Issue with multi-channel analog measrements using PXIe-6341

[cjsalvat]

Hello,

I am experiencing unexplained behavior with a PXIe-6341 card which is being used with a SCB-68A cable.

I have 3 sensors connected as follows:

AI 0 – Pyrometer

AI 1 – Pressure Sensor

AI 2 – Position Sensor

All 3 sensors are connected to the + and – inputs of each channel.

All 3 sensors have a power requirement of 24 volts (powered via a stand-alone PS) and draw 4ma – 20ma.

We have placed a 250 ohm resistor across each channels + and – inputs which results in a voltage measurement of 1 – 5 volts.

When I use the Test Panel feature of NI-Max and individually select each sensor from the pull-down menu – the resulting measured value displayed in the graph is always correct and everything works as expected. This is true every time for each of the 3 sensors.

However, if I create a NI-Max Task in which I try to simultaneously observe all 3 sensors measured data I experience the following behavior:

Scenario 1:

If the task contains just the Pyrometer and the Pressure sensor the measurements are always correct.

If I then add the Position sensor to the task its measured/displayed value is not correct. However, the voltage measured at the 68A + and – terminals is, in fact, correct.

Scenario 2:

If the task contains just the Position sensor the measurement is correct.

If I add either the Pyrometer or the Pressure sensor to the task their measured/displayed values are not correct. Their displayed data equals 0 volts. However, the voltage measured at the68A + and – terminals is, in fact, correct.

I am stumped.

Can anyone shed some light on possible causes of this type of behavior and recommend solutions?

Thank you.

[johnsold]

My guess is that the position sensor circuit has a different ground connection than the the other two sensors. The result is that when position and one of the others is connected, the common mode voltage limits for some of the channels is violated.  Measure the voltage to AI Ground from each input in the various Scenarios. I suspect that you will see a value outside the acceptable range when the "no-working" connections are made.

Lynn