Can I use AI clcock config.vi to set external time base

Question

DAQ PCI-MIO-16XE-50 board
According to NI website, I can use "Alternate Clcok rate spec in AI clock config.vi" to set scan clock using external timebase. Does this method in fact change the DAQ cards Master Timebase?
According to NI, I can use RTSI to change the DAQ cards Master Timebase, then all on board clock will be based on the external timebase.
Do the two methods have the same result of changing the DAQ card Master Timebase?

Answer 1

Hi May Lee,
I have some answers for you.
(1) I've never used SISOURCE before, but if it was found on our website, it should work. Assuming SISOURCE accepts an external clock source, that is correct. Scan clock will be set as the external scan rate, and Traditional NI-DAQ will select the fastest channel clock possible except for a minimal 10 microsecond interchannel delay (to allow for settling time for the amplifier and any unaccounted factors). If the scan rate is too fast for Traditional NI-DAQ to apply the 10 microsecond delay and still sample every channel before the next scan clock, than the delay will not be added. Also, if you specify the channel clock rate, the delay is not added.
(2) I'm confused what you mean by setting the channel clock using the internal timebase. If you use the internal timebase of 20MHz as your channel clock rate, this sets the interchannel delay at 50ns. To acquire accurate data, the signal must settle within an accuracy range before the A/D conversion takes place. This is called the settling time. Consult the settling time specifications of your device to see if it can acquire that quickly. Most boards typically have microsecond values.
If you're wondering about settling time issues with your board, consider the following. First, determine the maximum sampling rate and gains your application requires; you must select a DAQ board that settles to within your specifications. If you have a DAQ board already, which I think you do, run the DC or AC settling-time test to determine the settling time for your board. After you know the constraints of your DAQ board, consider the output impedance of your signals and arrange your signals if possible to minimize the swing in voltages between channels. Finally, select cables that are as short as possible with low resistance and capacitance. Using these guidelines, you can resolve all of your settling time concerns and be confident that your acquired data is accurate.
(3) You can set the scan clock and channel clock based on external timebases using the AI clock config VI. You can input the timing signals on any PFI pin or select the I/O connector option which looks for the signals to be input on the PFI pin dedicated for its output.
I've included an example from a PowerPoint presentation below. Let me know if this doesn't resolve the issue,
Chad AE
Applications Engineer - National Instruments
Attachments:
External_Clocking_(AI).ppt ‏143 KB