PCI-6123 input voltage range does not match test panel

[jchrisj]

I'm using a PCI-6123 to read voltage signal from 4 microphones. The raw signal is routed through a 10X FET preamp. When I look at the signal using the test panel I can see about +/- 8 volts. I created a global channel using -10 / +10 as the input range but no other scale applied. When I look at the same signal inside my program I'm seeing what appears to be about 1.3 mV. Any ideas about what I'm doing wrong?

thanks,

Chris

[jchrisj]

Here is some more detail on what I'm seeing:

As I mentioned, I've got 4 microphones routed through 4 10X FET amps. The input signal comes from an audio generator feeding a speaker through an equalizer and equalizer audio  power amplifier. Leaving all of the other gain controls alone I can control the amplitude of the 1 KHz signal with the output control on the synthesizer.

If I look at the signal coming into the 6123 using the test panel I can adjust it until the preamplifier clips. (I assume that it is the preamplifier since the clipping occurs at about +/- 8 V, and the power supply to the amps is +/- 9V)

I rewrote my program to replace the global channels for the microphones with a set of "Create Channel" vis with the limits set to +/- 10.0. My program reads the microphone inputs and adjusts the output control on the generator to get an input amplitude between 75% and 80% of full scale.  When I run the program after the first step it reads +/- 16 V  and no matter how low the output of the generator is set it stays at about +/- 16V.

Either there is a gain function on the 6123 that is coming into play somehow or the generator output isn't changing, although it sounds like it its. (I can hear the 1KHz output signal and it sounds like it is getting quieter every step.)

thanks,

Chris

[hoffiz]

Hi Chris,

I would be more than glad to help you. Is there any chance you can post a very simplify version of your code that emulates de behavior. It would be easier for me to see what is happening if I take a look at the code. 

[hoffiz]

Hi Chris,

After a couple more readings of your post I think I might have a clue of what is happening. These are the things I would like you to check: one, the test panel your terminal configuration for the 6123 will come by default as differential (the only one supported by S series) so make sure your global channel and/or your task is also set as differential.

Second, a neat trick to try would be creating the task in MAX, testing that everything works and the drag and drop the task name from MAX to LabVIEW. This will allow you to right click the purple drop down constant and select generate code, if you have one of the newer version of LabVIEW and the DAQmx driver.

Third, I’m kind of suspicious that you are reading +/- 16 volts that sounds the double of where your preamp was clipping. What accessory or connector block are you connecting the microphones to? It is more likely to have gain settings on connectors block messing your signal, than in the DAQ card. If your input range is set to -/+ 10 your 6123 gain setting should be 1. AE KB 3ELB62IT: What Are the Gain Settings On My S-series Board?.

I would also suggest taking a look at this knowledge base: AE KB 32MD61IO: How Do I Configure an S Series DAQ Board for the Full Input Range So It Does Not Cli...

Let me know if any of these KBs shoot some light on your problem.

[jchrisj]

Thanks for the advice, I'll look at both links you included.  I've attached the most recent version of the VI I use for the setup.

I may also be seeing a noise issue in the preamp -  I'll be taking a careful look at the preamp's power supply this weekend, too.

One other question - in my example code I've got a couple of ring variables to define  things like input modes, units, etc. Is there an easy way to make them available to other vis that I write besides cutting and pasting?

Thanks,

Chris

[hoffiz]

Hi Chris,

This is more of a Lab View question that should be in a LabVIEW discussion forum but I will give you a quick answer. You have a couple of ways of doing it: one, you can right click the input and select: “create constant” (BEST); two, you can find them on your functions palette, if you search for the name of the constant and three, copying pasting like you are doing it right now.