Simultaneous analog and digital inter channel delay

[PMH01]

I need a simultaneous 1 channel analog input and 1 channel digital input intending to do maximum speed acquisition possible. Of course I expect no inter channel delay if sampled both inputs at the same sample clock edge. But this example demonstrates an unexpected behavior. Restarting the acquisition regularly shows different inter channel delays. I hope there is someone out there to help me out. See example included.

[PMH01]

Working example.

[PMH01]

Screenshot

[ThiCop]

Hello PMH,

One question before providing feedback:

What hardware are you using?

[PMH01]

Thierry,

Thanks for your reply.

I use the NI USB-6251 BNC https://www.ni.com/en-us/support/model.usb-6251.html

Labview 32 bit version 13.0f2 with latest DAQmx

Computer Optiplex 7010 (I7 processor)

Kind Regards,

[PMH01 · Accepted solution]

I asked support from National Instruments and they came with an adequate solution.

The time stamp t(0) in the waveform is NOT the acquisition start time but it is the time the buffer is read.

To demonstrate this apply the same 'pulse train' to both channels and set t(0) of the waveforms to the same start time. Observe that the edges of both signals match exactly. Because one channel is a digital input we have to take the logic voltage levels in account.

 
Thanks to Henk Talsma
National Instruments
Applications Engineering

[ThiCop]

Hello PMH01,

My apologies for not being able to reply earlier. (I have been Out of Office due to sickness for several weeks.

I'm happy to see that Henk was able to help you in the meanwhile.

This explanation is indeed correct.