HOW TO PROVIDE EXCITE VOLTAGE USING SCC-68

[suni]

Hi

I am using scc-68 connector block with NI 6225M series daq to acquire signal from string potentiameter. For the pot i need provide excitation voltage. I was thinking to use +5v available in scc-68 block to use in this purpose. If i do this how can i connect my sensor signal to scc-68 in differential mode. Because i connected +5vdc and AI GND to signal then i connected signal+ to AI(i) channel. Can i connect AI GND to AI (i+8)?

[John_P1]

Hi suni,

Thanks for posting.  While you could connect AI GND to AI- and measure differentially, why not just use RSE which would internally route AI GND to the negative input of the A/D converter?  For more information, check out our Devzone article: Field Wiring and Noise Considerations.

-John 

[suni]

Hi Jhon,

Thanks for the reply. In my scc-86 i am using scc-sg04 and TC02 moduales. In the manual it says RSE in not recommended. Should i connect AIGND to AI- and measure signal differentialy? 

[John_P1]

Hi suni,

This makes sense now why you would want to measure your potentiometer differentially.  For the strain gauge and thermocouple measurements you should use differential as specified in the manual.  To incorporate the potentiometer into this system, you have two options:

1. You may connect AIGND to AI- externally and measure differentially
2. You may use RSE on the specific channel you are using for the potentiometer (see the following image):

Connecting AIGND to AI- is the same result as performing a RSE measurement, so either of these methods should work.

-John 

Message Edited by John P on 10-28-2008 01:12 PM

[suni]

HI

Thanks for your reply. Is it ok to get power (+5VDC) avilable in the SCC-68 connector block to excite potentiameter? I am planning to use this +5vdc to excite two potentiameters? In the daq specifications manual it said max current i can draw is 1A form the daq device. Potentiameters only draw 12ma max. i think this should be ok. If that is the case i can connect +5vdc form scc-68 to EXC+ of the potentiameter and connect AIGND to EXC- of the sensor(pot) and connect signal form the pot to AI channel and leave AI- not connected and use RSE mode.

[John_P1]

Hey sunni,

The 5V lines draw directly from your computer's power supply and are limited to a 1A current draw via fuse.  Refer to this KB for more info.  You should be fine with a 12 mA draw per potentiometer (24 mA total for 2 pots).

You are correct in that you may connect the 5V output of the 6225 as the excitation for your potentiometer, AI GND as the reference, and use RSE mode to measure the output of the pot on a single ai channel.  Using differential mode and wiring AI- to AI GND should also result in the same measurement.  If you have any issues be sure to let me know.

-John 

[suni]

Hi Jhon

Thanks for the reply. In SCC-68 there are terminal called shield (72,73,74,75). Are they equal to AIGND? Can I use them to ground sensor signal wire shielding?

[John_P1]

The shield terminals are are connected to the shielded 68 pin cable.  Although they are indirectly connected to AI-GND, I would recommend to just use the AIGND terminals instead.