multiple analog input ?problem?

[manos]

Hello!

I'm using the DAQ-700 card with CB50-LP board (on WinXP). I need to read from 4 analog channels at the same time. At first, when nothing is connected, i get a reading of +10V (highZ i think). Then, i connect a +5V at the first channel (3.pin). On the waveform chart, there is a nice smooth line of +5V, but the other 3 channels jump on +5V also, and keep oscilating there(+-2V).

Can anybody tell me why is that, and what i am doing wrong(i need them to stay on +10 or something)? I'm using 4 "AI Sample Channel" blocks with 4 waveform charts.

Thank you very much

[AnalogKid2DigitalMan]

The 700 does not support simultaneous sampling, that is the front end circuity is switched between inputs to the analog to digital converter.

Here's a link to the manual:

http://www.ni.com/pdf/manuals/320676d.pdf

What you are seeing is normal. Due to the high input impedance and a slight capacitance on the front end circuity, unconnected inputs may tend to float to values to their adjacent connected channels. If you connect the unused inputs to ground, they should settle out a 0 volts. To test your code and hardware, I suggest you try hooking up each channel to a different voltage and see if you graphs match the expected results.

[ROtake]

Your problem is normal. If it is a problem to you, just connect the unused channels to the GND.

[manos]

Thank you very much for your answers. I just don't understand why do the daq700's have 15 AInputs then?!
OK!

But, if i ground the other ones, how would i 'unground' them at the moment when i need to read from them(and at the same time read from the first simultaneously)??

[AnalogKid2DigitalMan]

They have 16 usable inputs in single ended mode, and 8 in differential mode.

Just connect your signals up to the appropraite channels and read them. Once they are connected to a source, the source will act to bleed the adjacent channel crosstalk away.

No need to ground them out and connect to the signals prior to reading.