SCXI-1102 process-current resistors

[Broken_Arrow]

In order to measure current with the SCXI-1102, I understand you have to install some resistors. My questions are:

• NI offers a pack, which has four resistors. (?). Why four? Can't you convert as many channels as you want to current, or is there just four possible channels with the resistor pads?
• How does the installation of these resistors effect the factory pullups, which send the channel to rail to detect open wires?
• The "process-current pack" is just resistors, correct? So if you had 249 ohm resistors 0.1% you would not need NI's "pack", correct?

Thanks for any help!

[Broken_Arrow]

Nevermind. I had someone gain access to the card and we've got this hammered out. And the 0.1% resistors are also 5ppm which is hard to find, so NI's price is not too far off.

Message Edited by Broken Arrow on 06-25-2009 02:11 PM

[John_P1]

Hi Broken Arrow,

To answer your questions:

•  I'm not exactly sure why we sell resistors in packs of four, but the 1102 does have a pad for each channel (32 total pads).  I'd imagine some people only want current measurements on a few channels, so this gives a bit more flexibility.
•  The pullups are actually on the 1303 terminal block.  If you use bias resistors with the 1303 you would not be able to use the open circuit detection feature.  The 249 ohm resistor connects AI+ and AI-, so the circuit would always be "closed").
• You are correct that the Resistor Kit is four high precision resistors.  You may provide your own, but I would recommend to match the specs of the resistors we use (249 Ω, 0.1%, 1/4 W).

We also have the 1308 for current measurements with the 1102 if you don't want to open the device up and solder resistors directly to it.  Keep in mind that you must remove the soldered resistors for calibrating the 1102.  I hope this answers your questions fully, but let me know if anything else comes up.  Thanks for posting!

-John

[Broken_Arrow]

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John P wrote:

You are correct that the Resistor Kit is four high precision resistors.  You may provide your own, but I would recommend to match the specs of the resistors we use (249 Ω, 0.1%, 1/4 W).

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Right, but they are also 5ppm, which is a very low temperature coefficient, and harder to find than the typical 50 to 100ppm. I ended up ordering them from NI because the usual sources don't stock 5ppm.

Thanks for your response. My questions were based on sight-unseen system, I just wanted to know what to expect when I get there.

[John_P1]

Hey Broken Arrow,

5 ppm is correct, sorry to leave that spec out of my last post. The best of luck with your customer visit--we're always here if any other questions come up involving our products.  Have a great day!

-John