Different FFT result of LJLogM and LJStreamM

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Fri, 01/14/2022 - 06:46

Diego Luna

Different FFT result of LJLogM and LJStreamM

Hi,

I have tested Accelerometer, ADXL354 using Labjack T7-PRO, sampling rate 1000 Hz.

I have logged the data with LJLogM and LJStreamM and compared FFT result using SigView.

I expected that FFT result will be same between LJLogM data and LJStreamM data.

However, FFT Frequency is too different as attachment of LJLogM 04.png and LJStreamM 04.png.

I wonder if LogM is not suitable for high speed sampling like acceleration measurement.

I would appreciate any advice on a phenomenon like this.

File Attachment:

LJLogM 01(Screen Capture).jpg

LJLogM 02(FFT analysis).png

LJStreamM 01(Screen Capture).jpg

LJStreamM 02(FFT analysis).png

Fri, 01/14/2022 - 10:39

LabJack Support

We recommend LJLogM for

We recommend LJLogM for acquisition slower than 100 scans per second and LJStreamM for higher rates. LJLogM uses command response communications, and typical command response overhead is between 0.5-2ms depending on the connection type. There is also the actual AIN sampling time for each channel and other potential acquisition loop overhead when using LJLogM. This limits the max rate and it also results in less consistent timing than the hardware timed acquisition of stream mode (as used in LJStreamM). Our data rates page has some other relevant information about command response vs. stream and maximum data rates:

https://labjack.com/support/datasheets/t-series/appendix-a-1