Using SCXI-1125 as Standalone Amplifier

[Lancer]

Is there a way to use the SCXI-1125 module as a standalone amplifier? I want to be able input in a small signal (approximately 1-mV) into the SCXI-1125, use Measurement/Automation Explorer to control the SCXI-1125 and access the amplified output from the SCXI-1125 in order to display the signal on an oscilloscope.

Thanks for the help

Lancer

[Logan_K]

Technically yes; but there are some caveats. You would need to have a multifunction DAQ board connected directly to the SCXI-1125 so that you can configure it in parallel mode and control the programmatic gain settings per channel. Then, you could use the 50-pin connector on the SCXI-1349 adapter to directly access the channels of the SCXI-1125 and measure those amplified outputs with the oscilliscope. Now for the big caveat - the signals you will be measuring are raw, and will most likely have a large amount of error introduced from the amplifiers on the SCXI-1125. The error introduced by the SCXI-1125 is automatically corrected/compensated for by the NI-DAQ driver when it is acquired by the connected DAQ board because calibration data for each channel is stored on the SCXI-1125 and read back in software.

I am curious why you would need to use an oscilliscope when you will already have a DAQ board connected to the SCXI-1125? You can simply use your computer as the oscilliscope to capture the measurements - which would be less expensive, and would make saving/processing the acquired data quicker and easier as well. Your measurements will also be several times more accurate since the calibration data from the SCXI-1125 will automatically be applied to the measurements.

Logan

[Inpriser]

Hello Logan,

I just had a look at your post since I still have a problem concerning calibration of SCXI 1125, and I am a bit worried about what I understood. You explained that the output of SCXI 1125 at rear connector are raw values, and calibration constants stored in the SCXI card are use for software calibration, when measurement is actually done by the DAQ card.

I actually expected that the SCXI 1125 was able to correct internally the offset gain of the signal, by hardware...

Concerning the question of Lancer, I guess he wants to have a programmable amplifier using SCXI 1125. I used it the same way in one of my application, it works fine but I still have this calibration issue. I also encounter some stability problems (ripples on measurement) : the 1125 card is stated to be at least 0.5% accurate, but does that also mean that you might have 0.5% ripples ? Compared to the capability of a DAQ board, using SCXI conditioning degrades really too much the signal... Is that a problem associated to 1125 amplifiers, as you indicate they introduce errors ?

[Logan_K]

Accounting for gain and offset errors programmatically in hardware is very expensive, and typically not nearly as accurate as accounting for the errors in software, which is why we can not only increase the module's accuracy but also reduce cost by accounting for the module's errors in software.

Another word for "ripple" is noise, and the SCXI-1125 is will introduce the following amount of noise from it's onboard amplifiers, as specified in the module's user manual (http://www.ni.com/pdf/manuals/322425a.pdf):

<b>System noise</b>
4 Hz .................................................100 μVrms/gain RTI

10 kHz
Gain (1-100) .............................500 μVrms/gain RTI
Gain (200) ................................3.1 μVrms RTI
Gain (250) ................................3.2 μVrms RTI
Gain (500) ................................2.2 μVrms RTI
Gain (1000) ..............................2.0 μVrms RTI
Gain (2000) ..............................1.9 μVrms RTI

So, typically the noise introduced is going to be much smaller than the absolute accuracy of the module. The user manual also describes how much drift due to temperature that you can expect from the amplifiers, which may also help.

Logan