Accuracy of oscilloscope vs. DAQ (NI) at low frequencies...

Thread Starter

Alireza777

Joined Jun 3, 2018
2
Dear All,
Would someone guide me please, about:

I am mechanically exciting a piezoelectric polymer device at a very low frequency range (1-5Hz).
  • Using an Oscilloscope and a NI-DAQmx software, I received the same pk-pk Voltage magnitudes; but with different voltage-load phase configurations.

With an oscilloscope, the V-phase appears to fall out-of-phase and I cannot understand why V and Load phases are not overlapping?
This is not the case when using a NI-DAQmx though; and the voltage and applied load phases match exactly and perfectly on each others.

Therefore, I wondered if, generally, oscilloscopes are not a suitable way of acquiring data at low-frequency piezo excitations?
(or is there something else which I am not aware of about these two systems data acquisition method)?

Will really appreciate any idea and help.
Thanks in advance.
Ali
 

MrChips

Joined Oct 2, 2009
30,712
You need to tell us the make and model number of your oscilloscope.
You need to tell us what kind of probe you are using. Is it x1 or x10 probe? (not that this really matters).
You need to show us how your scope probes are set up and the front panel settings of your scope.

More importantly, you need to make sure that the INPUT coupling on your oscilloscope is set to DC (not AC).
 

Thread Starter

Alireza777

Joined Jun 3, 2018
2
Thanks for your comment, MrChips,
I am using an "Oscilloscope GW Instek GDS 2072A" with (I guess) x1 probe.

  • Connection-wise:
My piezo sensor output is attached via a BNC cable to a voltage amplifier; the output of the amplifier is connected via another BNC cable to the CH1 of the oscilloscope.

Please find attached the Voltage generated graph vs. load-displacement.

  • The Black graph = Voltage
  • The Blue graph = Displ.
  • The orange graph = Load

Many thanks,
Ali
 

Attachments

MrChips

Joined Oct 2, 2009
30,712
The GW Instek oscilloscope screen capture is showing a signal of 176mV pk-pk at a frequency of 1Hz. I don't see any problem.
 

Sensacell

Joined Jun 19, 2012
3,432
If you have the scope setup correctly- I would believe in what you see.

Scope measurements are good for clearing away confusion and assumptions about what is going on.
 

Wuerstchenhund

Joined Aug 31, 2017
189
First, if you really use a x1 scope probe and not just a cable then I'd use at least to an x10 probe as the x1 probe will put a lot of capacitive load on the circuit you are measuring which may well be the cause of what you see.

Also, NI's DAQmx is a software, but you didn't say where the software gets its data from. My guess is you're using some of NI's DAQs of which many have a higher vertical resolution than the just 8bits of your scope, although I doubt that this causes the phase difference.
 
Last edited:
Top