cDAQ-9184 Multiple channel measurements

[trevie1970]

I have a requirement to measure 3 phase input voltages on the cDAQ-9184. If I were to measure only 1 channel at a time, the result measurement is correct at 115VAC on each of the 3 phases. If we were to measure multiple channels simulatanously, then the first channel is measured correctly at 115VAC (phase A), the other two channels have a output less than the first channel. 109VAC (Phase B) and 108VAC (Phase C) respectively.  If we were to measure the other two channels seperately or the second and third channel. The output would be ok on the first channel measured (Phase B 115VAC) and the second channel would be less voltage.(Phase C 109VAC). I am unable to determine why the measurement error is there between the three channels. when confirmed its able to measure each channel independently correctly?

NI Hardware:  cDAQ-9184 device

I have attached a screen shot of the VI in use and the code below also, Labview 2016, Windows 7 OS.

[crossrulz]

What modules do you have in the cDAQ-9184?  What signal conditioning are you using?  This sounds somewhat like ghosting.

[trevie1970]

Hi,

I am using NI 9205, Using the module, I have attached the Signal conditioning circuit and the Input schematic to the 9205 module for clarity.

During High voltage measurement (115VAC), the range is set as shown in the drawing with the relay connected the 47k potential divider.

The measurement is set to differential as shown in the attachment.

Thanks for your help.

regards

Trevor.

[crossrulz · Accepted solution]

Voltage dividers can easily cause ghosting since they become high impedance sources.  My recommendation is to get a bunch of op-amps configured as a Voltage Follower.  The inputs should be connected to the divider and the output to your DAQ.  This will give the DAQ a low impedance source which will reduce your ghosting effect.

[trevie1970]

This sounds like a viable solution, thanks for your advice. I will follow this up .

regards

Trevor.