pseudo differential vs NRSE

[Mike Parker]

Hi Chaps,

can anyone tell me the difference between NRSE and pseudo differential. ive got a system with ground referenced signals and im trying to reduce the noise in the system. i had designed the system for NRSE but we recently purcharsed a 6120 which only does pseudo differential. the system is a flash tester, duration of the flash is about 1ms and im sampling as fast as possible during the flash.

cheers

Mike

[devchander]

Before answering your main query,

Are you speaking about hardware: USB 6210??

If yes, this device supports RSE, NRSE and differential signal connection, but not Pseudo differential

Where did u find/read that "this device does only pseudo differential"??

please let us know, so that we can guide you better

Regards,

Dev

[GrahamG]

Hi Mike

A pseudo differential input similar to a true differential input has two lines between which the potential difference is taken. The difference is that the bottom line is connected via a capacitor to the ground. This filters all the high frequency AC signals and drops them to ground while leaving the DC current thus reducing noise.

In your situation I cannot see that this configuration will be a problem for you as it should not effect your sampling rate or noise.

For a more detailed explanation I have found this site http://www.maxim-ic.com/appnotes.cfm/an_pk/1108 is quite good but if you would like any further assistance or advise I would be happy to advise further.

Best regards

[Mike Parker]

Im using a PCI 6120 with a BNC-2090A connector box.

Previously i was using a little usb daq card, since switching to the new system it looks like theres more noise on the signal, and although its not a massive amount id like to find where its coming from. One thought was that there may be a ground loop between the daq card and the system ground, ideally id like the whole measurement system to be floating and use the external ground.

In the documentation on page 4-10 is figure 4-3 which shows the recommended connection for ground referenced signals and on that picture theres a load of components like a 50ohm resistor and 0.1muff capacitor, the way its drawn looks like its internal components on the card itself that are somehow selectable.. does anyone know how to do this?

many thanks

Mike

[GrahamG]

Hi Mike

If you were using a USB DAQ Device then these are by the nature of thier shielding very low on noise. PCI cards are within the computer and so subject to a noisier background.

Diagram 4-3 that you mentioned above is the internal configuration of the inputs as they are currently set up as Pseudo differential. This is hardwired within the card and is not an option that you can either deselect or select.

To confirm that this is indeed noise from within your computer I would recommend testing this by removing your other PCI cards and running with only the PCI 6120 and see if the noise reduces. it maybe that you have something in there which is incorrectly shielded.

I hope this is helpful for you

best regards