GPIB-120B causes noises

[arthurliu6]

Hi, everyone.

We recently attempted to install GPIB-120B between the equipments and the computer.

But GPIB-120B seems to cause severe noises on our measurement.

To be concise, we measured an amplified DC voltage of a field effect transistor.

At the amplification stage, voltage preamplifier and lock-in amplifier were used.

With GPIB-120B installed, the voltage oscillations are much larger.

The noises are more severe with a longer distance between the computer and the multimeter.

 For the long distance, the length of GPIB cable is about 5m, and about 2m for the shorter one.

The test results are shown as the follwoing images. 

I can't think of any reason that causes this problem, can anybody help me?

Thanks alot

 

 

[Albert.Geven]

This can only be due to a ground loop.

GPIB does not influence the measurement directly, butt current can flow via the grounding.

[AnalogKid2DigitalMan]

That's odd because the 120B is a GPIB Isolator/Expander providing electrical isolation between equipment.

Are there any other GPIB instruments connected?

Link to manual:

http://www.ni.com/pdf/manuals/371851a.pdf

-AK2DM

[Albert.Geven]

The it is clear also: the grounding is interrupted!

[Steven_T]

Arthur,

I am very interested in understanding how you are using the GPIB-120B with your scope.

1.  What scope are you using?  Hopefully I can find a similar instrument here.

2.  What are the GPIB cable lengths between the computer and GPIB-120B and from the 120B to the instrument?

3.  Are the cables sheilded, do you have the part numbers?

4.  How does each node in this system get power?  Is everything grounded through a wall plug?

I would also like to know if other instruments have this problem.  Perhaps we can reproduce the problem here.

Here is how I am imagining the connections on the GPIB network.

<Computer> --------- <GPIB-120B> ----------- <Instrument>

Please let me know if there are any additional connections or instruments on the GPIB. 

Thanks for letting us know!

Albert,

I'm afraid that I'm not following your argument.  Would you expect this behavior with the isolator connected?

Thanks,

Steven T.

[Albert.Geven]

Hi Steven.

I've seen a lot of measurement problems when grounding was either so good that a current path existed thatwas not intended,

and/or grounding was taking another path because one of the paths was isolated, but still the grounding was not correct.

Having an antenna because the measurement circuit itself was not grounded is as bad as grounding via the measurement signal can be.

In either case your measurement is not what you want to measure.

[arthurliu6]

Hi. Steven.

I further checked the setup in detail and drew a schematic configuration for those equipments that are controlled via GPIB. The data shown previously was recorded by Keithley 2000 multimeter.

1.Power

The computer and those equipments are powered by two separate UPS(Uninterrupted Power Supply). The two UPS are then connected to wall plugs. I think everything is grounded through 3-prong plugs.

2.Cables

As for the GPIB cables, they are shielded. The part number is E119932.

3.Length

If I shorten the length between GPIB-120B and the instruments to 2m, the noises become severe as mentioned previously.

I also tried to power GPIB-120B through the two UPS and directly through the wall plug, but no satisfactory result was reached. I’m not sure whether Keithley 182 will have the same problem. It may be checked later on.

 

[arthurliu6]

Hi, Albert:

Installation of GIPB isolator is our intent to improve measurement accuracies.
I heard of laboratories use isolators for this purpose. We thought that GPIB isolator can suppress unintentional ground loop since it isolates electrical contacts of the instruments from the computer. Do we have an incorrect perception?

[Albert.Geven]

Your perception is OK. But you probably have now a grounding problem or better said, a measurement problem.

Was the keithley used in isolation mode? (no earthed connection)

And how is the dc amplifier connected?

When looking at your measurement I see that the  noise is about the same but the dc level has shifted to a higher value.

[Steven_T]

Arthur and Albert,

Albert's point of view is something that I have not considered before.  Since the grounds are isolated at the GPIB-120B, you are arguing that its like having an antenna connected to the ground of the instrument.  If Arthur were to take measurements using the front panel with a GPIB cable connected to nothing (except the instrument), would you expect to see the same results?

I believe that this would show that the problem has to do with grounding and not necessarily the GPIB-120B.

Let me know what you think.

I will see if we are able to reproduce this issue here.

Thanks,

Steven T.

Message Edited by Steven T on 03-27-2009 04:34 PM