Why wouldn't we short ai ground and ao ground together?

[Sean.C]

Hello,

I am setting up a system to make a number of measurements on a board where the signal being measured is going to be coming from the analog output on the daq card (PCI-6225).  Normally, I would try to keep the various daq grounds separated, but the profusion of grounds is causing complications (there are also two grounds on the DUT). 

Since AI ground and AO ground are actually connected inside the card itself, is it ok to have them grounded where they break out as well?  Alternately, would it be ok to just leave the ao ground unconnected and act as if ai ground were ao ground?

Thanks,

Sean

[John_P1]

Do you need to measure the AO voltage using an AI channel on the same DAQ card?  You can actually route this internally without having to make any external connections at all by using "Dev1/_ao0_vs_aognd" as the channel in your ai task. 

You would need to right-click on the channel constant or control, and select IO Name Filtering to choose the option to select Internal Channels:

       

The terminal configuration in this case must be "Differential", which is the default.  This will route AO_GND to the (-) terminal of the ADC, and AO0 to the (+) terminal.

Best Regards,

[Sean.C]

Hello John,

I have two quick questions about what you describe:

1) Can you measure ai lines with ao_gnd as the reference, or would you need to connect ao_gnd and ai_gnd externally and make an RSE measurement?

2) Why does it require the measurement be set to differential, since you are using an on-card reference for the measurement?

Thanks,

Sean

[John_P1]

Choosing _ao_vs_ao_gnd performs a differential measurement between AO and AO GND.  You have to select differential in the this case since by definition an RSE measurement is referenced to AI GND.

Technically, AI GND and AO GND are separate terminals.  In practice, AO GND and AI GND are tied together internally anyway on non-isolated boards so there shouldn't be much of a difference.

Best Regards,

[oilyfingers]

Hi - I have an additional question. What if I want to float one of the analog inputs (say AI4-) using the AO1+ output. Then I want to connect a measured voltage to the other side of AI4. This is so I can remove an offset voltage using the AO1 output. How would I do this with an internal connection like you mention? Thanks Dave

[Sarci1]

Hello oilyfingers,

Using _ao0_vs_aognd doesn't actually route the signal to a specific input channel.  Instead selecting the internal connections take a measurement similar to a differential signal coming into the board.  Because you are not necessarily able route the signal to a specific input channel, you cannot do what you are asking internally.

Regards,

Lynn