08-29-2012 08:45 AM
I am using a NI PXIe-1073 with a PXIe-4496 for several applications. In one of them, I would need to measure voltage, keeping the dc component (it`s a MEMS accelemeter). If it is vertical upright, it gives, for example, +2V, and I need to keep reading that almost constant value.
However, with the DAQ assisstant in LabView, it seems that it reads only the changing component of the signal, as if it doesn´t move, it reads almost 0. If it moves, the, the DAQ system takes the change but then goes back to zero.
Is this normal? Can I read the constant part of the signal?
(I am a mechnical engineer and I am not at all an expert in signals or data acquisition, so I am probably just missing something basic)
I attach an example in which, after a change, it goes back to zero, and the configuration in the DAQ assistant
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08-29-2012 08:56 AM
It looks like you have a grounding issue or your incoming signal is AC coupled. Do you have a wiring diagram of how the sensor is hooked up to the DAQ? I'm not sure what PseudoDifferential really means (the descriptions I have found don't say much). You probably want to use Referenced Single Ended (RSE) for your setup. But it's hard to say without knowing how the sensor should be hooked up.
08-29-2012 11:15 AM
Thank you for your response.
I attach a wiring diagram (the accelerometer is a MEMS DE-ACCM3D, reading for example the X channel).
a) If I read with a voltimeter, instead of the NI DAQ, y have my constant tension of aprox. 2V.
b) If I replace the output of the accelerometer by a voltage constant tension of 0.3 V for example, I have the same issue. I read 0 V unless I change it (case in which it reads the change and then goes to zero again)
08-29-2012 11:28 AM - edited 08-29-2012 11:31 AM
What signal is the ground attached to on the DAQ card? You should be using AI GND and then you can use the RSE configuration on your DAQ.
If the GND is going to the negative of a differential channel, then just change your configuration to "Differential" instead of "PseudoDifferential".
08-29-2012 11:38 AM
The negative of the BNC that goes to the DAQ card is the ground on the accelerometer which is the negative of the power supply.
I think that this card doesn't have a AI GND (does it?)
Looking again at the specs, it says:
Input configuration: Pseudodifferential (50 Ω between negative input and chassis ground)
Input coupling: AC only for NI-4496
I really don't know what this last thing means, but could it have something to do with it?
08-29-2012 11:45 AM
AC coupling is the culprit- it will block all steady state DC components.
08-29-2012 11:48 AM
I see, nothing to do then!
08-29-2012 11:48 AM
Yeah, maybe I should look at the specs of the cards you are using...
Ok, your problem is that the PXIe-4496 is AC coupled. In short, this means that it knocks out the DC component of your signal before sampling. This is done since some sensors output signals with a DC offset (usually the excitation voltage). Sorry, but you need a different DAQ card. Get a hold of you local NI Rep and discuss your project with them. I'm sure they will come up with something that will work for you.