Mydaq & Labview connection problem

[MaddoxX]

Hey,

at first sorry for my english it is not really good but i hope someone can help me.

Problem:

I use/have the MyDaq ( 2 weeks ) & Labview and now i have a little project.

I would reading a Mems Sensor with the Mydaq (it is mounted on a fan) and will process the data in Labview.

Now i have the X,Y,Z coordinate but the Mydaq has only 2 Analog In. I thought I take the audio input but these Input may only Max. 2 Volts.

The Question is now, how can i use 3 Analog Signals with the Mydaq?

Is there a solution in Labview who i can say : A/i 0+ is X-coordinate & A/i 0- is Y-coordinate & A/i 1+ is Z-coordinate ? Is this possible? ( I do not need the differntial-mode)

Thanks for reading & reply!

[Norbert_B]

Sorry, but the AI of the myDAQ are fixed differential.

Sure, you can connect X to AI0+ and Y to AI0-, but you can only acquire AI0. So the value you receive is the DIFFERENCE between the voltage level of X and Y. You will NOT get the voltage level of X and Y individually!

(Read: Voltage Level => electrical potential of the channel/pin compared to a common ground potential)

Norbert

[MaddoxX]

Mh.. Okay. I didnt know that they are fixed differential.

Well.. I think i use the DMM. It is annoying because the DMM are very slowly but it is the only solution.

Thanks a lot , Norbert.

[Bob_Schor]

I wonder if the DMM uses a separate A/D converter from the 2-channel A/D -- if it doesn't, you won't get a third channel that way.

There is, however, a way to get more channels of A/D using your myDAQ and some additional circuitry -- build a multiplexer.  You basically need a circuit that accepts two (or three) voltage signals in (let's call them X, Y, Z, and W -- with this circuit, you actually get 4 inputs) plus two digital circuits (call them 0 and 1), and one output (called Output).  Your two digital inputs give you a 2-bit number that you can think of as 0, 1, 2, or 3.  Based on the value of this two-bit number, you feed input X, Y, Z, or W to the output, Output.

I'm not an engineer, so I can't tell you how to build such a circuit, but I know that it is fairly standard, quite fast (the output should switch in less than a microsecond, I think), and there may even be chips out there that do this.

So to use this, you would wire your myDAQ up with two digital lines controlling the multiplexor.  To sample X, send 0 to the DIO and then sample.  To read Y, send 1 and sample. And so on.

I did this with a single channel and a two-bit (not $0.25) counter, but you could also make a one-bit counter and sample two channels.

Bob Schor

P.S. -- I don't know about the internals of myDAQ's A/D, but it is quite possible, maybe even likely, that there is a single A/D converter being fed by a 1-bit multipler circuit switching between A/D 0 and A/D 1.