offset voltage without connecting any signal

[Neo6]

Hello All,

I am tring to read a signal given from a signal generator to a resistor of 50 ohms using NI hardware. Mine is very simple circuit. I connected signal generator and DAQ  parallel to a resistor of 50 ohms. 

Signal from function generator is 2 KHz and 20 mV p-p .  The signal which I am getting in the signal express starts ocsillating between 2 and 3 volts. I am attaching the picture of that . Then the range of amplitude starts getting higher and higher. 

Even when the DAQ terminals are left free (without connecting any signal input),  signal express is reading some voltage. Picture attached.

My hardware

1) scb-68 Connection box

2)NI Pxie-6361 DAQ

3)NI Pxie-8360 controller

4)NI Pxie-1071 chasis

5) Agilent 33250A function generator.

I have gone through one discussion which discussed the the exact problem I am facing now. I tried with all the solutions in that post, but i cant figure out the problem.

This is the post describing the same problem I am facing

http://forums.ni.com/t5/Measurement-Studio-for-VB6/why-i-do-get-an-offset-with-analog-input/m-p/6168...

Please help me. Thank you

[AnalogKid2DigitalMan]

With no signal connected, what you see is normal.

I noticed with your signal connected, you have the task set up as RSE. Did you connect the fgen's ground to the analog input ground too?

I did not see an increase in your signal as you mentioned. It looks like there is a constant DC offset. Most fgen's can supply this so make sure the DC offset is set to 0 on it..

If that helps, then decrease your input range from +/-10V to a smaller setting for better accuracy and resolution.

-AK2DM

[AnalogKid2DigitalMan]

Not rated to your offset issue, but I just noticed that your sampling rate of 1Ks/sec is too low. You must sample at least 2 times as fast as the frequeny of interest to prevent aliasing.

Look up the term Nyquist.

So change your sample rate to at least 4Ks/sec.

-AK2DM

[Neo6]

Hallo AK2DM,

Thanx for the response. Seems like I got some new points in your reply. I only connected output of Fg across resistor and DAQ AI+ and AI-, I havent connected  Fg chasis ground to AI ground.  I will modify these chages and report you back

thank you 

[Neo6]

Hallo AK2DM,

I have connected the Fg chasis ground to the AI GND, increased my sampling rate, set my Fg offset to zero, But there is no improvement. I am attaching graph of voltage. Have a look at it, then you can understand what I mean that voltage keeps on  increasing with time. And also in your previous post you mentioned that , signal without connecting any input looks normal. But what about the waveform which is shown on the graph ???

Input frequency 2 KHz, amplitude 10 mV pp ,  

Thank you

[Neo6]

Hallo Ak2DM,

My problem( Voltage increment ) is not yet resolved. I am also experiencing lot of 1/f noise. how to eliminate that ??

Thanks

Vaidhin

[AnalogKid2DigitalMan]

I am not sure what is going on at this point.

Can you post a sketch of how you wave connected everything up? How do you make the connections to the function generator (ie. coaxial cable to a pigtail connection)?

Can you also post the settings for your acquisition.

Do you have an oscilloscope to view the output of the function generator?

Sorry, more questions than answers.

-AK2DM

[Neo6]

1)Sketch of my ciircuit is very very simple. I have soldered output of a function generator and input to a oscilloscope and Input to a daq across 50 ohms resistor. This is my circuit.

2) Regarding cables, I used this cable (http://www.pomonaelectronics.com/pdf/3957_100.pdf). I used two cables of this type, one from FG and one to the oscilloscope. I cut the banana ends of this cable and soldered cross resistor.

3) I am posting some screenshots which shows my settings

4) I am using oscilloscope in the circuit.output on the oscilloscope was perfect without noise. parameters like voltage and frequency were same as function generator.

5) I also connected FG chasis ground to the AI ground.

I am in confusion whether everything is Ok with the daq. so I am uploading screenshots of signal reception. I am uploading voltage time log data, voltage display graph and power spectrum graphs with and without signal connected. I have also tried in Differential, RSE, NRSE modes.

[Neo6]

These are the screen shots from signal express, when there is no input to the daq.

[johnsold]

It looks like you may still have a grounding problem. What terminal on the DAQ device is connected to the black wires in your cables?

Power spectrum with input:  The DC component is >50 dB above the 5 kHz component.

Recorded voltages data with signal connected: This looks like the drift of an input which is not grounded properly or a differential connection without appropriate bias resistors.

Voltage signals when connected: This clearly shows a 50 Hz component (probably power line frequency).

Power spectrum with no signal connected: DC component > 100 dB above baseline noise.

Recorded voltage data with no signal connected: Same drift as mentioned above.

Voltage signal when DAQ terminals r not connected any signal:    ''

All of these are typical of a system where grouding and biasing are not connected properly.

Please tell us exactly how everything is connected. Your description did not include ground, reference, or common connections. It did not describe the  function generator settings and A/D sampling rates. (You mentioned 2 kHz earlier but your spectrum shows 5 kHz). If your DAQ device is set to the +/-10 V range as needed to handle the DC component, then your resolution is about 0.3 mV which will not give you very good results on a 10 mV signal.  This is approximately equivalent to a 6-bit A/D converter, if you have no noise.  The last image shows noise of about 1 mVpp.

What is your final goal?  What characteristics of the signal do you need to measure.  What kind of interference or noise do you expect?

Lynn