problem on the source signal when acquiring with usb6251

[NICC]

Hi, I have a problem when I launch my VI to acquire two signals from a lock in amplifier. When the VI is off, the signal on the oscilloscope is clearly clean and good but when I turn on the VI on the oscilloscope the signal becomes very dirty and noisy. I already tried to change reference connections but nothing happened.

Could anybody give me a help?

Thanks

[MarcoMar]

Hi NICC,

I Don't know the output parameters of your source, so I can't know what could be the cause of this unexpected behavior.
Anyway, I suggest you to read the chapter 4 of the device manual, that you can find at the following address:

https://www.ni.com/docs/en-US/bundle/pci-pcie-pxi-pxie-usb-62xx-features/resource/371022l.pdf

Have a great day!

[John_P1]

The 6251 is a multiplexed board.  If you're using an external oscilloscope, you're probably picking up the transitions of the multiplexer.  Ideally you'd want to wait for the channel to settle before taking a sample.

Best Regards,

[NICC]

Thank you for your replies, but I wonder why the same signal is not noised when I use a usb9162. Anyway I try to apply some of your tip

[John_P1]

You didn't mention the 9162 before, but this is actually just the model number for the USB C Series carrier.  The front-end behavior is going to depend on what module you are using with it.  For example, modules like the 9215, 9237, and 9229/9239 are simultaneously sampled and wouldn't have the same behavior.  Modules like the 9201 and 9221 are multiplexed like the 6251.

Best Regards,