why i do get an offset with analog input

[amezam]

Hello Vaidhin,

Do you know if the signal you are inputting from your source has a DC offset? Have you test it in MAX with a test panel? It seems that the ground is wired correctly, do you know if you can use AC coupling? Maybe that can help us minimize the offset, if the problem is related with the DC signal. You said you tested all the modes (NRSE, RSE,..), did all have the same offset?

Thanks,

[Neo6]

Hello all,

Thanks for your support. With all the solutions mentioned in this thread, I corrected one by one like this

1) I checked for proper ground connections

2) I used proper biased resistors ( 68 ohms)

3) I resoldered everything.

4) DC offset in the function generator was set to zero.

Now voltage waveform is good, exactly sinewave and even peak to peak values are exactly matching with function generator.

But in the frequency spectrum, power at the input frequency is very low, it is only -70db and y axis in the power spectrum is in  v square/ Hz rms. 

why the power is still low  and what is this unit  /hz represents ???

Thank you.

[amezam]

Hello Vaidhin,

This unit represents the power spectral density of noise. Check the following links:

Can someone please explain noise per root hertz?

Part III: Resistor Noise and Sample Calculations

Noise in Audio Amplifiers

What is your expected value for power at the input frequency?

Have a good day!

[Neo6]

Hi Amezam,

Thanks for the links. Very informative.Understood the concept.

But in my case, I have just added a Voltage acquisition step and a Power spectrum analyzer step in Signal express. I havent adjusted any parameters regarding frequency range or cut-off frequency. So in my case on what frequency ranges Labview is calculating this / root (HZ). What is the default frequency interval ?????

Thank you.

[macncheese]

Hello Vaidhin,

How did you add the V/root Hz scale that you mention? I was doing your ser up here in Signal Express and I couldn’t find that function.

[Neo6]

Hello Mr.O

After adding power spectrum step in signal express and if you run the program, in the power spectrum graph on y-axis you can see " db rms ref 1.0E+0 V/ sqrt(hz).

Can you please explain me what do this mean. What is the interval this sqrt(hz) consider as default value??

Thanks

-Vaidhin

[macncheese]

Hello vaidhin,

that scale comes form the concept of power.The density is noise power/bandwidth.  To get voltage units you have to take the square root of both.  The square root of power is voltage, the square root of Hz is the square root of Hz.

Now, remember that we have a density here, so that scale is not indicating any bandwidth, but if you want to find the total power in a specific bandwidth then you have to do an integrate of that curve using the bandwidth where you need to obtain the total power, so (W/Hz)*Hz will give you Watts. Does it make sense?

You also can look for Power Spectral Density concepts also to help you understand better.  

[Neo6]

Hallo Mr.O

I been refering to many literatures. But I am not getting clear idea. Can you provide me some links from NI Knowledgebase or Papers .

Thank you.

[Neo6]

Hallo Mr.O,

Do you mean that reference is noise voltage reference ?  So are we comparing our signal with noise of 1V noise signal ?????

Thanks

Vaidhin.

[macncheese]

Hello vaidhin,

I know Power Spectral Density is not an easy concept to understand. Probably NI documentation is not the best place to get a good understanding of this. I would recommend you to go for an Electrical Communications book. Anyways I found this document talking about PSD. Now, I don’t think that the PSD is taking as a reference the noise; probably you are seeing the noise power there as well. That is why even if you generate a single tone you also see spectral components in other frequencies.