Difference between unbalanced-differential and single ended

[Ian_Ren]

What is the difference between unbalanced-differential (eg. NI 447X)and single ended?

I understand that the negative of NI 447X channels are connected to the ground via 50om resistors.

Thanks

Ian

[RossC]

Ian,

The difference between unbalanced-differential and single-ended is as follows:

A single-ended channel has only one input, and is referenced directly to a common line (usually ground). An unbalanced-differential input requires a positive and negative input, but the resistance between the positive signal to ground and the negative signal to ground is not matched (hence, unbalanced).

The unbalanced-differential input is used because it can accept most signal sources (floating or grounded) without issues.

If you need more information about this, please post here and we can discuss it further.

Sincerely,

Ross C
Applications Engineer
National Instruments

[Ian_Ren]

Ross

Thanks for your help.

If NI 447X is used to measure a grounded signal, will the unbalanced differetial suffer the groud loop issue as with the sigle-ended or the 50om resister would take care of it?

What is the advantage of unbalanced differential over balanced differential? We recently run into some difficulties when we try to measure several signals with negative at different levels. We in the end had to use a 10+ year old HP analyzer (ok, I guess we can used 447X if we build or buy additional signal condition modules), but it would be really nice if we can connect these signals directly to the 447X board.

Regards

Ian

[RossC]

Ian,

You are correct, the 50 ohm resistor is there to help prevent ground loops. The advantage of the unbalanced-differential input over balanced is that it can accept both grounded and floating signals. Unfortunately, it looks like you found a case where unbalanced-differential is not optimal (signals with different negative-end levels). Unfortuantely, the 50 ohm resistors are not especially easy to replace if I recall correctly (and would void the warranty), so external signal conditioning may be the best course of action.

Let me know if I you have further questions on this topic.

[Ian_Ren]

Thanks, Ross

I thought balanced differential can also be used to measure both grounded and floating signals.

It is probably beyond the scope of this discussion, but is there a possibility that NI will make both balanced differential and unbalanced differential available for NI 447X (through software control or hardware jumper, or...)?

Old HP analyzers and VXI DSA modules usually are "true" differential, by which I mean the impedance between the negatives of different channels are sufficiently high. It can be very useful for some DSA applications.


Regards

Ian



Ian

[RossC]

Ian,

What you referred to as true differential is what I meant by balanced differential. Just a discrepancy in terminology. Balanced/true differential can also be used with both signal types, but I believe the board is geared more towards floating signals, and unbalanced differential must provide an advantage in that area.

As for NI making a balanced differential product, that decision is beyond my control. However, posting here definitely helps, as it shows there is a desire for such a product out there. You can submit a product suggestion about this here.

Sincerely,

Ross C
Applications Engineer
National Instruments