provide safety protection.

Lynn