What is the combined effect of the SCXI-1520 Gain and NI-Max Gain?

[Electromecha]

LabVIEW 6.1
SCXI-1000
SCXI-1520
SCXI-1314
PCI-6031E
Windows 2000

Greetings-
I'm trying to determine how the gain that is settable on a given channel of an SCXI-1520 module is combined with the gain that NI Max assigns to a Virtual Channel.

For example, if you set a gain of 100 on ch0 of an SCXI-1520, and you then create a Virtual Channel to read the mV output of the Ch0 sensor, (in this case a full bridge load cell), should the AI limits (range) that you specify in the Virtual Channel creation reflect the gain of 100 that the 1520 module is already applying to the signal? And after the 1520 module applies it's gain, does NI Max attempt to apply another gain if there is still capacity to amplify the signal within the AI range of the daq device?

Also, when measuring a full bridge sensor as an analog input voltage signal, and after performing a Null on the sensor, there is usually some residual offset. Does one then programmatically (in LabVIEW) tare, or zero this residual offset by subtracting its value from all subsequent measurements?

Thanks for your help.

[Sean_C.]

Hello Steve,

How are you applying gain if not through a virtual channel?
When you set the input signal ranges in MAX for your virtual channel, SCXI and DAQ board gains are applied. For example, if you create a voltage channel to read the mV output of a sensor, and set the range to +/- 10mV, the SCXI Module would apply a gain of 1000 to the signal and send the resulting +/-10V to the MIO DAQ device. The DAQ device will then attenuate this signal to +/-5V (the range of the ADC) by applying a gain of 0.5. Now, if you set the input signal range to +/-1mV, the SCXI Module will apply its maximum gain of 1000 and send the resulting +/-1V to the MIO device. The DAQ device will then amplify this signal to +/-5V by applying a gain of 5.

If you are still seeing a residual offset after null calibration, you should use the Auto Zero property. This property measures ground before an acquisition and then subtracts that value from every subsequent measurement. You can set this in a strain task, or with a Channel Property Node in LabVIEW.

Let me know if you have any further questions.

Sean C.

[Electromecha]

Sean-
Thanks for your reply and clear examples, that's very helpful. My confusion results from the fact that you can right click on the SCXI-1520 module in MAX, and by selecting Properties you can set the gain individually on each of the 8 channels.

I was manually setting the gain on the channels through the properties of the 1520 module and I was then creating a virtual channel in MAX which I knew applied a gain as well. So it was the combined effect that I was unsure about.

Sounds like the only time I should manually set the gain on the 1520 module should be when I'm not using virtual channels?

Thanks for you help.
Steve

[Sean_C.]

Hi Steve,

"Sounds like the only time I should manually set the gain on the 1520 module should be when I'm not using virtual channels?"
This is correct. If you set the gains manually, then call a virtual channel, the virtal channel will overwrite the manual gain settings.

Regards,
Sean C.