oscilloscope causes a noise in DAQ ?

[Johnnykoo]

Hello, 

I'm using USB-6009 as my daq and.. 

I'm wondering if there is any case or possibility that connecting the oscilloscope causes a noise in daq 

simply what I have here is 

---RES-----    -

                        USB-6009 

---------------   +

and I just measure the voltatge across the resistor during the acquisition 

and I see crazy noises on the chart screen as well as my data ( well, same thing) 

Is there sometihng I'm doing wrong ? ? ? : ( 

have no clue on this.. please help ! 

[johnsold]

Your description is somewhat vague.

What is connected here? At the left of your diagram?  Without a connection there you do not have a circuit and have nothing meaningful to measure.

 |

 |

V 

   ?       ---RES-----    -

   ?                               USB-6009 

   ?       ---------------   +

Connecting an oscilloscope usually does not introduce significant noise.  What is the signal voltage and frequency? What is the input range and sampling rate for the USB-6009? How is the input configured? Differential or single ended? What is connected to the other inputs?

Lynn

[Johnnykoo]

Hi, Lynn 

basically on the left side, there is a PCB (device) and on the right hand side, 

there is a Calibrator for current input to PCB

and I'm measuring the voltage across the RES in the '-' wire 

It's DC voltage I'm measuring.. about 3V (RES=240ohm)

I've seen the crazy noise every time I connected the GND of oscilloscope to the wire 

however,, now.. it seems weird.. 

because it's a vibration testing, 

there is a random noise from PCB however, now every time I connecto my GND of oscilloscope 

to the wire, all the noise disappear.. I feel like all the noises are removed by the GND of oscilloscope ? 

what I'm not getting here is.. as far as I understand, any testing instrument should not affect to NI device ? (well I'm still not sure about this)

while measuring voltage (for any test), If I just measure the voltage using a multimeter, 

NI device and Labview should not be affected.. right ? it's just a multimeter,, 

and should be the same logic for oscilloscope ? 

[johnsold]

You probably have a ground loop.  This can occur when multiple devices (such as your DAQ and PCB setup and the oscilloscope) connect to ground at two or more points.  If the "grounds" are at different potentials, then currents flow between the points.  Since the wires connecting those points do not have exactly zero impedance, some voltage drop occurs by Ohm's Law.  You see the noise.

Can you post a real schematic diagram, not an ASCII simulation, of your entire system showing the PCB device, DAQ board connections, power supplies, oscilloscope, ...? Even a hand drawn diagram scanned into a .jpg or .png file or a .pdf file from a schematic capture program.  Do not post .bmp files. They are too large and are prohibited on the Forum.  It is too difficult to tell exactly how you have everything connected from a text description.

Lynn

[Johnnykoo]

here is the rough schematic.. 

for wiring diagram, I followed the NI DAQ assistant 

I was thinking about common ground as well.. 

should I do something on USB-609 ? 

I guess I'm not perfectly understanding about ground point in testing setup overall. 

(well, I understood what you tried to explain, thanks ) 

[johnsold]

That helps a lot.

A few more questions: Are the two power supplies connected to each other in any way? Do the power supplies have any connections to ground?  Which terminal of the oscilloscope is grounded?  Do you have anything connected to the USB-6009 grounds?  Are there any other connections to the PCBs? The USB-6009 has an internal connection to ground on each of its analog inputs.  This means that currents to ground may be flowing in places you do not expect.

Lynn