Re : Are the signals in channels sincronized?

[Asier_gast]

Hi,
I am taking two AC(a sine wave and a signal from a pressure transducer) signals with a DAQ card. I am taking them in different channels and proccessing them with labview. I have several issues with the time reference for each signal.
I have the DAQ assistant placed in a while loop. I have realised that any operation I make to the different data in the channels(even separating them and bringing them together) result in a delay on the graph I am plotting respect to the original one. First, I would like to know what´s the proper way to handdle this problem.  I have noticed as well, using an analog oscilloscope, that the two signals in different channels have a different phase shift to the one appearing on the oscilloscope.
Can anyone give some advise on how to proceed on getting signals properly?

[Dennis_Knutson]

The word you are looking for is simultaneous. The two channels of a scope have independent A/D converters so you can perform an accurate phase shift measurement. The majority of NI DAQ boards (yuo don't mention which one you have, though), share a single A/D and all signals are muxed to it.

[Asier_gast]

So is there any way I can solve this?

[Wilfried_71]

I think Dennis is right : tell us the model of the card first ... and we will check if the channel are simultaneous or multiplexed ...

[Dennis_Knutson]

Exactly how much phase shift are you seeing and what is acceptable? According to this the default inter-channel delay is 10us. There are links there that also explain how to check what it actually is and how to change it. You can't do any of this with the DAQ Assistant though. If you cannot tolerate any phase shift of the acquired signals, then you have have to get one of the simultaneous sampling DAQ boards.

Message Edited by Dennis Knutson on 11-13-2007 09:12 AM

[Asier_gast]

sorry the model is :NI  SCB-68

[Dennis_Knutson]

Well, no wonder. You are acquiring data with a terminal block. With just a terminal block, you can't acquire anything.

You need to provide the model of the DAQ card itself.

[Asier_gast]

I am seeing a delay of 700us between channels anything one magnitude smaller is ok(70us pe). On the other hand, how can I handle the problem with operating the data without delaying the signal?The time is not important but the relative time position between signals is.

[Dennis_Knutson]

You are seeing 700us phase shift with the raw data? That seems very high but maybe there is some weird setting when you use the DAQ Assistant. I never use this. I would suggest you try one of the DAQmx shipping examples.

If you are seeing delays when manipulating the data, then how are you manipulating the data? Any filters on one signal that you don't do to the other?

Why don't you post your VI for someone to look at? It's much easier than trying to guess what might be going on.

p.s. You still haven't provided the model of the DAQ card you are using.

[Asier_gast]

Ok I think I have it now
the card is a NI PCI 6220. I also attach the VI
thanks,
Asier