1 sample delay on first AI channel

[Matt_W]

Hi,

I'm using an NI 6221, Labview 7.1 and DAQmx to output a tone burst from AO0. I then need to measure this signal on four analogue inputs. The AO is triggered from the Start Trigger of the Analogue input. The problem I'm finding is there seems to be a delay of one clock cycle on the first sample of the first channel. There is no noticeable delay between the last three channels. In fact the first sample seems to be random, generally floating around full scale.

I've tried reversing the trigger, so the AI is triggered from the AO but the same problem occurs.

Can anyone help?

[reddog]

I'm pretty sure your problem is that your sampling your first channel before the output from your AO channel has updated and settled. See the attached picture for a better look at the timing signals. By default, the first AI channel conversion will take place 100 ns after the sample clock. However, for the 6221, the slew rate of the DAC is 15V/us. Depending on the step sizes of your output waveform and the desired settling time, you will have to push the first conversion out further from the start of the sample clock if you want all channels to sample the same data. I'm not sure which API you're using, but in LabVIEW you can tweak this by using the More->Delay From Sample Clock->Delay and Delay Units properties in the Timing Property Node. In the other text based ADE's, you should find similarly named get/set function names (e.g. DAQmxSetDelayFromSampClkDelay).

[Matt_W]

Played with this a bit yesterday, works just fine. A delay of around 60 - 70 'ticks' seems to work well.

Many thanks for your help.