Voltage leaking between channels on SCB-68-A connected to a PXIe6363

[MHen7]

Hello,

We have been trying to measure the voltage output generated by our force balance. This force balance puts out 8 channels with different voltages. We have connected these to a NI SCB-68A connector block (see attachment 1: SCB-68A), I have included the pinout in attachment 2. We have attached the connector block to a PC using a PXIe6363 card and are able to get a signal.

However, when we use a signal generator there seems to be a voltage leaking between the channels. For example, we apply 4 volts over AI0+ and AI0- (68 and 34), but the matlab measurement shows that other channels are also influenced when there is no voltage applied to them, see attachment 3.

I have attached our matlab code as that could also be part of the problem. Is use the "testing_scriptje.m" to create the plot and "NImaakmeetsessie.m" to setup the daq session.

Does anybody have an idea what could be a fault in our system?

I look forward to you responses and ideas.

With kind regards, Mark

ps. I was not allowed to upload the NImaakmeetsessie.m therefore I have added it below:

function s = NImaakmeetsessie(kaartenarray,PXI1Channels,PXI2Channels,PXI3Channels,PXI4Channels,PXI5Channels,samplefreq,duration,voltage)

s=daq.createSession('ni'); %creeert een meetsessie met de kast
s.Rate=samplefreq; %sample frequentie
s.DurationInSeconds= duration; %meetduur in s

if kaartenarray(3)>0
ch1=addAnalogInputChannel(s,'PXI1Slot3',PXI3Channels,'Voltage'); %voegt kanalen toe vanuit 3e kaart
% voer voltage range in
for i=1:size(PXI1Channels,2)
ch1(1,i).Range=[-voltage voltage];
ch1(1,i).TerminalConfig = 'Differential';
end
end

end

[RHadley]

There are a number of possible causes for this, but as a general principle, no signal being supplied to a channel does not mean the channel will be at a constant 0 value. For a more detailed look into this, I recommend reviewing our article on Incorrect Readings on Unconnected or Open Channels of DAQ Device.

However, as this is a behaviour of open channels, this will not affect the values of your measurements once these channels are connected up.

That said, if you do have any issues with unexpected measurements once you have connected the channels, let us know and we will definitely help you look into it!

Kind regards,

Richard Hadley CLAD

NIUK

Technical Support Engineer

[MHen7]

Hello Richard,

thanks for your response. Due to covid I could not test it earlier at the office. It was indeed ghosting of the empty channels.

I do experience a second misreading which could be related to this. I should measure 5 volts at channels 1 - 6, 0.2 volts on the 7th and 0 volts on the 8th. It seems that the 0.2 volt is pulled up towards the 5 volts of the other channels. Could this still be a problem of the ghosting? When I attach a voltreader to the pins of channel 7 I measure the 0.2 as expected but my software reads 5.

kind regards Mark