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NI 9237 - how does the gain amplifier work?


I have a few questions regarding the internal function of the NI 9237 strain gauge input module for CompactDAQ.


I understand that this module contains a programmable gain amplifier, which adapts the input signal with the purpose of using as big span as possible of the internal A/D converter.


I cannot find detailed information about this though. I guess that the amplification is calculated from the signal input range values set in NI MAX. Is that correct?


Is the amplification of the gain amplifier set in just a few steps (1, 2, 5 etc)? How do I know its setting?


And most important; how do I know what a full scale analog input to the module will actually read on the A/D converter?


The reason I'm interested in this is that I am trying to figure out the total accuracy of my application. I understand that the offset error is relative to the range, which is 25mV/V. I guess that is the range of the A/D converter. This accuracy can only be achieved if the gain amplifier can be set so that full scale output from my load cell gives 25 mV/V. If the gain can only be programmed in steps (1, 2, 5) etc, this is not possible. I then need to know what gain is actually set to be able to calculate the real offset error of the entire measurement chain.


There is a possibility I am getting this fundamentally wrong, but I need to figure this out.




Linus Nilsson
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