Diff with 10V<AI+/AI- <20V

[pgraebel]

Hallo,

i`m using PXI-6255 with 80SE / 40Diff for my measurementschane. One of it was the voltagedrop of a  +14V Cable:

I was sure that i can use my Channels in Diff.-Mode only with Inputvoltage +/-10V (http://www.ni.com/pdf/manuals/371291h.pdf  Page 1) ! That means, i`m using bleeder to divide my voltage!

I was searching in the forum and i read this answer in a thread:

Each input is limited to +/-10V from ground in DIFF mode, so:

(-10V) - (10V) = -20V

(10V) - (-10V) = +20V

Thus +/- 20V range. Unless I am totally missing the boat here.

I looked up the manual, it tends to agree (portions copied/pasted here):

http://www.ni.com/pdf/manuals/371303g.pdf

The differential input mode can measure ±20 V signals in the ±20 V range.

However, the maximum voltage on any one pin is ±10 V with respect to

GND. For example, if AI 1 is +10 V and AI 5 is –10 V, then the

measurement returned from the device is +20 V.

Input range

Single-ended ................................... ±10 V

Differential...................................... ±20 V, ±10 V, ±5 V, ±4 V,

±2.5 V, ±2 V, ±1.25 V, ±1 V

Working voltage..................................... ±10 V

Now i`m confused, can i use my AI+ /AI- directly without a bleeder (e.g. 14V AI+, 13,8 AI-)??

I tried it once and it works without problem.

Thanks in advantage

G. Jilwan

Message Edited by pgraebel on 02-22-2010 07:06 AM

[jyang72211]

If I understand your question correctly, my answer would be no, you can't.  Let me make sure I understand you.  You are thinking about putting 14V on AI+ and 13.8V on AI-, and from that configuration you intend to get 0.2V in differential mode.  Is that correct?  This setting doesn't exceed the differential limit, but exceed the single-ended setting.  The spec clearly states that each pin shall not have voltage of greater than +-10V.  The voltage 14V and 13.8V are greater than +-10V, so you can't do that.

Yik

[pgraebel]

Correct!

Since i was sure that i cannot use an input range higher than 10V, I was confused reading this message in the forum with a USER GUIDE AND SPECIFICATIONS USB-6008/6009. I didnt see the difference between the 2 devices (6255 and 6008)But it`s ok i`ll not stop using my bleeder!

thanks

regards

[Henrik_Volkers]

Depending on the output impedance and signal bandwidth you can use a simple resistor voltage divider to expand the input range.

[pgraebel]

Yap that what i meant with bleeder = Resistor voltage divider!

i`ll stay on this point!

Thanks a lot!

Regards

[Henrik_Volkers]

If you want more see Fig. 30 /P12  in Linears AN98