High and low limits for p_pos and p_neg?

[RLD]

How should AI Config's high and low input limits be set when measuring p_pos and p_neg on a SCXI-1520?

For example if "channels" array is assigned with index 0 = "ob0!sc1!md1!3", index 1 = "ob0!sc1!md1!p_pos3" and index 2 = "ob0!sc1!md1!p_neg3". And input limits index 0 is assigned +0.1/-0.1, the p_pos and p_neg readings are limited to +/-0.1 volt. If elements of input limit array are assigned to +/-0.1, +/-5, +/-5 then the p_pos & p_neg values are correct but the values for channel 3 are wrong.

[RIO-Todd]

Greetings,

Try rearranging the way you are scanning through the channels and read the excitation voltages first. For instance, setup the channel array with p_pos3, then p_neg3, then channel 3. Then setup the voltage input limit array as +/-5 V, then +/-5 V, and finally +/-0.1 V.

This should work for you. Let me know if you are still having problems.

Regards,

Todd D.
NI Applications Engineer

[RLD]

This arrangment seems to work! Can you explain why?

If I change over to a PXI-4220 will it behave the same?

I went through the 1520 manual pretty carefully, but didn't see any indication that measurement order was critical. Did I just miss it?

Rich

[RIO-Todd]

Hello Rich,

The actual excitation voltages you are reading don't go through the amplifier on the SCXI-1520. Therefore, when the excitation signals are included in the scan list, that can confuse NI-DAQ. If you look at the reading from the channel of the SCXI-1520, it will be off, but it will be off by a factor. What's happening is that when you read from the channel and then read from the excitation signals, no gain is being applied to the channel (even if you insert a range of +/- .1 V) but NI-DAQ thinks a gain is applied. So, NI-DAQ divides down the voltage even though it doesn't need to.

For instance, if you read from the channel of the SCXI-1520 first and include a limit of +/- 0.1 V, NI-DAQ thinks a gain of 100 will be used. The 1520 appl
ies no gain so your reading in LabVIEW will be 100 time less than it should be. This just happens when you are trying to apply a gain to a channel prior to reading from the excitation signals. I haven't found a source that says this, but my testing has pointed this out.

With regards to the PXI-4220, it should act the same way. If you take a look at the block diagram of the SCXI-1520 and the PXI-4220, both have the excitation signals (P+, P-) going directly to the multiplexer without passing through the amplifier. So the setup of both devices look the same. You can find the block diagram of the devices in the user manual's which I've linked below.

SCXI-1520
http://digital.ni.com/manuals.nsf/webAdvsearch/9CB6AB6E04D6510586256DF3007AD256?OpenDocument&vid=niwc&node=132100_US

NI PXI-4220 User Manual
http://digital.ni.com/manuals.nsf/webAdvsearch/F93CCA9A0B4BA19B86256D600066CD03?OpenDocument&vid=niwc&node=132100_US

Regards,

Todd D.
NI Applications Engineer