Dual Ch input data aquisition

[pomme]

I have tried to aquire two voltage signals into my BNC 2090. When I record the signal, it does not work well. The signal from one channel affect the signal in another channel. it is something like signal leaking between channel. Maybe I did somethin g work with my code. Can you guy help me check it out? I have used AI channel 0 and channel 1.

                // AI (analog input)  measurement microphone
                DAQmxCreateTask ("Cal_Res_Task", &cal_in_th); // assign a task name and task id
                DAQmxCreateAIVoltageChan (cal_in_th, "Dev1/ai0", "",DAQmx_Val_Cfg_Default, -10, 10,DAQmx_Val_Volts, NULL); // min $ max -10-10V
                DAQmxCreateAIVoltageChan (cal_in_th, "Dev1/ai1", "",DAQmx_Val_Cfg_Default, -10, 10,DAQmx_Val_Volts, NULL); // min $ max -10-10V
                DAQmxCfgSampClkTiming (cal_in_th, "OnboardClock", cal_type_stim.samrate, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps, cal_type_stim.npts);
                // set up external triggering
                DAQmxCfgDigEdgeStartTrig (cal_in_th, "/Dev1/PFI0", DAQmx_Val_Rising);       

[e-Cash-Money]

Hello pomme,

On the BNC 2090 I believe there are switches that configure which terminal configuration you are using for your channels.  What do you have your switches set to for channel ai0 and ai1?
Which DAQ board are you using?
If you just hook up channel 0, does it read in the correct signal?

It doesn't seem like there is anything in your code to cause the cross-talk between the channels, my guess is that it is a configuration/connection issue.

[Ward]

Hello Pomme,

When you supply a voltage to channel one, do you connect the other channel to ground? If you leave it 'open' I've noticed there is indeed a reasonably large amount of cross-talk...

Best regards,

Ward Jennekens

[Tom_W_[DE]]

Hello All-

The configuration settings E.Lee mentioned are described in this KB.  Please make sure you are using the correct switch settings and that you are not scanning open channels- this is known to cause problems as Ward suggested.

Thanks-

[elmo]

Hello all,

I am also using a BNC2090 with an Mseries DAQ device NI-6259 using Labview 7.1.

Whilst I read in more than one analog input signal without a problem (so far), I do have the issue that when you open circuit a channel, which can represent a sensor being disconnected, the voltage fluctuates and jumps all over the place without settling down.  Ideally, you would want this voltage to return to zero.

Is there any way around this (additional circuitry?), since you dont want to be displaying these oscillating values if an open circuit occurs.  Also I would like to be able to detect the open circuit.

Is this not just a property of the Mseries device and my default circuitry, in how the voltage falls at the pins to the amp during an open circuit?

Kev

[e-Cash-Money]

Hello Kev,

I would suggest using a pull-up resistor in order to prevent crosstalk as well as detect if an open circuit occurs.  Putting a 1 MOhm resistor between the positive input channel and +Vref should pull the input value to +Vref when there is an open circuit, thus preventing any cross-talk from other channels.  If you are doing a differential measurement, put a 1 MOhm resistor between the negative input channel and ground as well.