DAQ reading problem

[bjbdts]

Hi,
I am using a DAQ board to read the voltage across a load cell to a computer. I also have a 2.2-2.5GHz power amplifier sitting close to the DAQ board. My problem is when the amplifier is on, my DAQ reading to the computer shifts about 6mV, which means with nothing attaching to it, my labview program shows the load cell has about 5 lbs force! But there is no change before or after turning on the amp if I use a voltmeter to measure the voltage at the DAQ board directly. Any suggestion is appreciated.

 

[zappedagain]

You are experiencing electromagnetic coupling between the PA and the DAQ. Fields are probably radiating from the PA and getting rectified by circuitry within the DAQ; that is why you see a DC offset.

The first thing to try is to separate them and see if that gives enough improvement; more distance reduces coupling effect typically.

If you twist the wires running to the load cell that will help to cancel out some coupling. Same thing goes for the power supply wires for the PA, if applicable.

You might also need to put a choke on the power supply leads. Stward has some nice ones that will clamp on the cable, and some good app. notes to explain why you are doing this.

If you can do it safely, isolate the load cell or the PA from GND. If there is a DC path between the two you may be forming GND loops that can also cause offset voltages.

Welcome to the world of EMC/EMI!

 

[analogkid2digitalman]

Zappedagain has some good pointers. Additionally, if your load cell output is floating, I would suggest setting up the DAQ card for differential mode input.

If this is a USB based DAQ card, they are tradionally more susceptible to picking up EMI/RFI than cards residing within a PC.

Your DMM may have filtering, averaging, or better CMRR than the DAQ. Also, the sampling rate of the DAQ may make it display more noise than the DMM.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[bjbdts]

I check out the reason is Vout- of the load cell shares the same gnd on the DAQ board with Vout- of a power detector board. The power detector board connects the output of the PA to monitor the output power.
My DAQ is now working as single-ended-nonreferenced mode. Is it easy to change it to differential mode?

 

[zappedagain]

That depends on the DAQ card you are using and the software. Check the manual.

 

[bjbdts]

Zappedagain:
I managed to configure the DAQ to diff mode this morning and now I have no problem with the load cell anymore. Thank you guys for the help.

bj

 

[analogkid2digitalman]

Cool. Sounds like it made all the 'differential'. Happy DAQ'ing.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[Higgler]

I'm curious if the real problem was just the increased IR voltage drop from the current pulled by the power amp.
i.e. as opposed to RF coupling being the problem. There wasn't any mention of a signal in the RF amp, just that it was turned on.

Any thoughts?

kch