Real acquisition frequency does not match with sampling frequency setup in Labview

[YangYang]

I am using four Kulite XTEL-190-SM pressure sensors to measure dynamic pressure inside twin screw compressor. Four KSC-1 BU4 conditioners are used to process the data. When we setup sampling rate of 50 kHz in Labview , the Real acquisition frequency is around 25 kHz (by counting the number of data in cycles). When the sampling rate is set as 20 kHz in Labview, the data shows the real sampling rate is around 12 kHz. The PC is 8 cores and 16 GB RAM. The CPU is 100 % when I did the test.

May I ask what is the reason caused by the mismatching between real acquisition frequency and the sampling frequency setup in Labview.

Thanks.

Yang

[GerdW]

Hi YangYang,

how do you set the sampling frequency?

Which DAQ hardware do you use?

Does your DAQ hardware support the requested sample rates?

(I hope you DON'T try to read each samples as "one after the other", as you should read them in larger blocks of samples…)

[YangYang]

Hi GerdW,

Thanks for your reply.

It's an ADC c-module reading 10v signal, this is processed by FPGA to save to FIFO buffer. The module is 250kS/s running 16 channels. The LabVIEW is accessing FIFO to get the sets of data and processes it to display diagrams and save files. Should I get DAQ hardware from NI?

Best regards,

Yang

[GerdW]

Hi Yang,

you still don't tell which DAQ hardware you actually use…

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@YangYang wrote:

It's an ADC c-module reading 10v signal, this is processed by FPGA to save to FIFO buffer. The module is 250kS/s running 16 channels. The LabVIEW is accessing FIFO to get the sets of data and processes it to display diagrams and save files.

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• Which "ADC c-module"?
• Which FPGA?
• Which LabVIEW version?

[YangYang]

Hi GredW,

Sorry for missing your questions. The DAQ is hardware is NI9205. The ADC c-module and FBGA are functions in Labview. The version of Labview is 2020.

Thanks.

Best regards,

Yang

[GerdW]

Hi Yang,

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@YangYang wrote:

The DAQ is hardware is NI9205.

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This is an AI module for voltage signals, supporting 16 DIFF or 32 SE channels - and using a single ADC with a MUX.

So the given sample rate of 250kS/s has to be divided by the number of channels in use! When you are reading 16 channels (as noted in the first message), then the max sample rate is 250/16 kS/s = 15.625 kS/s…

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@YangYang wrote:

The ADC c-module and FBGA are functions in Labview.

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The ADC is located within the NI9205 module. Your "c module" probably is a cRIO chassis, that contains a RT CPU and a FPGA…

[YangYang]

Hi GredW,

Thanks. I believe you are absolutely right about the ADC and c module. May I ask two questions.

1. As shown in the attached picture, I am using 4 channels. Will the max sample rate is 250/4 kS/s = 62 kS/s?

2. When I set sampling rate as 50 kHz, it gives real sampling rate around 25 kHz. Is the hardware cause the difference or the labview I did not program it correctly?

Thanks.

Best regards,

Yang

[GerdW]

Hi Yang,

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@YangYang wrote:

1. As shown in the attached picture, I am using 4 channels. Will the max sample rate is 250/4 kS/s = 62 kS/s?

2. When I set sampling rate as 50 kHz, it gives real sampling rate around 25 kHz. Is the hardware cause the difference or the labview I did not program it correctly?

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1. Yes, using/reading 4 channels will result in 62.5kS/s max samplerate…
2. How should we know when you don't show your LabVIEW code?

(When attaching code I suggest to downconvert from recent LabVIEW versions down to LV2019 using File->Save for previous.)

[YangYang]

Dear GerdW,

I am preparing the Labview code and will share here later. May I ask a question about KSC-1 BU4 conditioner.

- It has 4-pole low-pass filters with four selectable cutoffs (BU4: 100 Hz, 5 kHz, 25 kHz and 50 kHz). What is the maximum sampling frequency can this conditioner deal with?

- Should we select 5 kHz filter during our test? Our harmonic frequency is from 120 Hz to 140 Hz and we just need first four orders of the information.

Thanks.

Best regards,

Yang

Best regards,

Yang

[GerdW]

Hi Yang,

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@YangYang wrote:

- It has 4-pole low-pass filters with four selectable cutoffs (BU4: 100 Hz, 5 kHz, 25 kHz and 50 kHz). What is the maximum sampling frequency can this conditioner deal with?

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This question should be answered in the manual of your signal conditioner device…

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@YangYang wrote:

- Should we select 5 kHz filter during our test? Our harmonic frequency is from 120 Hz to 140 Hz and we just need first four orders of the information.

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So it sounds the 5kHz filter should be fine for your requirements…