Setting gain when using an E Series DAQ device connected to SCXI Filter

[SE_Hoard]

I am using a 16 channel E Series DAQ device to record data from an SCXI filter. This is how the system is set up to work:

The filter will spread an incoming signal over its +/-5V output range. I can set the input range for the filter from +/-5.0V, which corresponds to a gain of 1, down to +/-50mV, which corresponds to a gain of 100. The +/-5V output of the filter is the input to the DAQ card. The DAQ card will then spead the incoming +/-5.0V signal over its +/-10V range, which corresponds to a gain of 2. That means that my overall gain has a range of 2 to 200.

This is my problem:

The range of the incoming signals may be much smaller than +/-50mV, let's say +/-5mV. So even if I set the filter gain to its maximum, 100, the output will only be a +/-0.5V signal. Since this setup is designed to work as a "system" the gain of the DAQ card is always forced to 2. I would like to adjust the gain of the DAQ card to a gain of 20, which corresponds to an input range of +/-0.5V. This is frustrating because when these devices are used together as a "system" their capabilities become limited. If I were to use the DAQ card to read signals output by a different filter I would be able to adjust the gain of the DAQ card and get a higher overall gain out of the system.

I have tried to use channel property nodes in order to force the DAQ to a different input range (gain) but I have not had any success. LabVIEW will not allow me to configure the DAQ card in the same program that configures the filter.

I would appreciate any suggestions about how to accomplish setting these gains independently, if it is even possible.

[Salvador_Santolucito]

Hi,

I'd like to know a little more about your system...

1) What version of LabVIEW are you running?
2) What SCXI hardware do you have?
3) What DAQ board are you using?
4) Are you using DAQmx or Traditional DAQ?

In general, when using SCXI with DAQmx, all you need to do is create a virtual channel on your SCXI hardware and tell the channel what your signal range will be. DAQmx will automatically configure the SCXI and DAQ card gains to give you the greatest resolution.

-Sal

[SE_Hoard]

Thanks for your advice Sal. Here is a little more information about my system:

1) What version of LabVIEW are you running?
LabVIEW 7.1
2) What SCXI hardware do you have?
SCXI 1000 chassis with SCXI 1142 filter
3) What DAQ board are you using?
PCI-MIO-16E-10
4) Are you using DAQmx or Traditional DAQ?
I use DAQmx, but I have also tried using Traditional DAQ.

Based on your reply are you saying that I can set my input range to less than what the specs for the SCXI 1142 say? So if I were to set the input range to something less than the +/-50mV range (say +/-5mV) it would give me greater resolution than if I were to set the input range to +/-50mV, the smallest range the 1142 is capable of? (The filter would then have a gain of 100 and the DAQ card would have a gain of 20?)

My problem has been that DAQmx does automatically configure both devices. It seems to me that if both devices are capable of applying gains up to 100 that I should be able to get more than a gain of 200 out of the system when they are connected to each other.

[Sean_C.]

Hello,

You are correct. Though the smallest range of the SCXI-1142 is +/- 50mV, you can still apply a gain to that signal at the MIO board.

"So if I were to set the input range to something less than the +/-50mV range (say +/-5mV) it would give me greater resolution than if I were to set the input range to +/-50mV, the smallest range the 1142 is capable of? (The filter would then have a gain of 100 and the DAQ card would have a gain of 20?)"
This is exactly what happens. Try running the attached VI to verify this. If you set the input signal range to +/-50mV, you will see that the SCXI Gain is 100 and the MIO Range is +/-5V (which corresponds to an MIO Gain of 1). Now, if you set the input signal range to +/-5mV, you will see an SCXI Gain of 100 and an MIO Range of +/-0.5V (an MIO Gain of 10).

Let me know if you have any further questions.

Regards,
Sean C.

[Sean_C.]

Here is the VI...

[RVallieu]

I have to do something VERY similar with an S-Series Daq card (6143) in parallel mode with an SCXI-1142 filter card.  Would this same sort of configuration in LabVIEW be possible with this proposed hardware?  Would I be able to set the gain individually for each channel?

Thank you,

Ryan

[Sean_C.]

Hello Ryan,

Yes this configuration is possible with your hardware.  You set the gains of each individual channel by specifying the ranges with the DAQmx Create Virtual Channel.vi or in MAX using the DAQ Assistant to create global channels and tasks.

Let me know if you have any further questions.

Regards,

Sean C.