I Am Getting Inaccurate Readings When Some Channels of the SCXI-1520/SCXI-1314 Are Open Circuits

[S_Kanchap]

 Am Getting Inaccurate Readings When Some Channels of the SCXI-1520/SCXI-1314 Are Open Circuits

My hardware is :-

PCI MIO 16E-1 - 1 no.

SCXI-1000 chassis - 1 no.

SCXI-1520 - 4 nos.

SCXI-1314 - 4 nos.

MAX is showing all hardware ( DAQ card + Chassis + SCXIs), set in Multiplexer mode.

I am using MAX task to configure channels ( as per requirement ) and then use the same task in Labview 7.1

When I am trying to generate task only for connected channels then reading are coming some times....... but when i generate task for all channels including (not connected channels) then the reading are either on - side or on + side (max or min).

I have tried to ground the +S but not work

tell me some solution

Thanks

Surender Kumar

[devchander]

Hi,

Are you using DAQmx or traditional DAQ device drivers??

[S_Kanchap]

Hi Dev,

I am using DAQmx.

Shouls I need to use traditional or some other way.

The values shift to +/- range if any channel opens.

Thanks for reply.

[Elijah_K]

I'm a little unclear as to what your problem is.  As I understand it, you're attempting to generate an analog output, but it's not setting the channel to the correct value, correct?

How, if it's an open circuit, are you measuring the voltage out? 

Are the values ever correct?

What are you setting the range of the card to be and are you exceeding it with the value you're attempting to read?

[devchander]

No, DAQmx is superior and I strongly recommend you used DAQmx

Elijah, I guess I understood the problem that Sunder is facing, coz even I had experienced it 2 years back

What happens is if you scan only the channels to which sensors/strain gauges are connected, all is fine and you get good strain readings.

But if you happen to include an unconnected channel into the scan list, all the subsequent channels after the open channel will show saturated readings.

Right, Surender??

The problem we found, lies in the settling time of the E series DAQ card's ADC. Even we were using PXI 6071 E

Once the ADC saturates on reading an open channel, it cannot settle fast enough to read the next channel.Hence, the next channel also reads saturated value, instead of the correct value.

This is irrespective of the sampling rate because, this happened even when we did continuous sampling at low scan rates of 10 to 100 samples /sec

We found 2 solutions to this problem

• Disable Simultaneous sampling and hold for the SCXI 1520 module ( using a DAQmx property node)
• Use a M series DAQcard, which did not have this settling time problem

Hope this helps,

Regards,

Dev