DAQ phase delay between two lines physically tied together on DAQ NI USB-6289

[Sami.B]

Hello,

 Currently, we have two signals that are physically and logically tied together on the same input line of the DAQ. The first signal, "I", and the second signal, "Q", are both tied to "Ai6" (pin 23) and "Ai GND" (ground). However, when we probe the two lines coming our of a "signal split" function, I leads Q by about 0.5ms. My question is, if these two lines are in fact tied together physically and logically, then why are they phase shifted if they're both the same channel? They should be exact matches. I thought inter-channel phase delay only occurs with respect to different channels being read off the DAQ. I've attached some screen shots as well.

Incoming Signal is 20kHz. Sample rate is 40.5kHz, # of samples/point is 1000.

Thank you.

Best,

Sami

System Engineer - Ultrawave Labs

[Aaron O]

Can you confirm that the signals are in phase when you tie them together by probing the input of the DAQ?

[Sami.B]

Hello,

 Thanks for the reply. Well, what we did was physically, we only have 1 line going into the DAQ. We then split this 1 line, logically, into 2 signals, "I" and "Q". These 2 signals, "I" and "Q", were then slightly out of phase. We ended up fixing this issue in Labview code. We still don't know the reason that "I" and "Q" were initially out of phase. Maybe because of the interchannel delay?

Thanks,

Sami

[Seth_B. · Accepted solution]

The board you are using is a multiplexed board.  If you create multiple virtual channels pointing to the same physical channel, we will sample that channel once for each channel.  So, what was basically happening is that the physical channel was sampled once for channe "I" and then again for channel "Q" with a delay of a few microseconds between the two.