difference between oscilloscope and labview voltage reading

[Premashis]

I am acquiring two constant dc voltage with DAQ borad. But when I look the voltage directly in the oscilloscope, it is diffrent from the vltage I am mesuring with my labview program and they are not constant (voltage is drifting). Do you have any idea why this ocuurs? Is this the problem with my code or it is the DAQ board I am using. The vi is attached.

Premashis

[LV_Pro]

How different are they? Usually when you see DAQ inputs drift is indicates that they aren't properly connected. How are your inputs configured? Differential, Single-Ended?

[Premashis]

The inputs are configured as default. I don't know whether it is differential or single ended.

[Premashis]

I change the input configuration to RSE and that fixed the problem.

Thanks,

Prem

[mikeporter]

Glad to see you figured out the problem. For future reference, there are a couple other things that could potentially cause problems.

First, many 'scopes have a variable volts/div control. If that's not all the way in the calibrated position, it can change the apparent amplitude of the signal you are looking at.

Second, 'scopes also allow you to select different kinds of coupling on the input. If you are looking at a signal with a DC offset or a very slowly drifting bias that setting can create a significantly different result.

Mike...

[Norbert_B]

And to add some other options for false measurements:

- Incorrect impedance setup between source and measurement device

- Using probes (e.g. 10:1) without configuring correct scaling

- Different connection wires (BNC vs. "lab measurement lead")

- ....

Norbert

[mikeporter]

Excellent points all! Back in the day o'scopes had a well-deserved reputation for loading down circuits. While the problem has become less noticable as 'scopes have made the transition from analog to digital. Still there is an awful lot of old hardware out there, and if you lab istructions include the phrase, "...pull over the oscilloscope cart..." you may have loading problems.

Likewise, I can't count (or wouldn't want to count) the number of times I have been bit by a probe with the built in attenuator in the wrong position. Sheesh...

Mike...

[Norbert_B]

Hehe, nice Mike. I think that happened to all of us at least once 🙂

Well, one additional point coming up right now is

- Measurement device influences circuit.

Meaning: In-Circuit measurements could require guard amps for proper results....

Norbert

[LV_Pro]

Ok, I was trying to work through the simple stuff first. Now, when it comes to scopes, and circuit loading, the speed of the signals being observed is also important as the scope probe impedance isn't entirely resistive. You will note on most scope probes their capacitance as well as their impedance. While the capacitance is not particularly important at lower frequencies (<10MHz) as the frequency increases there will be more loading on the circuit.

It isn't magic, but there are a lot of factors involved in making accurate measurement.

[Norbert_B]

Hmmmm,

resistance + capacitance (EDIT: or inductivity) = impedance

....

Norbert