6008, measurement of analog out with oscilloscope through voltage divider

[ronan35]

Hi

I 'm having a problem with my usb 6008 card.

I am trying to output a voltage on an analog output, then to differentiate that voltage using some voltage dividers. (cf schematic)

The problem i am facing is that the noise seems to be terrible with this configuration (up to 100mV) when measured with an oscilloscpe (1MΩ input)

-Notice that i am fully aware that the maximum current drive is 5mA, and therefore  i use 2 1.7kΩ for my first voltage divider (voltage doesnt go higher than 2V)

I tried different parameters with the test panel (Measurement Automation). This noise appears even when i set the output to 0V!

-Notice also that this noise issue does not exist when i measure the output (after my voltage divider) with a multimeter: voltage remains stable

I would really like to be able to visualise those signals with an oscilloscope though.

Help will be greatly appreciated

thanks

Ronan

[Nathan_R]

Hi ronan35,

The schematic you posted shows that you are measuring the voltage from the analog output to the middle of the voltage divider. I would recommend measuring from ground to the middle instead. This is because your oscilloscope may be trying to ground the middle of the voltage divider and causing the noise problem. Also I would guess the noise is too high a frequency for your multi-meter to measure. It is most likely just averaging the noise out.

If the noise is still present when measuring the bottom of the voltage divider, I have some questions that will let us narrow down where the problem is.

1.     Are you outputting a DC voltage when you measure the noise or is the output voltage changing? If the voltage is changing you may be seeing glitches on the output related to changing the voltage.

2.     Is the noise present if you measure just the analog output without the voltage divider? This will let us know if the voltage divider is causing the noise.

3.     Does the noise seem random or is it periodic?

4.     If it is periodic what is the frequency of the noise?

Characterizing the noise more will help to narrow down its source. Let me know if you need any more help.

Thanks,

Nathan

[ronan35]

Nathan
Thnaks for the answer
this is exactly the problem that i was getting. My oscilloscope indeed tries to ground the middle of the voltage divider, and i actually got rid of the problem a couple of hours just inverting the two  probes of the oscilloscope. I had no idea the way voltage was measured  mattered, and i assumed the voltage would be inverted, but just as clean.
Thanks a lot
Ronan