why in the multichannel daq there is always wrong datas in the begining of signals?

[CUHK]

i use the 6036E for a four channel retrigger DAQ,but i found that my signal is very wierd that  there is alwarys a big rising and falling train in the begining of result data and following with the correct data from No200 data .which i complement this task with 50khz sampling rate and samples of 5000.

i want to know how to avoid this uncorrect reading in this vi , thank you!

scott

[Jeffrey_P]

Scott,

Thank you for contacting National Instruments!  Can you give me a little more information about your physical set-up?  How much is the rising and falling train?  Does it rail to +10 and -10V or is it just a little.  How many data points is it?  Also, what exactly do you mean by "No200 data"?  What are you using to trigger the four channels?  Can you post just the DAQ part of your code?  What kind of signals are you reading?  What are the ranges of the signals that you are reading?

Also, does this always happen?  That is, does it happen upon every retrigger or just the first time the program runs?  Are your programming in LabVIEW, C, VB, CVI?

Answering these questions will help me to better answer YOUR question

-Jeff P.

National Instruments Applications Engineer

[CUHK]

thank you, jeffery.

i am just a little bit frustrated with the performance of 6063E right now,, i wish my problem is just a wrong with my code.

ok, for the understanding, is right to make clear what i am doing. i use the ai0~ai3 for the input and ao0 for the output. the input voltage has very big DC to AC ratio, say the 2~3 volts of DC and about 15mv of AC(due to high backgroud, this is another physical problem), all of therm are positive. the signal come from the 4 independent photodiodes. and the output is a triangle wave with range of -2~2 volts with frequency 10hz. in my setup, i need the triangle wave to drive the PZT to move forward and backward and result in the light to interference in the Photodiode which capture the intensity variation of light.

 i also use the /Ctr0InternalOutput for the internal trigger source. in each trigger duriation, i will acquire 5000 samples with rate of 50k( cause the max rate is 200k for 6036E). and output  several  triangle wave. for the purpose of synchronize the AO and Ai. i try the connection of RSE and DIFF.

the question is  every time of the trigger, the signal of ai0 and ai1 rise from the 0 volts and ai2 and ai3 fall from 10 volts to the real signal about 2~3 volts, and the rising and falling train will rot the first 200 datas( means that i can not use them). the demo code is in attachment. can tell me what's the problem? anyhelp will be appreciated!

scott

[devchander]

Hi Scott,

Take a look at the "Cont Acq&Chart Samples-Int Clk-Dig Start.vi" that is in the Examples shipped with labview.

You will observe that only the AI read function is inside the while loop.

In your application, you are starting( DAQmx start task) and stopping( DAQmx stop)  the AI read and AO write functions in each iteration

Plus you also do not really need DAQmx wait until done in your VI.

Modify your VI on the lines of the example VI mentioned above and your code should work

Regards

Dev

[CUHK]

hi,Dev

thank your very much for your reply.

i try your idea but there is a error that the AO can not work if the AO write is behind the start which lead to a empty buffer , and also how to synchronize the AO and Ai with the start out the loop? BTW, i also cannt found the vi example you mentioned. but if without the synchronize and the AO, i know that put the start and stop out loop can work out , and the wait unitil done is for the avoid of the error of stop loop suddenly.

can you give me little bit detail hints of your suggestion, maybe i dont got the point.

thankyou and regards

scott

[devchander]

Hi Scot,

Here is a modified version of your VI

Run it and let me know how it works

Regards

Dev

[CUHK]

hi, Dev

thank you

the modified vi have a error----"Attempted to read a sample beyond the final sample acquired. The acquisition has stopped, therefore the sample specified by the combination of position and offset will never be available." maybe some other comfiguration is exected

but really thank you anyhow,

scott

[devchander]

Sorry scott,

I do not have a multifunction DAQ card with me today, so could not test the code before sending.

Will see if i can get my hands on a daq card and try to get this VI to run

[CUHK]

Hi, Dev

it's ok,

what i am worry now is that whether it is related with the hardware, say the capacity in the DAQ card cannot decharge as soon as possible cause the setting which i am not sure.

cause there is a case that if i disconnect the AI2,3, from the photodiode , the data of Ai2,3 acquire will be the same with AI0,1 respectively......it looks like very unnormal,maybe due to some unclear cause, but i sure that the card is ok itself.

thank you for so many of time, and look forward the good suggestion from you.

scott

[Tom_W_[DE]]

Hi Scott-

It's not uncommon to see severely ghosted data on disconnected channels.  After all, as you said, there is no way to discharge the PGIA and analog input circuitry when there is no supplied path to some reference.  You should always connect signals to the channels you're scanning or your data will be quite unpredictable.

Hopefully this helps-