Why Cannot NI PXIe-4340 Excitation Output Be Measured by Oscilloscope

Issue Details

I'm using PXIe-4340 and TB-4340 to measure an LVDT sensor signal. The measurement seems to work well. I want to verify whether the excitation signal is as expected. However,

• When using an oscilloscope channel to measure the excitation signal (EX+/-),  no signal can be detected. Meanwhile, the measurement doesn't work.
• When using an oscilloscope channel to measure the EXCITATION VERIFICATION pins on TB-4340, the signal can be detected, but it is only about half of the configuration. Meanwhile, the measurement can work.

Why is this happening? How can I measure the excitation signal of PXIe-4340?

Solution

Reason

The reason for this issue is that the excitation signal of the PXIe-4340 is a differential signal. And for most of the oscilloscopes, the channels' negative terminals are directly grounded.

For example, when the excitation is configurated as 3 Vrms, the EX+ pin has a +1.5 Vrms potential to ground, and the EX- pin has a -1.5 Vrms potential to ground. The oscilloscope channel's negative terminal is directly grounded. If we connect the negative terminal to the EX- pin, the EX- potential will be pulled to 0 V, which means the EX- is shortened to ground, and so is the EX+ pin. In this condition, the PXIe-4340 will not be able to output excitation. Therefore, no signal can be detected, and neither can the measurements work.  

Walkaround

You can use any of the following methods to measure the differential signal,

• Use a ground-isolated measurement device to measure the signal.
• Use a differential active probe to measure the signal, for example, the SA2500X.
• Use two oscilloscope channels to measure the signal. 
  1. Connect oscilloscope CH1+ to EX+, connect oscilloscope CH2+ to EX-, and leave CH1- and CH2- unconnected.
  2. Use the oscilloscope MATH channel to obtain the subtraction between CH1 and CH2, which is the EX+/- signal.