compact daq multiple power supplies into analog input

[Monse]

I'm trying to read several values from different sensors into a 9205 voltage input module using a compact DAQ.  My sensors use different power supplies, they all output voltage between 0-10Vdc.  I can read the value off of any one sensor fine, but once I hook up another both values start to shift and osscillate, however either one by themselves gives me a steady DC value.  I am taking differential measurements from the sensor's outputs... I'm not sure if it would be ok to tie the grounds of the two power supplies together and feed them to my ground reference input of the module, if I take a voltage measurement between the two power supply grounds I see about 1Volt difference.

Any ides/suggestions would be appreciated.

Thanks,

M. 

[James_McN]

First suggestion would be to use a differential reading but I see that you are doing that already, that should eliminate issues with being on seperate grounds.  Firstly I would consider whether they could be interfering with each other or perhaps something else that you are doing when connecting them up is increasing noise i.e. proximity to the power supplies.  Do you have an approximate frequency of the osscillations?  this could identify the source.

Also what pins do you have the sensors connected to?  and what sensors are you using?

Regards,

[Monse]

Hello, thanks for responding

Right now I'm using a Humidity Sensor (Vaisala HMT100) and a CO2 sensor (Vaisala GMT220)... I have them hooked up to differential input ai0 (using pins 1 and 19) and to differential input ai1 (using pins 2 and 20).

Now because both of these sensors are running off two different power supplies (one sensor has a bulit in tranformer and the sensor plugs directly into the wall, while the other is going to a 24 volt power supply) I'm not sure what I should hook into my COM port (pin 17) on the DAQ.... I can get either sensor to work fine by themself, but once I hook both up the signals start to oscillate... I have 3 other sensors I will also need to hook up but right now I'm stuck on getting these two working.

Thanks

M. 

[James_McN]

Hi,

I have been looking at the sensors you are using and the pins seem to be correct so I just want to make sure that we are on the same page with the connections you are making.  This is assuming you have a spring terminal on your daq card rather than a DSUB.

Humidity Sensor - Pin 3 or 4 (CHn+) to AI0 (Pin 1) and Pin 6 (SGND) to AI8 (19)

CO2 Sensor - V to AI1 (pin 2) and 0 to AI9 (Pin 20).

In your software you should then be taking a differential voltage reading on AI0 and AI1.

Assuming that is all correct then this is very strange.  The wiring diagrams suggest that the signals are ground referenced so this should be sufficient however the 1V difference in the ground lines is perculiar if they are plugged into the similar place.  although the differential reading will eliminate error due to this one consideration is that if they are floating you may need to add bias resistors.  You can find out more about this here.  As I said it doesn't appear that this should be required but is worth a go if you have the kit to try it.

Otherwise it might be useful if you can send me some screenshots of the behavior on the graphs and perhaps your code and we can see if that helps.  Also if you are able to verify the functioning of your sensors some other way would also be useful i.e. with a multimeter.

Regards,

[Monse]

Thanks for your help, the problem has been solved, it was a grounding issue.... one of the negative leads of the sensors wasn't making good contact witht he com port of the module, this resolved the issue.

Thanks