Conversion Rate for Simultaneous DAQ

[Daniel K.]

I am using a PXIe-6366 MS/s/ch multifunction DAQ. The maximum sample rate for the analog inputs is 2MHz. Is the actual conversion time of the ADC always 0.5µs, or does it change with changing the sample rate? Can I set the conversion time? From reading, it seems like with muxed devices, you can set the convert clock and the sample clock separately. What about with simultaneous DAQ?

[GarPar · Accepted solution]

Hi Daniel K.-

     This is a great question. I wonder if you are getting any errant behavior - is your card not returning samples as you expect?

     Here is some information that might help you:

What Kind of Analog-to-Digital Converters Are Used in National Instruments Multifunction DAQ Devices...

Wikipedia: Successive approximation ADC

     I am unsure exactly what happens, but would venture an educated guess that the ADC digitizing time does not vary with sampling rate.  And it is also going to be at most 0.5 us - it is more than likely faster than that.

     I hope this helps.  Best of luck with your application!

[GarPar]

You also cannot set the convert clock on simultaneous sampling DAQ as they do not have one.  The convert clock is the amount of time between multiplexed measurements on muxed cards.

[Daniel K.]

The card is behaving correctly. I was asking because the conversion time of the ADC is an important consideration relative to the noise bandwidth of the voltage source. I understand your explanation for the simultaneous device. On the other hand, your answer is confusing relative to muxed DAQ devices. For instance, from the X-series manual, there is an explanation on page 4-25 that shows that the convert period is adjustable with the conversion clock. I think what NI calls "convert period" is a separate thing from the ADC conversion time. Am I correct?

[Seth_B.]

This is correct.  The ADC conversion time cannot be changed on X Series or most other DAQmx devices.  When NI refers to "convert period" this is the period between each channel's conversion on a multiplexed device.  "Conversion period" encompasses the multiplexing time, settling time, and ADC conversion time.