PCI 6250 Differential, NRSE

[wang]

Hi,

I have a question on how to measure PCI-6250 (CB-68LP) device on Differential, Non-Reference Single Ended mode.

For differential, there is two channels to measure. Let say I choose AI0 and AI8. Do I have put anything on AIGND pin or bias

resistor to measure?

For NRSE, there is ACH, AISENSE and AIGND. Do I need to put sth on AIGND to measure this mode?

[WM_John_Weeks]

That's actually a big topic that depends on the details of your signal source and grounding. Try this:

http://zone.ni.com/devzone/conceptd.nsf/2d17d611efb58b22862567a9006ffe76/01f147e156a1be15862568650057df15?OpenDocument

[Ayman K]

Wang,

If you are doing Differential measurements, then you would not need to wire anything to AIGND or connect any bias resistors unless the source signal is floating (not reference to ground). As far as NRSE, you need not to wire anything to AIGND when you select NRSE. NRSE references a channel to AISENSE. YOu would need to use bias resistors if your signal source is floating. Otherwise you don't need bias resistors.

Ayman K

[wang]

Dear Ayman K.

Thank you for your help. Actually it is a very good way to remeber.

Here is what you said:

If you are doing Differential measurements, then you would not need to wire anything to AIGND or connect any bias resistors unless the source signal is floating (not reference to ground). As far as NRSE, you need not to wire anything to AIGND when you select NRSE. NRSE references a channel to AISENSE. YOu would need to use bias resistors if your signal source is floating. Otherwise you don't need bias resistors.

Anyway,  I am trying to test if my test fixture output sources is a floating signal source or grounded signal source.

Can I determine by testing the differential mode and signle-ended-nonreferenced (NRSE) mode for grounded siganl

sources. If I can not get a good signal on ACH(+) and ACH(-) for differential mode in gounded signal source in differential mode and

ACH and AISENSE for NRSE mode in ground signal source. I can say my output souce of my test fixture is foating signal source..

Many thanks if you can reply promply.

[Doug M]

Wang,
Assume that if your source has anything that plugs into an earth ground line or a wall outlet, it is a grounded source.  If it runs off batter power or is a floating transducer, such as a thermocouple, it is probably floating.  The link that John mentioned is a great resource.  If your source is floating, the easiest connection would be to use an RSE configuration.  If your source is grounded, the best configuration is differential.  This way you don't need to use bias resistors.  You never want to use RSE with a grounded source, however, as this can cause ground loops.  It would probably be difficult just by looking at your readings to determine whether your source is grounded or floating.  The main consideration if your source is floating is that the common-mode voltage can saturate the amplifier in a differential setting, in which case you would definitely need to use bias resistors, which is why for a floating source RSE is the easiest method.

[birdhl]

Hi,

That means my way to determine the signal source wether it is floating source or grounded source is correct. I need a direct answer.

Thanks!

[Doug M]

Yes, if you connect your signal in differential or NRSE mode and the signal seems erratic or rails, your signal is floating and you need bias resistors.  However, it is not guaranteed that you will see erratic behavior or a railed voltage just because your source is floating.  In many cases it will looks totally normal, which is why this isn't a perfect method for determining whether your source is floating or grounded.  You will always be safe connecting your signal in differential mode whether it is floating or grounded, though.

Message Edited by Doug M on 09-07-2005 11:57 AM

[wang]

Dear Dough:

Thank you very much!

Why is not always be safe in using RSE method rather than Differential method? RSE is the easiest. What do you mean by safe?

[Doug M]

Wang,
The reason RSE isn't the safest connection is that if your source is grounded rather than floating, you can create a ground loop between your signal source and the data acquisition device, which in the best case will give you noisy readings, but in the worse case can damage your equipment.  Please see the following tutorial for a more in depth explanation.

[Doug M]

The link I gave in my last post was incorrect.  This is the correct link.