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Thanks for posting your question to the forum. I suspect there may be an issue with grounding. I recommend looking through the Developer Zone article, Field Wiring and Noise Considerations for Analog Signals, which describes in detail how to properly ground a signal.
The important things to note are whether you have a floating
signal source (not connected to building ground) or a grounded source and the
input configuration of your measurement (differential, referenced single-ended,
non-referenced single-ended). For
example, if you have a floating source and you are attempting to measure the
signal in differential mode, you would need to add two resistors (between
10kohms and 100kohms) to properly ground the signal. One resistor would connect AI+ to AI GND and
the other resistor would connect AI- to AI GND.
Post back with more detail about your signal source and input configuration if you need more assistance.
Thank you Hani for your help it really solved the problem as this solution was suggested to me by my friend and i used this but din know wheather its fine or not. Please classify that which inputs should be treated as floating and which inputs should be treated as RSE and NRSE inputs.
I am using a function generator which has a grounding signal along with original signal. I have connected 100k Ohm resistance between AI GND and AI0, AI1 inputs. Now when i acquire the input in RSE mode it gives me accurate results. But when i use differential mode i dont get it right. Whats the problem in that? Because you suggested that i should use differential mode. But i think function generator's input which provides a ground itself i should RSE input mode. What do you say about it? Please guide me if i am wrong. I will be very thankful to you for your guidence.
To measure a differential signal with
the PCI-6025E, you will want to use ACH<i> and ACH<i+8>. Therefore, if you are using AI0 for AI+, then
use AI8 for AI-. Remember that for a
grounded signal source, you do not need to use bias resistors.
I could not locate the Signal Compression VI, but I did find the Sample Compression VI (Express VI) in LabVIEW. If you have LabVIEW Application Builder you can save the express VI in a new VI and build a DLL that you can call in VB6. Remember that you would have to configure the inputs and outputs on the connector pane of the new VI before building the DLL.
It may be less work for you to use LabVIEW to program your application. In LabVIEW, you can find the same continuous acquisition example that you are using by going to Help » Find Examples then selecting Hardware Input and Output » DAQmx » Analog Measurements » Voltage. The example sets up the hardware to continuously read data and store it on the buffer. Data of N samples will be pulled off the buffer every iteration of the While Loop.
I am facing the same problem as this thread starter. I tried all the solutions prescribed above, and all the configrations available in manual, but cannnot solve the problem.
I am attaching the screen shot of signal acquired, when the AI were not connected with any signal. What might be the problem ????
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 gave a signal of 2 Khz 20 mV peak to peak across a resistor of 50 ohms. I am trying to read the signal across resistor.
All devices ( Pxie chasis and signal generator) are perfectly grounded
Can you elaborate a little more on how are you exactly wiring your device and what modes are you using?
Have a good day!
my setup was very simple. I have connected a FG( Agilent 33250), an oscilloscope and SCB-68 AI 0+ and AI 0- terminals parallely across a 50ohm resistor . To connect function generator and oscilloscope parallel across resistor I have used these cables(http://www.pomonaelectronics.com/pdf/3957_100.pdf). I cut the banana ends of cable and soldered across resistor. I have also connected FG chasis ground to the AI 0 ground on the SCB-68. This is how my circuit looks like.
Settings on FG : output frequency 5K, voltage 8 Vp-p, offset 0
output on the oscilloscope was perfect without noise. parameters like voltage and frequency were same as function generator.
1)But while reading the daq channels , voltage readings were not perfect. I am not getting correct peak to peak value. Also voltage graph seems increasing with time. I have attached the screenshot of it.
2)In the frequency spectrum of the signal gain is also very low , only -60db.
3) I also observed that there is 1/f noise. How to get rid of this ?
I am new to this DAQ.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.
These are the screen shots from signal express, when there is no input to the daq.