PXIe-4304 causing ripple on input signal?

[mysticfree]

I have a PXIe-1082 chassis that contains a PXIe-6341 and a PXI-4304 module.  To check my code, I've connected the analog input (channel 0) of the 4304 to the Digital output (PFI 12) of the 6341.  My VI program is showing a 0.2 Vpp ripple on the analog input that I'm not seeing using a scope. 

The wiring is PXIe-6341 [PFI 12, DGND] -> SCB-68A --> TB-4304 [AI0 +,-] -> PXIe-4304

I've attached pictures of the scope verses the VI graph.  The scope is measuring at the AI0+ and AI0- terminals on the TB-4304.
 

Is there an extra ground that I should be using, or is it normal for the -4304 to add ripple?

Thanks,

Ron

[Johnnyg17 · Accepted solution]

Short answer is that there is nothing wrong with what you are seeing.

You have connected a low output impedance digital signal to a high input impedance analog digitizer. Since a digital signal is essentially a time varying square wave, and square waves have transition edges that contain very high frequency information, you will almost always see some form of "ripple" (see the fourier synthesis animation of a square wave from this Wikipedia page ). As well, a digital output signal is more concerned with timing and leveling than being a perfect square wave.

Also, you may be seeing extra "ripple" due to differences between the PXIe-4304 and the oscope you showed; the oscope may have a combination of higher input bandwidth (which can come from a variety of sources like low sampling rate on the 4304 which would result in more high frequency information recorded by the oscope for smoother looking transitions) and possibly a lower input impedance (causing less, if any, signal reflection which would cause ringing of the signal).

[BigApple0]

I'm not sure I understand.

You have connected a low output impedance digital signal to a high input impedance analog digitizer. Since a digital signal is essentially a time varying square wave, and square waves have transition edges that contain very high frequency information, you will almost always see some form of "ripple" (see the fourier synthesis animation of a square wave from this Wikipedia page ). As well, a digital output signal is more concerned with timing and leveling than being a perfect square wave.

That doesn't explain the difference between the oscilloscope and the PXIe-4304.  An oscilloscope is an analog digitizer (ADC) as well.

Also, you may be seeing extra "ripple" due to differences between the PXIe-4304 and the oscope you showed; the oscope may have a combination of higher input bandwidth (which can come from a variety of sources like low sampling rate on the 4304 which would result in more high frequency information recorded by the oscope for smoother looking transitions) and possibly a lower input impedance (causing less, if any, signal reflection which would cause ringing of the signal).

This is getting closer to the real answer.

The real answer is that the PXIe-4304 has a low pass filter that is filtering off the higher frequency components of the square wave.  The oscilloscope also has a higher bandwidth. 

The result the original poster is seeing is what is shown in the wiki page.

https://en.wikipedia.org/wiki/Square_wave