AI basics - NI 9205 settings.

[rich24]

I am designing my first datalogging application on a cRIO with a NI 9205 and a sensor.  I have a few concerns regarding settings of the 9205 such as resolution and gain.

First of all, I have an anlog sensor that outputs 1.67mV steps from 0 to 3V.  I know that the 9205 has a 16 bit ADC.  Is it correct to think that if I set the nominal input range to +/- 5V (10 volt span) I will have a resolution of 10,000mV/(2^16-1) = 0.076mV?? If so this will be sufficient for my application.

But if the above equation is not the case, I assume I will need to add some gain to the input signal.  I have read that all channels of the 9205 share a programmable gain instrumentation amplifier and are multiplexed to the ADC.  However, I am unaware of how to programmatically set the gain.  Can I set individual gains for each channel??  Any help would be great.

[AndyClegg]

As you've worked out - your 16-bit ADC resolution is well withing the quantisation of the analogue signal.

[rich24]

Thank you for confirming my calculation.  I think I am starting to understand.

Is it correct to say that I am effectively changing the gain of the intrument amplifier when I set the nominal input voltage range??  Or is there a way to change this in MAX or programatically??

[AndyClegg]

Yes, when you change the range you effectively change the gain - but when talking about "gain" it is useful to think about between which signals it applies to. If you are working purely in engineering units (i.e. volts) throughout, then changing the range won't change the gain, but if you are expressing things in percentage, then it will. I'm not sure if that is clear - but have a think about it.

Regarding how it is configured ... somebody else more familiar with that module will probably have the answer to hand. I typically only configure these sort of settings from within the project item, but I am sure there are other ways to do it.

[rich24]

I see what you are saying.  The use of the work gain is not really appropriate.  The reason I am asking is because the manual mentions a instrument amplifier inside.  I was unsure if there is a way to change the gain of this instrument amplifier explicitly or if this is done on the back end by adjusting the nominal input voltage and thus increasing resolution (16-bits over a different range).  Seems to me that if the resolution is increasing or decreeing, there is something happening here.

[AndyClegg]

The change in range is effectively done by changing the amplifier gain - since it is scaling from volts (whether +/-10V or +/-200mV) to a fixed unitless range (0 to 65535). But since we normally use the volts within a VI, changing the range does not change the gain seen.

BTW 10,000mV/(2^16-1) = 0.15mV, but still within your quantisaton.