usb 6009 output changes

[MicahL]

Hi, I'm using a NI USB 6009 device.  I am using the analog output to drive very small currents (less than 1 micro Amp) through a 1 MOhm resistor.  I find that the output often changes significantly when I begin to acquire data, or if the computer is doing something, such as fitting data, or even if I am simply opening other files or programs.  Any advice?

Thanks!

Micah

[johnsold]

Micah,

It sounds like you may have a grounding problem.  Without more information it is just a guessing game.

Can you describe the circuit more completely? Does the output current pass through anything other than the resistor?  If so, what else is in the circuit?  How does the output change?  How are you measuring the change?  What is the connection of the acquisition circuit?  Does it share a common grounding conductor with the analog output?

Lynn 

[HeatherBaker]

Hi Micah,

Can you elaborate more on your setup?  What signal are you outputting?  How do you determine the output changes?  What types of changes occur?  How are you acquiring the data?  Are you using the NI USB-6009 for this?

This document may also be useful in verifying any grounding concerns as johnsold mentioned.

[MicahL]

Hi, thanks for the responses.

I'm outputing a DC voltage with values typically ranging from 0.1 V to 1 V.  The signal goes through a 1 MOhm resistor in series with a coil for producing very small magnetic fields applied to an atomic magnetometer.   It appears that the output changes by tens of millivolts, as recorded by a yokogawa oscilloscope with input impedance of 1 MOhm.  The coil and scope are both grounded.  I am using only a single input channel on the USB6009, coming from an SRS voltage preamp with output impedance of 50 Ohms

I can also infer that there are changes in the current when the computer "does something" by looking at the signal in the atomic magnetometer.   

Perhaps it would help if I shunted the output with some smaller resistor, say 10kOhm?

I will look at the document you mentioned.

Thanks again.

[johnsold]

When you say the output changes by tens of millivolts, are you talking about the output of the USB6009 or the voltage on the coil?  Do you see any change if you measure the voltage at any of the USB6009 GND terminals with respect to whatever you are using for ground?

If you break the connection between the 1 megohm resistor and the coil (so no current flows), and run the program, do you see any changes in the output or in the magnetometer? This will help you isolate what might be causing the problem.

Lynn