simultaneos generation and acquisition( i cannot see my generated signals att eh input of UUT)

[Chaitanya]

Hi,
I want to generate 5mv, 1 khz sine signal from labview to UUT(connected externally to DAQ) thru DAC0 and accept the signals from UUT thru ACH0. To check whether the signals I generated are going into the UUT, I connected a probe of oscilloscope at DAC0. But I could not see any signal at pin of UUT corresponding to DAC0.So my signal is not going to UUT at all. To troubleshoot the VI that I generated, I disconnected the UUT. But instead, I connected a probe from DAC0 of DAQ to ACH0 of DAQ. So i am redirecting my generated signal from DAC0 to ACH0 of DAQ itself and then I checked the received signal using oscilloscope at ACH0. I got the exact results. But only when I d
o this by including the UUT, my generated signals doesnt go in. Please help me. I am using PCI-6040 E series DAQ board. I am attaching the code I developed for this purpose. Please review it and advice me.Thank you all!!

[waldemar.hersacher]

Hi Chaitanya,

there are three things which will give you problems:

1. You want to generate a very small sine signal. If you use internal reference you would see that the resolution is near your signal of 5 mV.

2. The second problem is the load of the output. Any E - Series card can only drive a current of max. 5 mA. If the impedance of your UUT is to low the signal will go low too.

3. You are using two high level DAQ VIs. There is no control about which of the DAQ VIs will be executed first. There is no data flow dependence between them. If AO Waveform Gen will be executed first the waveform has been generated before AI Scan Waveform will start executing. Or you will first read in and then generate the waveform. In both cases you will not measu
re the signal.
Combine the examples under "Hardware Input and Output >>Getting started>>Getting started Analog input" and "Getting started analog output" in that way that AI Start will be executed before AO Start. Do this by using a sequence or a state machine. At AI Start use Leading edge for the slope and Digital trigger A for the trigger. Connect PFI6 with PFI0. This way you will start the generation and acquistion at the same time.
Read the LabVIEW Measurement Manual from the LabVIEW Bookshelf (should be installed together with your LabVIEW) for more information.

Waldemar

[Chaitanya]

Hi Mr.Waldemar,
Thank you for responding. My problem is that, I could not generate my waveforms in the first place.When checked by oscilloscope probes at the pins of circuit board connected to the output channel DAC0 of DAQ to check whether my signals are generated and reaching the pins or not, I could not see anything. So my signals are not generated itself. Is this problem attributed to the Step.1,2 that you described? If so, what do I need to do to rectify the problem? Is this problem related to the hardware part or software part of project. When I am successful in generating the signals, then I can focus on acquiring them simultaneously as described by you in step 3.
Please take time to help me.
Thank you.
Chaitanya Ausuri

[Chaitanya]

HI,
With regard to step 3 that u described if there is any problem in simultaneous generation or acquisition of VI's in my program, an error should result if AI starts before AO or AO overwrites the buffer before AI can read etc..But, my program doesnt give any error message when run. It runs continuously until I press the stop button.Also, as decribed initially, to test this process, I disconnected UUT. I redirected signals generated at DAC0 of DAQ to ACH0 of DAQ by using a probe.Then I checked the signals coming out of ACH0.IT gave me exact results. I generated 5mV at DAC0 and I got 5mV at ACH0. But when I connect UUT, the signals are not even going in at the input pins.
Please help.
Chaitanya Ausuri

[Chaitanya]

Hi...sorry for asking you so many questions. I am just a beginner in labview, so i did not understand what PF16 and PF10 are that u mentioned in step 3. I would appreciate if you can(if possible) provide me with some example for simultaneos generation and acquisition( generation starts first and then acquisition starts).
Thank you(...and sorry for bothering you so much).
Chaitanya Ausuri

[waldemar.hersacher]

Hi

sorry for my late response but yesterday I did some private work.
Back to your first response:
This is hardware part related. The 6040 card has a internal reference of +-10 Volt in bipolar mode. The DAC has a resolution of 12 bit. This will result in steps of 4.88 mV per bit. A sine with 5mV amplitude will give 0 mV, 4.88 mV, 0 mV, -4.88 mV which is more a rectangular wave than a sine. The best solution for this is to use external reference voltage. I do not have any experience which will be the smallest voltage you can use and the user manual, as far as I have seen, does not give any hint.
Next is the load. The user manual states that the card can drive -+5mA. If your load needs more current the signal will decrease. You say you
will have no signal when connecting the UUT I think there is a short circuit in the input of the UUT. Have you checked this?
Why have you measured 5 mV at the input of ACH0? The solution is the following: The last generated output will remain when the signal generation is ready. If your waveform ends with 5 mV then this voltage will stay after the genertion. The reading of the waveform will then read in this last sample.
You will not get any error by using AO and AI because both operations use different buffers.
PFI6 and PFI0 are signals also named WFTRIG and TRIG1. These signals are on the connector of the card. WFTRIG is set each time a waveform generation starts and TRIG1 will start a waiting waveform read.
For an example I have you to wait until tomorrow because I will test it on the PXI-System in the office.

I don't know in which time zone you life but I reside in Germany. Take this into account for "tomorrow".

Waldemar

[Chaitanya]

Hi Mr.Waldemar,
Thank you for your responses. regarding the resolution aspect which you explained, it appears that the DAQ i am using doesnt satisfy the signal levels that my test procedure recommends.So, is 6040 E series board not suited for this application?
I will check for any short circuit in UUT and let you know the details.
I mesaured 5mv at ACh0 just to see whether the vi i developed for simultaneos gen and acquisition is correct or not. Because, without any UUT connected to channels of DAQ, I connected a probe from DAC0 to ACH0 and run my VI. I generated 5mv at DAC0. So if my VI is correct, this generated 5mv gets redirected from DAC0 to ACHO and we can see it in labview inturn at AI part of VI. I saw a perfect 5mV in labvie
w. So my VI is correct.This is true even if i restarted the PC.
I look forward for your example.
Thank you for your time and interest in offering me your expertise.
Regards,
Chaitanya Ausuri.

[Chaitanya]

Hi,
As you said, is it not possible to measure such small voltages like the 5mv sine wave which I want to measure? If yes, then how can we do that?
Thank you for your time.
Regards,
Chaitanya Ausuri

[waldemar.hersacher]

Hi Chaitanya,

today I had the time to test your VI on the PXI-System in the office. I found out the following things:

My configuration was: PXI 8176 Controller with PXI-6040E card. The card was configured for bipolar AI and NRSE. I connected DAC0 with ACH0. I used a oscilloscope to check the signal.
Your while loop will do the four steps:
1. Generate the signal with a amplitude of -1.00 Volt to +1.00 Volt and 10 Hz.
2. Generate the output. You have to wait until AO Waveform Gen.vi will have the waveform generated once. Then the waveform will be generated continously.
3. The waveform will be read by AI Waveform Scan.vi. The start is done randomly at any point of the waveform.
4. The waveform is analyzed by the o
ther VIs.

This is done on each iteration of the while loop. On the oscillosope you can see that the sine wave will have a short time (depends on your system) where the generated value is constant. This is during the reinitalization at the beginning of AO Waveform Gen.vi.
The indicator "detected amplitudes" shows a value of 499.85m. In the indicator "from UUT to LabVIEW" you can see that the acquistion is not started with the waveform because the first sample is not 0. On my system it was always near 0.2 Volt.

My questions are:
What amplitude do you really want to generate? Now you generate a 2 Vpp amplitude or 0.7141 Veff.
Is it critical for your analysis if the AI is not started at the point where the sine goes through 0? If not you could leave your VI as it is.

For further discussions you can contact me at buero@hersacher.de.

Waldemar