Sampling and reading rate on NI9209

[mwally]

Hi all,

I am using a National Instruments Analog Input 9209 C-series to acquire and digitize 3 signals coming from a photodiode. I've read about the sampling and reading rate in NI DAQ devices, however, I want to double check that I understood the limitations of my equipment.

1. When using continuous sampling mode and High Resolution, this means that the sampling rate is 52 ms (as specified in the datasheet) times 3 channels = 156 ms or 6,41 Hz. Is this the fastest that I could read in this mode without reading the same value multiple times or ending with a buffer overflow (reading rate smaller than sampling rate)?

2. Similarly, when using continuous sampling mode and high speed, the sampling rate would be 2 ms (as specified in the datasheet) times 3 channels = 6 ms or 166.6 Hz. Is this the fastest that I could read without reading the same value multiple times or ending in buffer overflow?

I want to put my reading rate the same as the sampling rate, so that I get unique values on the buffer everytime the DAQ Assistant fetches them. Therefore, putting a reading rate faster than the sampling rate doesn't make sense.

Thanks in advance.

[cbutcher]

Regarding the sampling rates (and corresponding times) yes - that's what I also understand from the datasheet.

The title specifies "500 S/s aggregate", in constrast with some other NI devices that use terminology like "N S/s/ch" or similar, and phrases like "simultaneous sampling".

Regarding "reading rates" - I'm not exactly sure what you're referring to. If you mean the rate and which you run a loop containing something like a DAQmx Read node, then you'll find that the read takes the time you described (156ms or 6 ms) and so you can't wait for less than that time (for 3 samples, 1/ch), or a multiple if reading multiple samples/ch.

I haven't tried using Scan Mode, but I expect a similar behaviour. Maybe this uses the "reading rate" terminology (although I've only seen "Scan Rate" described previously)?

[TSE_Expert]

Hi mwally,

in high-resolution mode the convesion time per channel is really 52 ms, so the conversion rate is 19.23 S/s aggregate.

If you measure on 3 channels in parallel, you get 6.41 S/s for each channel, since the sampling rates per channel are equal. In high-speed mode Tconv=2 ms and the available sampling rate is 1/Tconv=500 S/s, as also explicitly stated in the datasheet, so it leads to 166.67 S/s physical sampling rate for each channel.

When you read data from the FIFO buffer, through the PC Buffer to the Application Memory, for example with the DAQmx Read VI, you automatically read each sample only once, if you don't read too slow. This is ensured by DAQmx:

http://zone.ni.com/reference/en-XX/help/370469AP-01/lvdaqmx/mxread/

So until you read a sample from the device, it's buffered. When you read it once it's gone, you cannot read it multiple times. The risk is, that you read too slow, and your PC buffer gets overwritten (Overwrite Error) or the data is overflown in the Device FIFO (Overflow Error).

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000PAraSAG

Best Regards,

NI TSE

Here you find the datasheet of the 9209:

http://www.ni.com/pdf/manuals/376909c_02.pdf

Best Regards,

NI TSE