The gain of the signal does not change if I vary the input level

[monicarojas]

I am using a DAQCard AI-16E-4 to meausre the signal from a pair of torque transducers. As the signal has a very low level, I need to amplify it and to do so, I am using the software programmable gain of the card. The problem is that if I change the input limit as specified on the manual, the gain does not change. I have checked it with signals>1V too but the problem is the same.
For the aquisition I am using the AI1-Scan VI and I am measuring the DC level of the signal, since the input is a DC signal. Why it is not working?
Thank you

[Jorge M.]

Hello monicarojas,

I'm pretty sure that the gain is being applied when you use the input limits. It may be that you think the VI will return you the amplified value, which is not what the driver does. Instead, the driver will amplify, digitalize, and then the software will divide, -before giving you the measurement-, the digitalized value, so YOU WILL ALWAYS GET THE REAL VOLTAGE (NOT THE AMPLIFIED VALUE).

Example:
daqcard with gains: 0.5, 1, 2, 5, 10, 20, 50, 100
input limits = +/- 1 Volt
In this case, the driver will choose the maximum best gain to be applied, in this case, gain will be 10 (considering the input limits the user set to +/- 1, a bigger gain could potentially saturate the ADC's input, which is +/- 10)
if you input 0.33 volts to your board, it will be amplified x10 times, so the ADC will get 3.3 Volts at its input, will convert, and give back the digitalized value. Then, the driver, as it knows that the gain applied is 10, it will divide 3.3 before giving it to you, so your vi will return 0.33 Volts.

I hope this clarifies your question,

Regards,

Jorge M.