Ni 9215 measure problem 4kHz frequency appear in data

[ASakhno]

Hi,

I have the simalar problem as http://forums.ni.com/ni/board/message?board.id=250&thread.id=44182&view=by_date_ascending&page=1.
I use NI 9215 (with Ni-9163/Enet) to measure 50Hz current signal, I have try different sample rates from 1k to 100k and have the same trouble, some periodic signal 4kHz on my 50Hz signal, my scheme and result are on the attachments. I try shemes with 1MOhm resistor between "com" and "AI0-" and without it, resuls are equal. I measure this signal with another daq device - Dataq company and with oscilloscope and there is no any 4kHz noise on my signal. What wrong in my scheme, I couldn't use such signal in my application. Please answer, if such problem was solved and what I should do.

Alexandr

Message Edited by ASakhno on 02-22-2009 07:04 AM

[Jordan F]

I would recommend to review the document below because it discussions reasons as to why you may be seeing signal distortion and proper connection methods.  It looks like you have a resistor to ground from the negative terminal but it is recommended to have a resistor to ground from the positive terminal also (with a value of 10kohms up to 100kHz).

Field Wiring and Noise Considerations for Analog Signals

[ASakhno]

Hellow, Jordan.
Thank You very match for Your reply. I have read Field Wiring and Noise Considerations for Analog Signals, and it seems like everything right in my scheme. I have tried also the scheme figure 11, with two 22kOhm resistors. The results were equal. Do You have one more ideas?
Thank You very match.
Alexandr.

[Jordan F]

Were you able to see if changing the sampling rate of your Daq device per Chris' advice on the other post that you have? 

"Perhaps it's not really 4kHz noise, but something that aliases to 4kHz.  The 9215 has a bandwidth beyond 400kHz.  (http://www.ni.com/pdf/manuals/373779e.pdf)  If there's 404kHz noise on the signal, for example, and you sample at 100kHz, or 50kHz, or 20kHz,... the 404kHz noise will appear to be at 4kHz.  Try sampling around 99kHz instead of 100kHz.   If the noise no longer appears to be at 4kHz, then it is aliased from a higher frequency.  What is the bandwidth of the Dataq device?"

https://forums.ni.com/t5/Multifunction-DAQ/certain-frequency-artifacts-appear-in-data/m-p/802143?vie...